THE JOURNAL OF INFECTIOUS DISEASES. VOL. 137. NO.3. MARCH 1978 © 1978 by The University of Chicago. 0022-1899/78/3703-0004$00.75

In Vitro Activity of Cephalosporins against Methicillin-Resistant, Coagulase-Negative Staphylococci From the Infectious Diseases Section, Department of Medicine, University of Minnesota Hospitals, Minneapolis, Minnesota

Michel Laverdiere, Phillip K. Peterson, Jan Verhoef,* David N. Williams,t and Leon D. Sabath

The pathogenic role of coagulase-negative staphylococci in human disease has been well established. This microorganism has been described as an etiologic agent in urinary tract [1, 2], wound [3], and ventriculoatrial shunt infections [4, 5]. Coagulase-negative staphylococcal endocarditis, particularly that involving prosthetic valves, has been reported to be an increasingly frequent and particularly serious disease [6-9]. The medical management of infections with coagulase-negative staphylococci is complicated by the high incidence of resistance to methicillin reported for these organisms [10]. Although cephalothin has been reported to be active in vitro

against methicillin-resistant staphylococci [8], the clinical efficacy of cephalosporins in the treatment of methicillin-resistant, coagulase-positive staphylococcal infections has been disappointing [11]. Recently, Richmond et al. [12] showed that testing the sensitivity of methicillin-resistant Staphylococcus aureus to cephalothin by a disk diffusion method was -unreliable. The majority of methicillin-resistant, coagulase-negative staphylococci have been shown to be sensitive to cephalothin by standardized disk diffusion [8, 13] and standardized agar dilution [14, 15] methods. In this study, susceptibility of 41 clinical isolates of methicillin-resistant, coagulasenegative staphylococci to five cephalosporins was determined by both disk diffusion and agar dilution methods with use of a large bacterial inoculum and modified microbiological media. The inhibitory and bactericidal activities of cephalothin were also determined by a tube dilution method. In addition, gentamicin and vancomycin were tested against a selected group of isolates.

Received for publication May 9,1977, and in revised form September 26,1977. This study was presented in part at the 17th Interscience Conference on Antimicrobial Agents and Chemotherapy, New York, New York, October 1977. We acknowledge the technical assistance of D. Welter, S. Hermel, and B. Zins, Please address requests for reprints to Dr. Phillip K. Peterson, Infectious Diseases Section, Department of Medicine, University of Minnesota Hospitals. University of Minnesota Medical School, 420 Delaware Street S.E., Minneapolis, Minnesota 55455. .. Present address: Laboratory for Microbiology, Utrecht, The Netherlands. t Present address: St. Louis Park Medical Center, St. Louis Park, Minnesota.

Materials and Methods

Bacterial strains. Forty-one clinical isolates of coagulase-negative staphylococci, shown to be resistant to methicillin by a standardized disk diffusion method [16], were obtained from four

245

Downloaded from http://jid.oxfordjournals.org/ at University of Iowa Libraries/Serials Acquisitions on June 1, 2015

The in vitro activity of five cephalosporins, gentamicin, and vancomycin was determined against 41 clinical isolates of methicillin-resistant, coagulase-negative staphylococci. Results obtained with disk diffusion and agar dilution methods failed to show complete cross-resistance between methicillin and four of the five cephalosporins despite the use of a high-salt medium and a large inoculum. Thirty-six (88%) of the 41 isolates were sensitive to cephalothin by a standardized disk diffusion method, whereas 23 isolates (56%) were sensitive to cephalothin with use of an agar dilution method and a large inoculum. Of these 23 isolates, only 11 (47%) were both inhibited and killed by ='6.25 JLg of cephalothin/ml. The inhibitory and bactericidal activity of gentamicin was greater than that of vancomycin and cephalothin. These results suggest that the disk diffusion technique has limitations in determining the in vitro activity of cephalosporins against methicillin-resistant, coagulase-negative staphylococci and that cephalothin exhibits poor bactericidal activity against these same isolates.

246

N.]. Antibiotic susceptibility testing. The susceptibility of the isolates to methicillin and cephalothin was first determined by a standardized disk diffusion method [16]. Susceptibility to methicillin and the five cephalosporins was then determined by disk diffusion and agar dilution methods on both nonsupplemented and 5% NaCl-supplemented Mueller-Hinton agar (MHA) with use of a large inoculum containing 108 organisms/ml (i.e., an undiluted overnight growth in Mueller-Hinton broth [MHB] ). For vancomycin and gentamicin, nonsupplemented MHA was used. In the disk diffusion method, commercial disks containing antibiotics were used when available, or the disks were freshly prepared. Diameters of the zones of inhibition were measured after incubation for 18-20 hr at 30 C. Susceptibility of the isolates to the antibiotics by the agar dilution technique was determined with use of the method of Steers et al. [17]. MICs were obtained by visual inspection of the surface of each plate after incubation for 18-20 hr at 30 C and were defined as the lowest concentration of antibiotic that produced no growth or less than four small isolated colonies. Broth dilution tests for susceptibility to ceph-

alothin, vancomycin, and gentarmcm were carried out for a selected group of isolates with use of 107 organisms/rnl of MHB. MICs were determined after incubation for 24 and 48 hr at 30 C. MBCs were also determined after incubation for 24 and 48 hr at 30 C and were defined as the concentration of antibiotic that killed ~99.9% of the organisms as demonstrated by subculture onto drug-free solid media. Results

Susceptibility by the disk diffusion method. With use of a standardized disk diffusion method with a bacterial inoculum of lOL106 organisms/ ml and nonsupplemented MHA, all of the 41 strains of coagulase-negative staphylococci were resistant to methicillin, whereas 36 strains (88%) were sensitive to cephalothin. The susceptibility patterns obtained with a disk diffusion method with a larger inoculum (10 8 organisms/rnl) for the five cephalosporins are shown in table 1. With use of nonsupplemented MHA, 78%, 68%, 83%, and 73% of the methicillin-resistant, coagulasenegative staphylococci were sensitive to cephalothin, cefazolin, cefamandole, and cephapirin, respectively; none of the 41 strains, however, were sensitive to cephalexin. The use of 5% NaClsupplemented MHA medium did not significantly modify the sensitivity of the staphylococci to cephalothin; only three strains that had been sensitive became intermediate in their susceptibility. The influence of disk potency on the diameters of the zones of inhibition obtained with methicillin and cephalothin was also studied. An increase in the potency of the methicillin disk from 5 to 30 fLg resulted in an increase of only 3 mm in the geometric mean diameter of the zones of inhibition. Although none of the strains were sensitive to methicillin with the recommended 5-fLg disk, four strains became sensitive with the 30-fLg disk. A decrease in the potency of the cephalothin disk to 5 fLg from the standard of 30 fLg produced a difference of 10 mm in the geometric mean diameter of the zones of inhibition. However, 25 (61%) of the 41 strains remained sensitive as compared with the 29 strains (71%) shown to be sensitive with the recommended 30-fLg disk. Susceptibility by the agar dilution method. When the agar dilution method was used with

Downloaded from http://jid.oxfordjournals.org/ at University of Iowa Libraries/Serials Acquisitions on June 1, 2015

centers: Boston City Hospital, Boston, Mass. (13 strains), St. Boniface General Hospital, Winnipeg, Canada (11), the Laboratory for Microbiology, Utrecht, The Netherlands (nine), and the University of Minnesota Hospitals, Minneapolis, Minn. (eight). Strains were identified on the basis of their morphology, gram stain, and negative tube coagulase test. Thirteen strains were isolated from patients with endocarditis, and 10 were isolated from patients with urinary tract infections. The remaining 18 strains were obtained from other sites of infection. A collection strain (S. aureus ATCC 25923; American Type Culture Collection, Rockville, Md.) was included as a control in each series of susceptibility tests. Antibiotics. Standard antibiotic powders were used and provided by their respective manufacturers: methicillin and cephapirin by Bristol Laboratories, Syracuse, N.Y.; cephalothin, cephalexin, cefazolin, cefamandole, and vancomycin by Eli Lilly and Company, Indianapolis, Ind.; and gentamicin by Schering Corp., Bloomfield,

Laverdiere et al,

Cephalosporins against Staphylococci

247

Table 1. Susceptibility of 41 clinical isolates of methicillin-resistant, coagulase-negative staphylococci to five cephalosporins. No. of strains with indicated susceptibility" Cephalexin Method, medium

8

R

Cefazolin

8

R

Cefamandole

8

R

8

Cephapirin R

8

40 40

8 6

1 6

32 29

11

2

28

7

34

7

NT

NT

NT

NT

NT

NT

NT

4 NT

NT

41 41

14 21

4 3

23 17

22 32

9 4

10 5

9 6

12 13

20 22

20 17

5 5

16 19

30

*By the disk diffusion method, zones of inhibition indicating susceptibility were as follows:

In vitro activity of cephalosporins against methicillin-resistant, coagulase-negative staphylococci.

THE JOURNAL OF INFECTIOUS DISEASES. VOL. 137. NO.3. MARCH 1978 © 1978 by The University of Chicago. 0022-1899/78/3703-0004$00.75 In Vitro Activity of...
468KB Sizes 0 Downloads 0 Views