SPECIAL

ARTICLE

Coagulase-negative staphylococci: Interplay of epidemiology and bench research Donald A. Goldmann, MD Boston, Massachusetts

Coagulase-negative staphylococci (CNS) are major nosocomial pathogens in patients with prostheses and indwelling devices such as central venous catheters. For Staphylococcus epidemidis the unique association with foreign-body infections appears to be due in part to a capsular polysaccharide adhesin that mediates attachment to silicon elastomer and other biomedical materials. In addition, staphylococcal “slime” may promote persistent colonization of indwelling devices and protect staphylococci from clearance by host defense mechanisms. Given these research findings, it seemed reasonable to assume that nosocomial CNS bacteremia in neonatal intensive care units might be associated with the use of indwelling vascular lines, as had been suggested by other investigators. We found that CNS cause the majority of nosocomial bacteremias in our neonatal intensive care units and that low birth weight and length of stay are major independent risk factors for these infections. In addition, we confirmed the association of central venous lines with CNS bacteremia but were surprised to find that intravenous administration of lipid emulsion was an even greater risk factor. These observations have brought our work back to the laboratory, where a rabbit model of CNS catheter infection is being studied to explore the relationship between lipid emulsion and catheter colonization and bacteremia. (AM J INFECTCONTROL1990; 18:211-21)

Suppose your every wish as an infection control professional were suddenly granted. Doctors and nurses no longer forgot to wash their hands. Precautions were selected appropriately and applied promptly and conscientiously. Surgical technique approached perfection. Antibiotics were selected with care and were administered only as long as absolutely necessary. Would nosocomial infections disappear? Would the incidence of infection even decline substantially? My suspicion is that nosocomial infections will remain a prominent feature on the hospiFrom the Infection Control Program, Department Children’s Hospital.

of Medicine,

Reprint requests: Donald Goldmann, MD, Division of Infectious Diseases, Children’s Hospital, 300 Longwood Ave., Boston, MA 02115. 17152118392

tal landscape no matter how comprehensive and aggressive an institution’s surveillance and control program. For many patientsthose who require intensive care, immunosuppressive therapy, or prosthetic implants, for example-substantial reductions in the risk of nosocomial infection may depend as much on progress in basic and applied research as on new infection control initiatives. My hope is that improved understanding of the pathogenesis of specific infections will lead to customized interventions designed to block critical steps in the attack of nosocomial microorganisms. However, if we in the epidemiology community expect investigators to ask relevant questions concerning the most important nosocomial infections, we will have to provide considerable guidance. Unfortunately, collaboration between hospital epidemiology programs and research laboratories has been sporadic and 211

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inconsistent at best. The time has come for workers at the “dry bench” (the epidemiology “laboratory”) to work more closely with their colleagues at the “wet bench” (the basic and applied research laboratories). My colleagues and I have tried to implement this collaborative approach to fundamental nosocomial infection problems at Children’s Hospital. To illustrate this model I will discuss our epidemiologic and laboratory studies of coagulase-negative staphylococcal infections. At this meeting,* at the Annual Meeting of the American Society for Microbiology, at the Interscience Conference on Antimicrobial Agents and Chemotherapy, and at virtually every major infectious diseases congress in the world, the refrain is similar: gram-positive bacteria, especially coagulase-negative staphylococci, have displaced gram-negative bacilli as the principal cause of nosocomial infections in a wide variety of clinical settings. Yet, just a decade or two ago, recovery of coagulase-negative staphylococci from clinical specimens was treated with near contempt by infection control specialists. Isolation of these organisms was assumed merely to reflect contamination of the culture, and the results often were ignored. Certainly I shared this skepticism when I was in the Hospital Infections Branch of the Centers for Disease Control in the early 1970s. If coagulase-negative staphylococci showed up on a line-listing from a National Nosocomial Infections Study hospital, I wondered whether the infection control practitioner was just regurgitating a microbiology laboratory report rather than critically examining the patient, and I entered the “infection” into the computerized data base with great reluctance. Now, of course, coagulase-negative staphylococci are something of the darlings of the infection control profession, and the epidemiology and pathogenesis of the infections they cause are the subject of intensive investigation. The key question, of course, is, “How did these harmless skin commensals suddenly become major nosocomial pathogens?” In large mea*National Foundation for Infectious Diseases Lecture presented at the Sixteenth National Conference of the Association for Practitioners in Infection Control, Reno, Nev., May 22, 1989.

American Journal of INFECTION CONTROL

sure the answer lies in the rapidly changing nature of patient care in recent years. Our patient population is older, or, in the case of neonates (the subjects of our research), smaller and increasingly premature. Our hospitalized patients generally are more severely ill and likely to be immunosuppressed. Their length of stay is longer-at least it,was before the introduction of prospective payment and diagnosisrelated groups. Patients are being exposed to an increasing array of invasive diagnostic and therapeutic procedures that breech normal physical barriers to microbial invasion. Surgery has become more complex and aggressive, often involving implantation of prosthetic materials or devices. Thus the general factors that convert a saprophyte into a pathogen seem clear. Take a critically ill elderly patient; put him in a hospital for a prolonged period; bypass the protective barriers of the skin with intravenous and intraarterial catheters, a urinary drainage catheter, an endotracheal tube, and other invasive devices; assault him surgically; and implant a prosthesis; and you have set the stage for a coagulase-negative staphylococcal infection. In the face of these overwhelming host risk factors, I suspect that it will be difficult to elucidate the more subtle microbiologic factors that help coagulase-negative staphylococci produce infections. After all, compared with its relative, Staphylococcus aureus, the coagulasenegative staphylococcus is notable for its lack of recognized virulence factors. We do, however, have one extremely important clue: the presence of a foreign body is virtually a prerequisite for invasive disease, except, perhaps, in the most immunocompromised patients. In animal models an infection is nearly impossible to produce in the absence of a foreign body, even by injecting large numbers of coagulasenegative staphylococci intravenously or intraperitoneally. In contrast, infection can be induced if a catheter or prosthesis is in place when staphylococci are inoculated. The same phenomenon can be documented in clinical medicine. For example, coagulase-negative staphylococcal infections are extraordinarily rare in neurosurgery unless a diverting cerebrospinal fluid shunt or other foreign material is employed. Even patients who undergo massive re-

Volume 18 Number 3 June 1990

sections for brain tumor almost never have a coagulase-negative staphylococcal wound infection . This unique association between coagulasenegative staphylococci-particularly Staphylococcus epidermidis-and foreign bodies has been seen with virtually every implantable device, including cerebrospinal fluid shunts, prosthetic heart valves, pacemakers, peritoneal dihemodialysis shunts, prosalysis catheters, thetic joints, vascular grafts, and, of course, intravascular catheters. The literature concerning coagulase-negative staphylococcal foreign body infections is far too vast to review here, but I return to the example of cerebrospinal fluid shunt infections, not only because they play such a prominent role in pediatrics but also because they illustrate several important points. First, as is the case with many foreign bodies, coagulase-negative staphylococci are the most frequently isolated pathogens.‘m4 Second, removal of the shunt, which is frequently necessary to cure the patient, is accompanied by predictable morbidity and prolongation of hospitalization. Third, at least in our hospital, coagulase-negative staphylococci have become increasingly resistant to oxacillin, and more than 85% of strains recovered from inpatients are not susceptible to this class of antibiotics. Thus vancomycin has become a mainstay of prophylaxis and therapy. Finally, and most significant, the preeminence of coagulase-negative staphylococci in neurosurgical infection is not a recent development. At the Children’s Medical Center in Dallas from 1975 to 198 1, coagulasenegative staphylococci were isolated from 27 of 59 (46%) of shunt infections2; in a series from Children’s Hospital in Boston more than a decade earlier (1959 to 1968), coagulase-negative staphylococci were responsible for 51 of 102 (50%) shunt infections.3 Therefore the widely held belief that nosocomial coagulase-negative staphylococcal infections are a relatively new and growing problem is not supported by the shunt infection literature. I believe that this can be attributed to the fact that, as opposed to patient populations in other surgical specialties and in intensive care units, this particular group of patients and their indications for shunt placement have not changed radically dur-

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ing the years. Coagulase-negative staphylococci have always been present on the skin, ready to latch onto foreign bodies such as cerebrospinal fluid shunts, and only when the use of catheters and prostheses increases or when the host becomes more vulnerable does the incidence of infection appear to increase. I will return to this hypothesis later when I discuss coagulasenegative staphylococcal infections in critically ill neonates. How can the intimate association between coagulase-negative staphylococci and foreign bodies be explained? The first step in the pathogenesis of these staphylococcal infections undoubtedly involves colonization of the prosthetic material. It is appealing to speculate that the colonization process requires specific adherence between an adhesin on the surface of the staphylococcus and a receptor on the foreign body. This mechanism of colonization is encountered frequently in the pathogenesis of infectious diseases. For example, so-called pili (fimbriae) on the surface of Escherichia coli mediate adherence to specific globoside receptors on the uroepithelium, thus facilitating the development of ascending urinary tract infection. Similarly, group A streptococcal lipoteichoic acid is important in colonization and infection of the pharynx. However, any theory concerning the mechanism of staphylococcal colonization of foreign bodies must account for the fact that commonly used devices are fashioned from an extraordinarily wide variety of materials, for example, silicon elastomer, polyethylene, polyurethane, methylmethacrylate, polytetrafluoroethylene (Teflon), stainless steel, and titanium. Surely, all these diverse materials do not bear a common receptor. Perhaps prostheses are coated with a host factor in vivo that serves as a specific bacterial receptor. For example, S. aweus adherence to prosthetic materials has been associated with binding to fibronectin,5 but the role of fibronectin in adherence of coagulase-negative staphylococci is far more controversial. Perhaps nonspecific factors play a major role in colonization of foreign bodies. The DVLO theory (named for Derjaquin, Landau, Verway, and Overbeek)6 may account for nonspecific close-range forces of attraction between bacteria and foreign body surfaces,

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and hydrophobic interactions may also play a role .’ Of course, it is entirely possible that colonization and infection require a m ix of specific and nonspecific adherence phenomena. Moreover, the factors that mediate initial attachment to a foreign body may be entirely different from those which favor bacterial multiplication and persistence on the prosthesis. Still other bacterial and host factors may be responsible for transforming colonization into invasive infection. Surely we are dealing with a complex, dynamic process. Of all the possible bacterial factors that m ight be involved in the pathogenesis of coagulasenegative staphylococcal infections, one striking property of this organism seems to have captured the imagination of most investigators in the field. Coagulase-negative staphylococci recovered from foreign body infections often produce large quantities of an extracellular material that generally is referred to as slime but is also known as exopolysaccharide, mucoid substance, glycocalyx, or biofilm. The idea that such extracellular bacterial products m ight be important in the attachment of m icroorganisms to foreign surfaces is not new. Years ago, aquatic bacteria such as Pseudomonas were noted to elaborate a slimy biofilm that helps them cling to and survive on surfaces in marine and freshwater environments. The relevance of this phenomenon to human disease has been popularized by Costerton et al.,’ who proposed an important role for glycocalyx in the pathogenesis of a number of infections, including gram-positive prosthetic joint infections, dialysis-associated peritonitis, pacemaker infections, and intravenous catheter infections, as well as Pseudomonas pulmonary infections in cystic fibrosis and catheter-associated urinary tract infections. With specific regard to coagulase-negative staphylococcal infections of medical devices, however, I think it is more appropriate to cite the observations of Bayston and Penny9 in 1972. These investigators examined cerebrospinal fluid shunts that had been removed because of coagulase-negative staphylococcal infection and noted m icrocolonies of staphylococci embedded in a mucoid layer closely adherent to the surface of the shunt valves. When

INFECTION CONTROL

the staphylococci isolated from these shunts were cultivated in vitro, they elaborated considerable quantities of mucoid material. Because this material stained with alcian blue, Bayston and Penny assumed it was an acidic polysaccharide and they hypothesized that it protected the staphylococci from attack by host defenses and antibiotics. Subsequently, a number of investigators have confirmed the presence of a slimy material on a variety of plastic catheters removed from patients with coagulase-negative staphylococcal infections. Peters et al.,” for example, performed scanning electron m icroscopy of intravenous catheters that had been exposed to coagulase-negative staphylococci in vitro. In a typical experiment, polyethylene cannulas were exposed to. very high concentrations of coagulase-negative staphylococci suspended in saline solution and examined frequently during a 4-day period of incubation at 37” C. After just a few m inutes, scattered bacterial cells were seen adhering to the inner and outer surfaces of the catheter, particularly at the site of tiny irregularities in the plastic. M icrocolonies formed after about an hour of incubation, and heavy, confluent colonization was seen after 6 to 12 hours. Eventually, multiple layers of bacteria formed and gradually were submerged in a sea of slime. Remarkably, the staphylococci seemed to pit the plastic, leading the investigators to speculate that the bacteria were feeding on plasticizers or stabilizers in the polyethylene. Similar results were obtained by Franson et al.,” who worked with polyvinyl chloride catheters and coagulase-negative staphylococci. These experiments illustrated the dynamic nature of the colonization process and suggested that slime m ight be more important in insuring the persistence of staphylococci on catheter surfaces than in establishing initial attachment of bacteria to the plastic. More recently, Christensen et a1.12-14 have attempted to solidify the association between slime production and catheter infection. While investigating an “outbreak” of S. epidemidis intravenous catheter infections, they found that nearly two thirds of clinically significant strains were slime producers, whereas only about one third of alleged blood culture contaminants and skin isolates elaborated slime. Slime produc-

Volume 18 Number 3 June 1990

tion was easily monitored by static culture in broth in plastic tubes-slime producers left a film on the surface of the tubes that was easily visible, especially after application of a polysaccharide strain. Slime producers colonized plastic catheters heavily in vitro, whereas nonslime producers were inefficient colonizers. Slime-producing strains also appeared to be more virulent in a mouse model of subcutaneous catheter infection,15, l6 although it must be noted that isogenic mutants rather than unrelated staphylococcal strains would have to be developed and tested to prove that slime is uniquely and specifically associated with virulence. Unfortunately, such genetic manipulations have not been performed successfully in coagulase-negative staphylococci. Slime production does vary, depending on environmental conditions and media-a phenomenon called phase variation-but the resulting phenotypes may not be stable on repeated passage. The association of slime production with foreign body infections has prompted some investigators to suggest that detection of slime might be a useful laboratory test to distinguish clinically significant strains from culture contaminants. Workers at the University of Virginia noted that 91% of “clinically significant” bloodstream isolates elaborated slime.” Speciation of a strain as S. epidermidis, together with detection of slime by the tube method described previously, had a sensitivity of 93%, a specificity of 85%, and a predictive value positive of 87% for the presence of a clinically significant bacteremia. Similarly, the University of Iowa group found that 81% of clinically significant isolates from prosthetic devices produced slime compared with only 50% of contaminants.‘* Infections caused by slime-producing strains appeared to be more difficult to eradicate without removing the prosthesis. More recently, the Iowa group demonstrated that both slime production and hydrophobicity were markers for clinically significant isolates of coagulasenegative staphylococci.lg Of course, both groups used somewhat arbitrary criteria to determine which infections were clinically significant, and both used a relatively crude, nonquantitative assay for slime. You have undoubtedly noted that we have come a fair distance from thinking of slime

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solely as an adherence factor. The epidemiologic studies I have reviewed may have left you wondering whether slime is a virulence factor, an adhesin, or both. In fact, crude preparations of slime impair the lymphoproliferative response of mononuclear cells, interfere with granulocyte chemotaxis and fi.mction,20, 21 and may impair the ability of antibiotics to kill staphylococci clinging to catheter surfaces. In reviewing the growing body of work on the epidemiology and pathogenesis of coagulasenegative staphylococcal infections of plastic devices, I believed that it was important to understand slime better and to determine whether it really does function as a specific adhesin. Therefore I turned to my colleague and friend Gerald Pier at the Channing Laboratory, down the street from Children’s Hospital. Dr. Pier is well known for his work with Pseudomonas exopolysaccharide and was intrigued by the possibility of studying the exopolysaccharide of coagulase-negative staphylococci. We realized that slime is a complex, poorly characterized material, and we decided to try to tease out, isolate, and purify a more specific extracellular product that might function as an adhesin to plastics. We elected to confine our initial experiments to a single plastic used in a variety of catheters, silicon elastomer (Silastic).” To pursue this work, we had to develop a simple, reproducible adherence assay. After preliminary experiments with an assay with radiolabeled bacteria, we settled on a procedure that was fast, reliable, and that did not require radioisotopes. Briefly, a 3 cm piece of Silastic catheter was fitted with a 21-gauge needle and dipped into a suspension of staphylococci for 15 minutes at room temperature. After washing and flushing the catheter vigorously with saline solution, a 1 cm piece of tubing was cut from the tip and discarded. The remaining 2 cm was rolled on agar in several directions, the culture plate was incubated at 37” C overnight, and the resulting colonies were counted. The correlation between the radiometric assay and the roll plate technique was excellent. To isolate the putative adhesin, we used a slime-producing, adherent strain called RP62A, supplied by Dr. Christensen. The details of this procedure have been published elsewhere.22 Briefly, we isolated a polysaccharide

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Table 1. Expression of slime and adhesin and adherence Silastic catheter tubing Strain RP-62A

RP-12 RP-14 F-3284 RP-62NA SP-2 CL

Journal of

INFECTION CONTROL

Species S. S. S. S. S. S. S.

epidermidis epidermidis hominis epidermidis epidermidis haemolyticus haemolyticus

Degree of slime production

of coagulase-negative

Adhesin production*

+++ +++ + ++ -

Pos Negt

Pos Pos Pas* Neg Neg

Modified from Tojo M. Yamashita N, Goldmann DA. Pier JB. J Infect Dis 1988;157:713-22. Neg, Negative; Pos, positive. *Determined by double immunodiffusion. tsubsequently shown to be closely related serologically to the adhesin of RP-62A. *Weakly positive for adhesin production.

adhesin composed of a complex mix of monosaccharides, with a predominance of galactose and glucosamine. Adherence of strain RP-62A to Silastic tubing could be inhibited competitively in a dose-response fashion by pretreating the catheter with purified adhesin. In addition, dose-response inhibition of adherence could be achieved by treating catheters with rabbit antibody raised to RP-62A purified adhesin. Finally, the adhesin itself bound avidly to polymeric silicone, as demonstrated by binding of antiadhesin rabbit antibody to adhesin-coated catheter material. These experiments conclusively demonstrated the specificity of the reaction between the RP-62A polysaccharide adhesin and silicone. We then examined other selected coagulasenegative staphylococcal strains for production of slime and adhesin and adherence to silicon elastomer (Table 1). The RP-12 strain of S. epidermidis produced abundant slime, adhered well to the silicone, and produced an adhesin that reacted serologically with antibody to the RP-62A adhesin. Although strain RP-14 (StaphyZococcus hominis) elaborated little slime, it produced an adhesin that was identical serologically to that produced by RP-62A and adhered well to the silicone. Adherence could be inhibited by RP-62A adhesin and antibody to RP-62A adhesin. Studies with this strain suggested a possible disassociation between slime production and expression of adhesin, and this has been confirmed with other strains. Strain

staphylococci

Copyright

to

No. of CFU adhering (Mean k SD) 233 + 295 + 167 + 144 + 68 t 7k7 19 f

20 40 24 3 30 5

1988, University of Chicago.

RP-62NA was of interest because Christensen had selected it as a non-slime-producing variant of RP-62A. As shown in Table 1, this strain indeed was slime negative, but it did express some adhesin and adhered moderately well to silicone. Examination of many other strains of coagulase-negative staphylococci has shown that most S. epidermidis strains express an adhesin serologically similar to that of RP-62A, and the vast majority of adhesin-positive strains adhere well to silicone. Transmission immunoelectron microscopy revealed that adhesin is part of the capsule of coagulase-negative staphylococci. The capsule could be visualized clearly with antibody to whole cells of RP-62A, but if the antisera were first adsorbed with purified adhesin from homologous or heterologous strains, the capsule could no longer be seen. More recently, we have established a rabbit model of coagulase-negative staphylococcal catheter infection in which a Silastic cannula is first exposed to a strain of S. epidermidis and is then inserted into the rabbit’s jugular vein.23 Immunization of rabbits with adhesin reduced bacteremia by the experimental strain, apparently by stimulating production of opsonophagocytic antibody. Polyclonal and monoclonal antiadhesin antibody infused via an uncontaminated catheter placed in the contralateral jugular vein also inhibited bacteremia. In the meantime we are continuing our efforts to develop the isogenic adhesin-negative and

Volume 18 Number 3 June 1990

slime-negative mutants that will help us determine the role of adhesin and/or slime in colonization and infection of plastic catheters. Armed with an extensive epidemiologic literature and our own applied research demonstrating the close link between plastic catheters and coagulase-negative staphylococcal infection, we turned our attention to the neonatal intensive care unit (NICU), where we had been studying the epidemiology of nosocomial infections for several years.24 The NICU has always appealed to us as a sort of epidemiologist’s laboratory in which nosocomial infections could be examined with unique clarity. As opposed to adult patients, babies enter the world unencumbered by a long list of diagnoses and free from a complex microbial flora. Thus it is possible to monitor clinical and microbiologic developments while accumulating detailed information about the risk factors for and consequences of nosocomial infection. Numerous investigators25-31 had already suggested that coagulase-negative staphylococci had emerged only recently as the principal pathogens in nosocomial bacteremia in critically ill babies. Furthermore, it had been suggested that central venous catheters were a major risk factor for coagulase-negative staphylococcal bacteremia.3g Our studies in the NICUs of the Brigham and Women’s and Children’s Hospitals, which were performed in collaboration with Jonathan Freeman, Richard Platt, Michael Epstein, David Sidebottom, and Jeanne Leclair, revealed three interesting conclusions that were somewhat at variance with the literature: 0 Coagulase-negative staphylococcal bacteremia is not a recent phenomenon but rather has been a problem in the NICU for years. l Not only are coagulase-negative staphylococci not a new cause of bacteremia, the infections they produce generally are not very serious, especially given the fragility of this patient population. l Although central venous lines have been touted as a major risk factor for coagulasenegative staphylococcal bacteremia, other factors are even more important determinants of risk.

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217

Of course, good epidemiologic studies require sound case definitions, and coagulase-negative staphylococcal bacteremia poses substantial difficulties. In adults it is possible to obtain multiple blood cultures and to include a requirement for at least two positive cultures in the definition of bacteremia. However, the blood of premature babies is precious, and venipunctures may be technically difficult. At least in our NICUs, the general practice is to perform only one blood culture per bacteremia workup, so definitions based on multiple positive cultures are not practical. In addition, the signs and symptoms of bloodstream infection are especially nonspecific in neonates. Apnea, bradycardia, feeding intolerance, and temperature instability certainly are not unique to bacteremia. Therefore whenever a tiny NICU baby looks ill-which happens frequently-a blood culture is performed and empiric antibiotic therapy is initiated. Given the uncertainty of the clinical diagnosis of bacteremia in neonates, it is not surprising that more than 90% of babies hospitalized in our NICUs receive antibiotics at some time during their stay. Faced with the lack of a “gold standard” for clinically relevant bacteremia, we decided to use a simple, objective definition.32 Any blood culture that yielded a single morphologic type or species of coagulase-negative staphylococci as the sole isolate was accepted as evidence of bacteremia with that organism, and bacteremias occurring 48 hours or more after birth were considered to be nosocomial. Blood cultures were taken percutaneously, except for cultures taken through umbilical catheters at the time of placement. This definition does not imply anything about the clinical condition of the baby; it merely requires the growth of coagulase-negative staphylococci in a blood culture bottle. It is, of course, possible that some of these positive blood cultures did not represent bacteremia at all but rather reflected contamination. However, we believe that contaminants were not a major factor. The vast majority of positive blood cultures occurred between the second and third weeks of hospitalization, as would be expected for a nosocomial bacteremia. The probability that a blood culture would yield coagulase-negative

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staphylococci increased from 2% to 3% in the first week to a peak of 12% at approximately 2 to 5 weeks of hospitalization.32 If contamination by skin bacteria were a significant problem, the positive culture rate should have been higher in the first week of life, because the skin of babies is heavily colonized with coagulasenegative staphylococci after just a few days in the NICU.32 In any event, misclassification of contaminated cultures as bacteremias in our data set tended to reduce the power of our study, resulting in underestimates of the magnitude of the differences we went on to demonstrate. With this background we can examine my first claim-that coagulase-negative staphylococcal bacteremia is not a new problem in neonates. Although many investigators have claimed that in the incidence of this infection in the NICU has increased dramatically, none have critically analyzed the longitudinal influence of several key variables on their data. These factors include blood culturing practice and frequency, interpretation of blood culture results, birth weight, length of stay in the NICU, and perhaps others. First, we took a crude look at blood culture data from Children’s Hospital collected during the 1S-year period from 1970 to 1984.32The proportion of blood culture workups that yielded coagulase-negative staphylococci ranged from 2.5 to 6.7 per 100 admissions, with a mean of 4.4. No discernable increase was observed over time, and this relatively steady rate has continued to the present. For a more refined analysis we compared detailed data sets from 1976 (the earliest year for which data concerning blood culture practices and patient characteristics were available) and 1982 (the first year in which the clinicians in our joint program in neonatology claimed that they were experiencing an outbreak of coagulase-negative staphylococcal bacteremia). First, we determined that there had not been a substantial change in the frequency with which blood cultures were obtained at Children’s Hospital in the two study years. After stratifying data by birth weight, we observed a small but statistically significant decline in the incidence density of blood cultures.

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When 1976 was compared with 1982, the Mantel-Haenszel adjusted summary relative risk was 1.20 (95% confidence interval [CI], 1.02 to 1.41). Thus the perceived outbreak could not be attributed to increased detection resulting from more diligent culturing. Was there, then, an epidemic of coagulasenegative staphylococcal bacteremia as alleged by our neonatologists? As any sensible hospital epidemiologist would do, I consulted our infection control epidemiologist, Jeanne Leclair, to find out what was happening at Children’s Hospital. She informed me that there had been in fact a fivefold increase in the number of nosocomial coagulase-negative staphylococcal bacteremias during the study period. Part of this increase could be attributed to an expansion of the number of NICU beds, but, even allowing for the increased census, there still was a 3.8fold increase in the number of bacteremias. It is important to recall, however, that these bacteremias had been classified with the use of relatively subjective criteria. The infection control team had relied heavily on the clinical impression of the physicians caring for patients with positive blood cultures. If the attending neonatologists believed that the coagulasenegative staphylococci growing in a blood culture bottle represented clinical bacteremia, then nosocomial infection was duly recorded by the infection control team. If the clinicians believed that the positive culture reflected contamination, an infection generally was not recorded. Of course, the neonatologists themselves were influenced by the clinical appearance of the baby and, as will become clear in a moment, by the neonate’s birth weight. At this point we returned to our objective definition of bacteremia, which was based solely on the results of blood cultures. Using this microbiologic definition and stratifying the data by birth weight, we found no change in the incidence density of coagulase-negative staphylococcal bacteremia from the mid-1970s until 1982, with the rate remaining at about 3.3 cases per 1000 patient days .33What had changed was not the rate of bacteremia but rather the type of neonate occupying beds in the NICU. In the decade from the 1970s to 198Os, the improve-

Volume 18 Number 3

Coaguhse-negative staphylococci

June 1990

Table 2. Probability of neonatal

Table 3. Interpretation of blood cultures

survival by birth weight, Children’s Hospital NICU, Boston*

positive for coagulase-negative staphylococci by birth weight in Children’s Hospital NICU, Boston*

No. survived/No. Birth weight (sm) 500-749

1975 o/4

admitted

(%)

1982

(0)

5/10(50.0) 21/28 (75.0) 19123 (82.6) 18121 (85.7) 23/26 (88.5) 27128 (96.4)

750-999 1000-1249 1250-1499

4111 (36.4) 10119 (52.6) 11114 (78.6)

1500-l

749

12/13

1750-1999 22000

1091127

(85.8)

2441277

159/204

(77.9)

3571413 (86.4)

Crude overall total

(92.3)

13116 (81.3)

(88.1)

Relative probability of survival, 198211975

Unbounded 2.06 1.57 1.09 0.96 1.19 1.02 1.11

Modified from JAMA 1987;158:2.548-52. Copyright 1987, American Medical Association. l ManteCHaenszel summary relative probability of survival for 1982/1975 adjusted for birth weight was 1 .I 1 (95% Cl, 1.03 to 1.19). Adjusted relative probability of survival for 198211975 for infants with birth weights less than 1000 gm was 2.56 (95% Cl, 1.32 to 4.95), and 1.07 for infants with birth weights 1000 gm or more (95% Cl, 0.99 to 1.15).

ment in the survival of very low birth weight babies had been dramatic (Table 2).33 Whereas the survival rate of larger babies with birth weights less than 2000 gm had been virtually unchanged, major advances had been made in the tiniest babies with birth weights less than 1000 gm. For example, infants with birth weights of less than 1000 gm were 2.56 times as likely to survive in 1982 as in 1975 (95% CI, 1.32 to 4.95; p < O.OOS).‘”In other terms, 60.9% of the infants with birth weights of less than 1000 grams who survived in 1982 would have died in 1975. These tiny, critically ill survivors stayed in the NICU for prolonged periods of time, often for several months. This resulted in a 62.3% relative increase in NICU bed use by infants with birth weights of less than 1000 gm. Whereas infants with birth weights less than 1000 gm accounted for only 13 .O% of all NICU patient care days in 1975, they accounted for 34.4% of patient care days in 1982 (relative risk, 2.65; 95% CI, 2.41 to 2.91).33 Thus the 1982 NICU was occupied by a population of remarkably small babies. When a positive blood culture was obtained from one of these very fragile infants, neonatologistsand, subsequently, the infection control team-

No. interpreted as bacteremia/No. positive Birth weight

(gm)

1975

219

(%)

1982

~1000

112 (50.0) 116 (16.7)

9/11 (81.8) 115 (20.0)

Crude overall total*

2/8 (25.0)

10116 (62.5)

500-999

Modified from JAMA 1987;156:2548-52. Copyright 1987, Medical Association. ‘ManteCHaenszel summary relative probability that a positive ture would be interpreted as bacteremia when taken from an birth weight of less than 1000 gm compared with infants weights more than 1000 gm was 3.8 (95% Cl, 1.3 to 11.2). no significant difference between 1975 and 1982.

American blood infant with There

culwith birth was

were far more likely to consider the isolate to be clinically significant than if the same organism had been recovered from a larger baby. Specifically, infants with birth weights less than 1000 gm were 3.8 times as likely as heavier infants to have had a positive blood culture interpreted as bacteremia (95% CI, 1.3 to 11.2; Table 3)‘ and this influence of birth weight on the interpretation of positive blood cultures was similar in 1975 and 1982. Thus the increase in the number of very low birth weight babies with prolonged hospital stays, coupled with the tendency of clinicians to interpret a positive blood culture as significant if taken from such small infants, may account for the reported increase in nosocomial coagulase-negative staphylococcal bacteremia in our NICUs. The alleged epidemic of bacteremia turned out to be little more than an “outbreak” of very low birth weight survivors and clinical anxiety concerning the importance of positive cultures in such critically ill babies. This brings me to my second assertion-that the stakes for babies with coagulase-negative staphylococcal bacteremia may not have been as high as our neonatologists feared. However, before proceeding to a description of the studies that support this claim, it is necessary to emphasize the importance of birth weight and length of stay as independent determinants of nosocomial coagulase-negative staphylococcal bacteremia in neonates. Both birth weight and

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length of stay can seriously confound comparative studies of the consequences of and risk factors for coagulase-negative staphylococcal bacteremia unless appropriate adjustments are made. To investigate the outcome of infants with coagulase-negative staphylococcal bacteremia, we performed a cohort study based on the 45 infants in our NICUs who had coagulase-negative staphylococcal bacteremia in 1982.34Because all seven infants with birth weights less than 700 gm who survived more than 48 hours had coagulase-negative staphylococcal bacteremia, no nonbacteremic comparison subjects were available for matching. The remaining 38 bacteremic infants each were matched with two comparison subjects by birth weight within 100 gm and nearest date of discharge. To adjust for duration of exposure to the hospital, it was also required that both comparison subjects remained in the NICU for as long as it took for bacteremia to occur in the case. We found that bacteremic infants remained in the NICU for an average of nearly 20 days longer than the comparison babies (p < 0.0001) and required 11 more days of antibiotic therapy.34 Vancomycin was given to 53% of bacteremic babies but to only 5% of comparison subjects. Most strikingly, 37 of the 38 bacteremic babies survived, as did all the comparison patients. Thus coagulase-negative staphylococcal bacteremia had a substantial impact on length of stay and antibiotic use but resulted in little, if any, excess mortality. Most recently, we have been examining potential risk factors for coagulase-negative staphylococcal bacteremia in our NICU population.35 After appropriate adjustment for birth weight and length of stay in a case-control study, only two factors were consistently and independently associated with an increased risk of bacteremia. As expected, bacteremic babies were more likely than comparison subjects to have had a central venous catheter. Surprisingly, however, the relative risk for lipid infusions, which were administered almost exclusively through peripheral catheters during the study period, was even greater, and the statistical association was even more compelling. Contamination of lipids with coagulasenegative staphylococci seems extraordinarily

unlikely given existing protocols for preparation and administration. It is tempting to speculate that lipid infusions through alreadycolonized catheters favor rapid proliferation of staphylococci and bloodstream invasion, as has been described for Mahssezia ~~~~~~~~~~~It is also possible that lipid infusions have an adverse effect on local host defense.3g Of course, we may not have to speculate for long. It is again time to have a cup of tea with my colleague Dr. Pier and to plan our next round of experiments in vitro and in our animal model, this time including lipids in the equation. The dry bench-wet bench collaboration has come full circle. References 1. Price EH. Staphylococcus epidemidis infections of cerebrospinal fluid shunts. J Hosp Infect 1984;5:7-17. 2. Odio C, McCracken GH, Nelson JD. CSF shunt infections in pediatrics: a seven-year experience. Am J Dis Child 1984;138:1103-8. 3. Schoenbaum SC, Gardner P, Shillito J. Infections in cerebrospinal fluid shunts: epidemiology, clinical manifestations and therapy. J Infect Dis 1975;131:543-52. 4. Meirovitch J, Kitai-Cohen Y, Keren G, Fiendler G, Rubinstein E. Cerebrospinal fluid shunt infections in children. Pediatr Infect Dis J 1987;6:921-4. 5. Hermann M, Vaudaux PE, Pittet D, et al. Fibronectin, fibrinogen, and laminin act as mediators of adherence of clinical staphylococcal isolates to foreign material. J Infect Dis 1988;158:693-701. 6. Marshall KC. Mechanisms of bacterial adhesion at solid water interfaces. In: Savage DC, Fletcher M, eds. Bacterial adhesion: mechanisms and physiological significance. New York: Plenum Publishing Corp, 1985:13161. 7. Hogt AH, Dankert J, Feijin J. Encapsulation, slime production and surface hydrophobicity of coagulasenegative staphylococci. FEMS Lett 1983;18:21 l-5. 8. Costerton JW, Irvin RT, Cheng K-J. The bacterial glycocalyx in nature and disease. Ann Rev Microbial 1981;35:299-324. 9. Bayston R, Penny SR. Excessive production of mucoid substance in staphylococcus SIIA: a possible factor in colonization of Holter shunts. Dev Med Child Neurol 1972;14(suppl):25-8. 10. Peters G, Locci R, Pulverer G. Microbial colonization of prosthetic devices. II. Scanning electron microscopy of naturally infected intravenous catheters. Zentralbl Bakteriol Mikrobiol Hyg [B] 1981;173:293-9. 11. Franson TR, Sheth NK, Rose HD, Sohnle PG. Scanning election microscopy of bacteria adherent to intravascular catheters. J Clin Microbial 1984;20:500-5. 12. Cristensen GD, Parisi JT, Bisno AL, Simpson WA, Beachey EH. Characterization of clinically significant strains of coagulase-negative staphylococci. J Clin Microbiol 1983;18:258-69. 13. Christensen GD, Simpson WA, Bisno AL, Beachey EH.

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Adherence of slime-producing strains of StaphyZococctls epidemidis to smooth surfaces. Infect Immun 1982;37: 318-26. 14. Christensen GD, Bisno AL, Parisi JT, McLaughlin B, Hester MG, Luther RW. Nosocomial septicemia due to Staphylococcus epidermimultiply antibiotic-resistant dis. Ann Intern Med 1982;96:1-10. 15. Christensen GD, Simpson WA, Bisno AL, Beachey EH. Experimental foreign body infections in mice chalStaphylococcus epidennilenged with slime-producing dis. Infect Immun 1983;40:407-IO. 16. Christensen GD, Baddour LM, Simpson WA. Phenotypic variation of Staphylococcus epidemidis slime production in vitro and in vivo. Infect Immun 1987;

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55:2870-7. 17. Ishak MA, Groschel DHM, Mandell SL, Wenzel RP. Association of slime with pathogenicity of coagulasenegative staphylococci causing nosocomial septicemia. J Clin Microbial 1985;22:1025-9. 18. Davenport DS, Massanari RM, Pfaller MA, Bal MJ, Streed SA, Hierholzer WJ Jr. Usefulness of a test for slime production as a marker for clinically significant infections with coagulase-negative staphylococci. J Infect Dis 1986;153:332-9. 19. Martin MA, Pfaller MA, Massauari RM, Wenzel RP. Use of cellular hydrophobicity, slime production, and species identification as markers for the clinical significance of coagulase-negative staphylococcal isolates. AM J INFECT CONTROL 1989;17:130-5. 20. Gray ED, Peters G, Verstegen M, Regelmann WE. Effect of extracellular slime substance from StaphyZococcus epidemidis on the human cellular immune response. Lancet 1984;1:365-7. 21. Johnson GM, Lee DA, Regelmann WE, Gray ED, Peters G, Quie PG. Interference with granulocyte function by StaphyZococcus epidemzidis slime. Infect Immun 1986; 54:13-20. 22. Tojo M, Yamashita N, Goldmann DA, Pier JB. Isolation and characterization of a capsular polysaccharide adhesin from Staphylococcus epidermidis. J Infect Dis 1988;157:713-22. 23. Kojima Y, Goldmann DA, Pier GB. Protection against coagulase-negative staphylococcal catheter infection and bacteremia in rabbits by antibody to the capsular polysaccharide adhesin. Abstracts of the Twenty-eighth Interscience Conference on Antimicrobial Agents and Chemotherapy, October 23-26, 1988, Los Angeles, Abstract No. 375. 24. Goldmann DA, Durbin WA, Freeman J. Nosocomial infections in a neonatal intensive care unit. J Infect Dis 1981;144:449-59. 25. Battisti 0, Mitchison R, Davies PO. Changing blood culture isolates in a referral neonatal intensive care unit. Arch Dis Child 1981;56:775-8. 26. Munson DP, Thompson TR, Johnson DE, Rhame FS, Van Drunen N, Ferrieri P. Coagulase-negative staphy-

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lococcal septicemia: experience in a newborn intensive care unit. J Pediatr 1982;101:602-5. Fleer A, Senders RC, Visser MR, et al. Septicemia due to coagulase-negative staphylococci in a neonatal intensive care unit: clinical and bacteriological features and contaminated parenteral fluids as a source of sepsis. Pediatr Infect Dis 1983;2:426-31. Baumgart S, Hall SE, Campos JM, Polin RA. Sepsis with coagulase-negative staphylococci in critically ill newborns. Am J Dis Child 1983;137:461-3. Cainen G, Campognone P, Peter G. Coagulase-negative staphylococcal bacteremia in newborns. Clin Pediatr 1984;23:542-4. Donowitz LG, Haley CE, Gregory WW, Wenzel RP. Neonatal intensive care unit bacteremia: emergence of gram-positive bacteria as major pathogens. Am J Infect Control 1987;15:141-7. Schmidt BK, Kirplani HM, Corey M, Low DE, Philip AGS, Ford-Jones EL. Coagulase-negative staphylococci as true pathogens in newborn infants: a cohort study. Pediatr Infect Dis J 1987;6: 1026-3 1. Sidebottom DG, Freeman J, Platt R, Epstein MF, Goldmann DA. Fifteen-year experience with bloodstream isolates of coagulase-negative staphylococci in neonatal intensive care. J Clin Microbial 1988;26:713-8. Freeman J, Platt R, Sidebottom DG, Leclair JM, Epstein MF, Goldmann DA. Coagulase-negative staphylococcal bacteremia in a changing neonatal intensive care unit population: is there an epidemic? JAMA 1987;258:2548-52. Freeman J, Epstein MF, Platt R, Sidebottom DG, Goldmann DA. Impact of coagulase-negative staphylococcal bacteremia on mortality and length of stay in the neonatal intensive care unit [Abstract]. Pediatr Res 1986; 20:378A. Freeman J, Goldmann D, Smith N, Sidebottom D, Epstein M, Platt R. Intravenous lipid emulsion as a major determinant of nosocomial coagulase-negative staphylococcal bacteremia in a neonatal intensive care unit population. Presented at the Twenty-ninth Interscience Conference on Antibiotics and Chemotherapy, Houston, September 17-20, 1989. Bell LM, Alpert G, Slight PH, Campos JM. Malassezia firfur skin colonization in infancy. Infect Control Hosp Epidemiol 1988;9:151-3. Powell DA, Aungst J, Snedden S, Hansen N, Brady M. Broviac catheter-related Malassezia fiirfiirsepsis in five infants receiving intravenous fat emulsions. J Pediatr 1984;105:987-90. Long JG, Keyserling HL. Catheter-related infection in infants due to an unusual lipophilic yeast-Malassezia furfur. Pediatrics 1985;76:896-900. Fischer GW, Wilson SR, Hunter KW, Mease AD. Diminished bacterial defences with intralipid. Lancet 1980;2:819-20.

Coagulase-negative staphylococci: interplay of epidemiology and bench research.

Coagulase-negative staphylococci (CNS) are major nosocomial pathogens in patients with prostheses and indwelling devices such as central venous cathet...
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