T. M . H o o t o n
The Epidemiology of Urinary Tract Infection and the Concept of Significant Bacteriuria Summary: Urinary tract infections ( u r I ) are among the most common infections afflicting man. Urinary tract infections in young adult women are usually uncomplicated, but are often recurrent and cause considerable morbidity. Urinary tract infections in pregnant women, elderly patients, and catheterized patients warrant special attention because of their association with increased morbidity and POssibly with increased mortality. Diagnosis of UTI is usually based on quantitation of uropathogens in voided urine. The traditional criteria for significant bacteriuria, ->!05 uropathogens per ml of voided urine, is insensitive for detecting acute symptomatic cystitis in men and women and should be replaced with a lower colony count threshold.
Zusammenfassung:Epidemiologie der Harnwegsinfektionen und das Konzept der signifikanten Bakteriurie. Harnwegsinfektionen geh6ren zu den h/iufigsten Infektions; krankheiten des Menschen. Bei jungen Frauen sind Harnwegsinfektionen in der Regel unkompliziert, doch sind rezidivierende Verl/iufe h/iufig, und die Morbidit/it ist betr/ichtlich. Bei Schwangeren, ~ilteren Personen und Kathetertr/igern auftretende Hamwegsinfektionen verdienen besondere Beachtung, da nicht nur die Morbidit~it erh6ht ist, sondern mfglicherweise auch die Mortatit/it. Die Diagnose einer Harnwegsinfektion basiert in der Regel auf der Keimzahlbestimmung im Spontanurin. Fiir die Diagnose einer akuten Zystitis bei M~innern und Frauen ist der traditionelle Richtwert fiir eine signifikante Bakteriurie von >10 5 uropathogenen Keimen pro ml zu wenig empfindlich, aus diesem Grund sollten die Grenzwerte ftir die Keimzahlen niedtiger angesetzt werden.
Epidemiology of Urinary Tract Infections Urinary tract infections (UTI) are a major public health problem worldwide. In the United States, U r l s account for approximately 5-6 million office visits annually [1]. Bacteriuria is much more common in females than males at all ages of life except in the first year when the prevalence in boys is somewhat higher than that in girls [2, 3]. Between the ages of 1 and 50, UTIs in males whose genitourinary tracts have not been instrumented are rare, but after age 50 the rate starts to rise. Insertive anal intercourse, particularly in homosexual males, may increase the risk of UTI by exposing the urethra to high quantities of fecal Escherichia coli [4]. T h e prevalence of UTIs in preschool and school girls is approximately 1-3% which is S 40
30-fold higher than that in boys [5]. It is estimated that approximately 5% of girls will have one or more episodes of UTIs during their school years [5]. The prevalence of bacteriuria increases with age in adult women such that in the reproductive years the prevalence of bacteriuria is approximately 50 times greater than that in males [6]. In the elderly, the prevalence of bacteriuria approaches 20-50% in women and 5-20% in men with institutionalized persons having the highest rate [7]. It has been estimated that as many as 20-30% of adult women experience one or more dysuria episodes yearly, and that most of these episodes represent UTI [8], Anatomic and functional abnormalities and instrumentation of the urinary tract predispose both sexes to develop UTI. Bacteriuria in the non-instrumented male almost always warrants an evaluation for such abnormalities. In elderly males, bacteriuria is usually related to abnormalities in the prostate gland. Although girls less than age 1 with bacteriuria are likely to have an abnormality in the genitourinary tract, school girls and adult women infrequently have detectable abnormalities in association with bacteriuria. Vesicoureteral reflux is commonly found in infant boys and girls and this abnormality in conjunction with bacteriuria is thought to predispose to renal scarring and chronic renal disease [3, 5]. Many women with UTI will develop one or more recurrences. Relapse should be considered when an infection recurs within a week or so following treatment. Relapse usually occurs in the setting of upper tract infection, a functionally or anatomically abnormal urinary tract, or, in the male, chronic bacterial prostatitis. Approximately 80% of recurrent infections are thought to represent reinfection from perineal flora rather than relapse [9]. Reinfection in adult women, however, tends not to be associated with abnormalities in the urinary tract at least as detected by excretory urograms or cystoscopy [10]. It is not possible with present technology to reliably demonstrate that a recurrent UTI is a relapse since an identical strain from fecal flora may cause a reinfection. Recurrent Lrl'I in young women with anatomically normal urinary tracts appears to be due, at least in part, to a greater propensity for adherence of E. coli to their uroepithelial cells [11] and a higher prevalence of vaginal colonization with E. coli compared to women without recurrent UTI [12]. Extrinsic factors which may lead to recurrent LrrI in an otherwise healthy woman include sexual activity [13] and diaphragm use [i4], Persistence of vaginal colonizT. M. Hooton, M.D., Department of Medicine, University of Washington, Harborview Medical Center, 325 Ninth Avenue, Seattle, Washington 98104, USA.
Infection 18 (1990) Suppl. 2 © MMV Medizin Verlag GmbH Miinchen, Miinchen 1990
T. M. Hooton: Epidemiology of UTI
ation with E. coli after treatment of UTI may be a risk factor for early recurrence of infection [15]. Bacteriuria in pregnancy, in the elderly, and in institutionalized persons warrants special mention when discussing the epidemiology of UTI. Bacteriuria in pregnancy is a potentially serious problem occurring in approximately 5% of pregnant women in the first trimester and increasing with age and parity [16, 17]. Pregnancy itself appears not to be a risk factor for developing UTI, and the prevalence of bacteriuria does not increase as the pregnancy progresses. However, left untreated, approximately one fourth of pregnant women with bacteriuria early in pregnancy will develop symptomatic UTI (including acute pyelonephritis) [16] whereas treatment reduces the subsequent risk by 80-90% [17, 18]. In addition, persistent bacteriuria is associated with premature delivery and increased perinatal mortality [19, 20], Thus, pregnancy is one Of the few conditions for which routine screening for and treatment of asymptomatic bacteriuria is recommended. Bacteriuria in the elderly poses a significant problem given the very high prevalence found in both sexes, and recent reports that suggest an association between bacteriuria and increased mortality [7]. Treatment of bacteriuria in the elderly is associated with a high rate of recurrence, increased drug side effects, and increased antimicrobial resistance. Further study is indicated on whether bacteriuric elderly have a truly increased mortality risk or whether bacteriuria is simply an indicator of the severity of the patient's underlying disease state. Routine screening and treatment of asymptomatic bacteriuria in this population is unwarranted unless this association is confirmed by further carefully conducted investigation. Nosocomial UTIs occur in approximately 2-3% of hospitalized patients in the United States and account for 42% of all nosocomial infections [21]. Two thirds of these infections are associated with urinary catheterization [22] and catheter-associated UTIs are the most common source of gram-negative bacteremia in hospitalized patients [23]. Recent studies suggest that catheterized patients with bacteriuria have a significantly increased mortality compared with non-bacteriuric catheterized patients even after controlling for several potential risk factors [24]. As with the elderly, further studies are indicated to confirm this association to determine underlying mechanisms and whether therapy of asymptomatic bacteriuria is warranted. Efforts to prevent nosocomial UTIs are largely centered around recommendations for catheter care [25]. The microbial etiology of UTI differs between uncomplicated and complicated infections. Uncomplicated infections in young adult women are caused by E. coli in approximately 80-90% of cases and S. saprophyticus in approximately 10-20% [26, 27]. Uncomplicated UTI are occasionally caused by other gram-negative bacilli such as Proteus mirabilis and Klebsiella species. These organisms are generally susceptible to most of the commonly used antimicrobials [28]. Complicated infections, especially nosocomial UTI and those occurring in previously treated
patients with abnormal urinary tracts, are caused by a greater variety of organisms which tend to be resistant to multiple antibiotics [28]. The spectrum of organisms found in complicated UTI include, in addition to E. coli, Pseudomonas aeruginosa, Proteus species, Klebsiella species, Serratia, Enterobacter, other gram-negative rods, enterococcus, yeasts, and many other organisms. It is obvious, then, that UTIs are a significant health problem worldwide. It has been estimated that the evaluation and treatment of uncomplicated UTI in ambulatory women account for approximately $1 billion of expenditures in the United States alone [28]. It is further estimated that nosocomial UTIs cost United States hospitals more than half a billion dollars annually because of associated increased morbidity and increased lengths of stay in the hospital [29]. Obviously, the increased mortality reported to be associated with bacteriuria in the elderly and in catheterized patients cannot be so easily quantitated. Improved methods of prevention and therapy for Lrrls are certainly indicated to decrease the frequency and sequelae of this major health problem.
The Concept of Significant Bacteriuria Urinary tract infection is defined as bacteriuria in the bladder or upper urinary tract. Reliable urine specimen collection methods for assessing the presence of bladder bacteriuria, bladder aspirate or urethral catheterization, are invasive and uncomfortable. Voided urine, on the other hand, is easy to collect, but often difficult to interpret because of the inevitable problem with contamination from periurethral flora. Significant bacteriuria can be defined as that quantity of uropathogens in urine which reliably distinguishes bladder bacteriuria from contamination. The need to quantitate uropathogens in voided urines to distinguish between bladder bacteriuria and bacterial contamination has been recognized for many years. Not, however, until the work of E. 11. Kass and others in the 1950s was a systematic attempt made to correlate UTI syndromes with the quantity of organisms in urine. Kass reported that colony counts -->105 uropathogens per ml of urine discriminated between contamination and infection in women with acute symptomatic pyelonephritis [30, 31] and, when confirmed on consecutive urine specimens, in women with asymptomatic bacteriuria [31]. Lesser colony counts in asymptomatic women rarely were persistent suggesting contamination [31]. This threshold criteria for significant bacteriuria was widely generalized for the diagnosis of UTIs in all patient populations even though it was not systematically evaluated in other populations. Studies over the last 20 to 30 years have demonstrated that many women with the dysuria/frequency syndrome have bladder bacteriuria with quantities < 105 uropathogens per ml [32]. More recent studies in women with the dysuria/ frequency syndrome have demonstrated that the traditional criteria for significant bacteriuria (>---105cfu per ml) has
Infection 18 (1990) Suppl. 2 © MMV Medizin Verlag GmbH Miinchen, Miinchen 1990
S 41
T. M. Hooton: Epidemiology of UTI
high specificity (.99), but low sensitivity (0.51) in detecting bladder bacteriuria while a threshold of -->102 uropathogens per ml is a more sensitive indicator of bladder bacteriuria (0.95) while being only slightly less specific (0.85) [33]. Evidence that "low count" bacteriuria is significant includes confirmation by suprapubic aspirates, presence of UTI symptoms and pyuria, persistence when untreated, response to treatment, and pattern of organisms' virulence characteristics [32, 33]. While controversy surrounds this concept [34], there is no doubt that many symptomatic women considered uninfected using the traditional threshold in fact have true bladder bacteriuria with or without subclinical upper tract infection. Presence of multiple organisms in similar quantities and predominance of organisms generally considered to be normal flora (lactobaciUi, diphtheroids) increase the probability that a voided specimen does not represent conditions in bladder urine. Men are less likely to contaminate voided urine specimens and, therefore, lower coiony counts in voided specimens have generally been considered to represent significant bacteriuria. It has long been known that men, as with women, can occasionally be found to have low count bladder bacteriuria as determined by bladder aspirates. A recent study in older men with irritative urethral symptoms seen in a urology clinic were evaluated with first void and midstream urines, suprapubic aspirates, and urethral catheterization [35]. In this study of 66 men itwas found that the criterion for midstream urine that best differentiated sterile from infected bladder urine w a s 103 cfu of a predominant species per ml (sensitivity 0.97 and specificity 0.97). In this study there was good correlation of colonY counts in uncleansed first void specimens compared with those in midstream urines. A wide range of colony counts, ranging from 102 to 105 cfu per ml, has been used to represent significant bacteriuria in catheterized patients. A recent study of such patients demonstrated that those with colony counts of 102 to 104 cfu of uropathogenic bacteria and fungi per ml, if followed untreated, almost all had counts rising to >105 cfu per ml within three days suggesting that these low counts represented true bladder bacteriuria [36]. Most authorities consider any quantity of organisms isolated in a bladder aspirate specimen to represent true bladder bacteriuria although there may be a slight risk of contamination of such specimens [37], presumably on the basis of urethral organisms being sucked into the bladder with negative pressure during the aspiration attempt, However, bladder aspirates represent the "gold standard" for determining-the presence of bladder bacteriuria and the presence of any organisms generally should be considered "significant". There are no published data on the significance of bacterial colony counts in urine following antimicrobial treatment. Thus, it is not at all clear whether colony counts
The Epidemiology of Urinary Tract Infection and the Concept of Significant Bacteriuria Summary: Urinary tract infections ( u r I ) are among the most common infections afflicting man. Urinary tract infections in young adult women are usually uncomplicated, but are often recurrent and cause considerable morbidity. Urinary tract infections in pregnant women, elderly patients, and catheterized patients warrant special attention because of their association with increased morbidity and POssibly with increased mortality. Diagnosis of UTI is usually based on quantitation of uropathogens in voided urine. The traditional criteria for significant bacteriuria, ->!05 uropathogens per ml of voided urine, is insensitive for detecting acute symptomatic cystitis in men and women and should be replaced with a lower colony count threshold.
Zusammenfassung:Epidemiologie der Harnwegsinfektionen und das Konzept der signifikanten Bakteriurie. Harnwegsinfektionen geh6ren zu den h/iufigsten Infektions; krankheiten des Menschen. Bei jungen Frauen sind Harnwegsinfektionen in der Regel unkompliziert, doch sind rezidivierende Verl/iufe h/iufig, und die Morbidit/it ist betr/ichtlich. Bei Schwangeren, ~ilteren Personen und Kathetertr/igern auftretende Hamwegsinfektionen verdienen besondere Beachtung, da nicht nur die Morbidit~it erh6ht ist, sondern mfglicherweise auch die Mortatit/it. Die Diagnose einer Harnwegsinfektion basiert in der Regel auf der Keimzahlbestimmung im Spontanurin. Fiir die Diagnose einer akuten Zystitis bei M~innern und Frauen ist der traditionelle Richtwert fiir eine signifikante Bakteriurie von >10 5 uropathogenen Keimen pro ml zu wenig empfindlich, aus diesem Grund sollten die Grenzwerte ftir die Keimzahlen niedtiger angesetzt werden.
Epidemiology of Urinary Tract Infections Urinary tract infections (UTI) are a major public health problem worldwide. In the United States, U r l s account for approximately 5-6 million office visits annually [1]. Bacteriuria is much more common in females than males at all ages of life except in the first year when the prevalence in boys is somewhat higher than that in girls [2, 3]. Between the ages of 1 and 50, UTIs in males whose genitourinary tracts have not been instrumented are rare, but after age 50 the rate starts to rise. Insertive anal intercourse, particularly in homosexual males, may increase the risk of UTI by exposing the urethra to high quantities of fecal Escherichia coli [4]. T h e prevalence of UTIs in preschool and school girls is approximately 1-3% which is S 40
30-fold higher than that in boys [5]. It is estimated that approximately 5% of girls will have one or more episodes of UTIs during their school years [5]. The prevalence of bacteriuria increases with age in adult women such that in the reproductive years the prevalence of bacteriuria is approximately 50 times greater than that in males [6]. In the elderly, the prevalence of bacteriuria approaches 20-50% in women and 5-20% in men with institutionalized persons having the highest rate [7]. It has been estimated that as many as 20-30% of adult women experience one or more dysuria episodes yearly, and that most of these episodes represent UTI [8], Anatomic and functional abnormalities and instrumentation of the urinary tract predispose both sexes to develop UTI. Bacteriuria in the non-instrumented male almost always warrants an evaluation for such abnormalities. In elderly males, bacteriuria is usually related to abnormalities in the prostate gland. Although girls less than age 1 with bacteriuria are likely to have an abnormality in the genitourinary tract, school girls and adult women infrequently have detectable abnormalities in association with bacteriuria. Vesicoureteral reflux is commonly found in infant boys and girls and this abnormality in conjunction with bacteriuria is thought to predispose to renal scarring and chronic renal disease [3, 5]. Many women with UTI will develop one or more recurrences. Relapse should be considered when an infection recurs within a week or so following treatment. Relapse usually occurs in the setting of upper tract infection, a functionally or anatomically abnormal urinary tract, or, in the male, chronic bacterial prostatitis. Approximately 80% of recurrent infections are thought to represent reinfection from perineal flora rather than relapse [9]. Reinfection in adult women, however, tends not to be associated with abnormalities in the urinary tract at least as detected by excretory urograms or cystoscopy [10]. It is not possible with present technology to reliably demonstrate that a recurrent UTI is a relapse since an identical strain from fecal flora may cause a reinfection. Recurrent Lrl'I in young women with anatomically normal urinary tracts appears to be due, at least in part, to a greater propensity for adherence of E. coli to their uroepithelial cells [11] and a higher prevalence of vaginal colonization with E. coli compared to women without recurrent UTI [12]. Extrinsic factors which may lead to recurrent LrrI in an otherwise healthy woman include sexual activity [13] and diaphragm use [i4], Persistence of vaginal colonizT. M. Hooton, M.D., Department of Medicine, University of Washington, Harborview Medical Center, 325 Ninth Avenue, Seattle, Washington 98104, USA.
Infection 18 (1990) Suppl. 2 © MMV Medizin Verlag GmbH Miinchen, Miinchen 1990
T. M. Hooton: Epidemiology of UTI
ation with E. coli after treatment of UTI may be a risk factor for early recurrence of infection [15]. Bacteriuria in pregnancy, in the elderly, and in institutionalized persons warrants special mention when discussing the epidemiology of UTI. Bacteriuria in pregnancy is a potentially serious problem occurring in approximately 5% of pregnant women in the first trimester and increasing with age and parity [16, 17]. Pregnancy itself appears not to be a risk factor for developing UTI, and the prevalence of bacteriuria does not increase as the pregnancy progresses. However, left untreated, approximately one fourth of pregnant women with bacteriuria early in pregnancy will develop symptomatic UTI (including acute pyelonephritis) [16] whereas treatment reduces the subsequent risk by 80-90% [17, 18]. In addition, persistent bacteriuria is associated with premature delivery and increased perinatal mortality [19, 20], Thus, pregnancy is one Of the few conditions for which routine screening for and treatment of asymptomatic bacteriuria is recommended. Bacteriuria in the elderly poses a significant problem given the very high prevalence found in both sexes, and recent reports that suggest an association between bacteriuria and increased mortality [7]. Treatment of bacteriuria in the elderly is associated with a high rate of recurrence, increased drug side effects, and increased antimicrobial resistance. Further study is indicated on whether bacteriuric elderly have a truly increased mortality risk or whether bacteriuria is simply an indicator of the severity of the patient's underlying disease state. Routine screening and treatment of asymptomatic bacteriuria in this population is unwarranted unless this association is confirmed by further carefully conducted investigation. Nosocomial UTIs occur in approximately 2-3% of hospitalized patients in the United States and account for 42% of all nosocomial infections [21]. Two thirds of these infections are associated with urinary catheterization [22] and catheter-associated UTIs are the most common source of gram-negative bacteremia in hospitalized patients [23]. Recent studies suggest that catheterized patients with bacteriuria have a significantly increased mortality compared with non-bacteriuric catheterized patients even after controlling for several potential risk factors [24]. As with the elderly, further studies are indicated to confirm this association to determine underlying mechanisms and whether therapy of asymptomatic bacteriuria is warranted. Efforts to prevent nosocomial UTIs are largely centered around recommendations for catheter care [25]. The microbial etiology of UTI differs between uncomplicated and complicated infections. Uncomplicated infections in young adult women are caused by E. coli in approximately 80-90% of cases and S. saprophyticus in approximately 10-20% [26, 27]. Uncomplicated UTI are occasionally caused by other gram-negative bacilli such as Proteus mirabilis and Klebsiella species. These organisms are generally susceptible to most of the commonly used antimicrobials [28]. Complicated infections, especially nosocomial UTI and those occurring in previously treated
patients with abnormal urinary tracts, are caused by a greater variety of organisms which tend to be resistant to multiple antibiotics [28]. The spectrum of organisms found in complicated UTI include, in addition to E. coli, Pseudomonas aeruginosa, Proteus species, Klebsiella species, Serratia, Enterobacter, other gram-negative rods, enterococcus, yeasts, and many other organisms. It is obvious, then, that UTIs are a significant health problem worldwide. It has been estimated that the evaluation and treatment of uncomplicated UTI in ambulatory women account for approximately $1 billion of expenditures in the United States alone [28]. It is further estimated that nosocomial UTIs cost United States hospitals more than half a billion dollars annually because of associated increased morbidity and increased lengths of stay in the hospital [29]. Obviously, the increased mortality reported to be associated with bacteriuria in the elderly and in catheterized patients cannot be so easily quantitated. Improved methods of prevention and therapy for Lrrls are certainly indicated to decrease the frequency and sequelae of this major health problem.
The Concept of Significant Bacteriuria Urinary tract infection is defined as bacteriuria in the bladder or upper urinary tract. Reliable urine specimen collection methods for assessing the presence of bladder bacteriuria, bladder aspirate or urethral catheterization, are invasive and uncomfortable. Voided urine, on the other hand, is easy to collect, but often difficult to interpret because of the inevitable problem with contamination from periurethral flora. Significant bacteriuria can be defined as that quantity of uropathogens in urine which reliably distinguishes bladder bacteriuria from contamination. The need to quantitate uropathogens in voided urines to distinguish between bladder bacteriuria and bacterial contamination has been recognized for many years. Not, however, until the work of E. 11. Kass and others in the 1950s was a systematic attempt made to correlate UTI syndromes with the quantity of organisms in urine. Kass reported that colony counts -->105 uropathogens per ml of urine discriminated between contamination and infection in women with acute symptomatic pyelonephritis [30, 31] and, when confirmed on consecutive urine specimens, in women with asymptomatic bacteriuria [31]. Lesser colony counts in asymptomatic women rarely were persistent suggesting contamination [31]. This threshold criteria for significant bacteriuria was widely generalized for the diagnosis of UTIs in all patient populations even though it was not systematically evaluated in other populations. Studies over the last 20 to 30 years have demonstrated that many women with the dysuria/frequency syndrome have bladder bacteriuria with quantities < 105 uropathogens per ml [32]. More recent studies in women with the dysuria/ frequency syndrome have demonstrated that the traditional criteria for significant bacteriuria (>---105cfu per ml) has
Infection 18 (1990) Suppl. 2 © MMV Medizin Verlag GmbH Miinchen, Miinchen 1990
S 41
T. M. Hooton: Epidemiology of UTI
high specificity (.99), but low sensitivity (0.51) in detecting bladder bacteriuria while a threshold of -->102 uropathogens per ml is a more sensitive indicator of bladder bacteriuria (0.95) while being only slightly less specific (0.85) [33]. Evidence that "low count" bacteriuria is significant includes confirmation by suprapubic aspirates, presence of UTI symptoms and pyuria, persistence when untreated, response to treatment, and pattern of organisms' virulence characteristics [32, 33]. While controversy surrounds this concept [34], there is no doubt that many symptomatic women considered uninfected using the traditional threshold in fact have true bladder bacteriuria with or without subclinical upper tract infection. Presence of multiple organisms in similar quantities and predominance of organisms generally considered to be normal flora (lactobaciUi, diphtheroids) increase the probability that a voided specimen does not represent conditions in bladder urine. Men are less likely to contaminate voided urine specimens and, therefore, lower coiony counts in voided specimens have generally been considered to represent significant bacteriuria. It has long been known that men, as with women, can occasionally be found to have low count bladder bacteriuria as determined by bladder aspirates. A recent study in older men with irritative urethral symptoms seen in a urology clinic were evaluated with first void and midstream urines, suprapubic aspirates, and urethral catheterization [35]. In this study of 66 men itwas found that the criterion for midstream urine that best differentiated sterile from infected bladder urine w a s 103 cfu of a predominant species per ml (sensitivity 0.97 and specificity 0.97). In this study there was good correlation of colonY counts in uncleansed first void specimens compared with those in midstream urines. A wide range of colony counts, ranging from 102 to 105 cfu per ml, has been used to represent significant bacteriuria in catheterized patients. A recent study of such patients demonstrated that those with colony counts of 102 to 104 cfu of uropathogenic bacteria and fungi per ml, if followed untreated, almost all had counts rising to >105 cfu per ml within three days suggesting that these low counts represented true bladder bacteriuria [36]. Most authorities consider any quantity of organisms isolated in a bladder aspirate specimen to represent true bladder bacteriuria although there may be a slight risk of contamination of such specimens [37], presumably on the basis of urethral organisms being sucked into the bladder with negative pressure during the aspiration attempt, However, bladder aspirates represent the "gold standard" for determining-the presence of bladder bacteriuria and the presence of any organisms generally should be considered "significant". There are no published data on the significance of bacterial colony counts in urine following antimicrobial treatment. Thus, it is not at all clear whether colony counts
