Epidemiology and clinical significance of blood cultures positive for coagulase-negative staphylococcus

Bacteraemia during direct laryngoscopy and endotracheal intubation: a study using a multiple culture, large volume technique

Bacteraemia secondary to orotracheal intubation has been reported to occur in 0-5.3% of patients. Bacteraemia detection is dependent upon several factors including the volume of blood per culture and the number of cultures. Prior studies used small volumes of blood and one or two cultures, and may therefore have underestimated the incidence of bacteraemia. Sixty-two adult patients who underwent direct laryngoscopy and endotracheal intubation were studied. Baseline blood cultures were sterile in all patients. After intubation, four blood cultures were obtained in ten minutes, with 10 ml being evenly divided between aerobic and anaerobic media. Two patients (3.2%) became bacteraemic. This is a lower incidence than occurs in association with other procedures for which The American Heart Association does not recommend administration of prophylactic antibiotics. Therefore, prophylactic antibiotics are not recommended prior to direct laryngoscopy. However, when a prophylactic antibiotic is administered prior to surgery, it would be best to administer the antibiotic prior to direct laryngoscopy and intubation.

Defining intravascular catheter-related infections: a plea for uniformity

This article defines the complex interaction between catheterized patients and invading microbial pathogens. Catheter colonization reflects significant growth of a microbe on a catheter component. Localized intravascular catheter-related infection denotes infection at the exit site, tunnel tract, or pocket, in the absence of bloodstream infection. Systemic intravascular catheter-related infection is a complication of colonization or localized infection, usually documented by invasion of the bloodstream. Catheter sepsis is a systemic infection that is difficult to define because symptoms associated with bloodstream infection caused by the most common pathogens to infect catheterized patients, coagulase-negative staphylococci, may not meet the previously published criteria of sepsis. It is hoped that the information contained here will lead to greater uniformity in the definitions used by the many investigators in this fascinating field.

Pediatric primary gram-negative nosocomial bacteremia: a possible relationship with infusate contamination

OBJECTIVE

To evaluate the potential contribution of "extrinsic" contamination of intravenous fluids in hospital bacteremia and infection.

DESIGN

Prospective cross-sectional survey of infusate contamination, December 1992 to December 1993.

SETTING

A pediatric department (1,500 admissions per year) in a general, urban teaching hospital, serving low-income patients. SAMPLES AND PATIENTS: Infusate samples (0.5 to 1.0 mL) from the injection port used by the staff were taken for cultures from all febrile or septic patients in hospital wards. At least four samples were taken each day; if no febrile or septic patients were available, other patients were sampled at convenience.

RESULTS

A 6.8% positive culture rate (87 contaminates in 1,277 infusates) was obtained, without significant differences among the wards. Gram-negative organisms were recovered from 56 samples (62.9%), mainly of the tribe Klebsielleae (56.1%). Coagulase-negative staphylococci were isolated in 30 samples (33.7%). There was no significant difference between the febrile-septic group and the asymptomatic group in the rate of infusate contamination (P = .59). In eight patients, the same organisms were recovered from infusate and blood culture. The overall bacteremia rate was 2.5 per 100 discharges.

CONCLUSIONS

Compared to previous reports, higher infusate contamination rates and different organisms (mainly gram-negative) were observed. In hospitals of underdeveloped countries, nosocomial infection control frequently is disregarded. Infusate contamination may be common and could lead to gram-negative bacteremia. In such settings, it seems advisable to perform surveillance studies to identify infusate contamination, because a single infusate contamination could be a signal for an epidemic.

Coagulase-negative Staphylococci in Danish blood cultures: species distribution and antibiotic susceptibility

The distribution and antibiotic susceptibility of coagulase-negative staphylococci (CoNS) isolated from blood cultures was examined in samples from hospitals covering most of Denmark. A total of 499 CoNS isolates were detected in 477 blood cultures from 340 patients and speciated as Staphylococcus epidermidis, 285; Staphylococcus hominis, 61; Staphylococcus haemolyticus, 43; Staphylococcus warneri, 12; Staphylococcus cohnii, 7; Staphylococcus saprophyticus, 4; Staphylococcus capitis, 2 and Staphylococcus lugdunensis, 1. Seventy-eight isolates could not be identified to species level and six were Micrococcus spp. In 108 (22.6%) blood culture sets, more than one CoNS strain were found, as detected by species identification, antibiogram and biotyping. Significantly more blood cultures from patients in university hospitals were drawn from central venous catheters. Comparing university and non-university hospitals, the overall antibiotic susceptibility among CoNS was only slightly different, except for methicillin and amikacin. The prevalence of methicillin-resistant strains was 35.1% in the university hospital strains vs. 25.3% in the non-university hospital strains. The overall prevalence of methicillin resistance was 32%. Great geographic variation in both species distribution and antibiotic resistance was observed. The high prevalence of S. epidermidis makes subtyping of this species important.

Nosocomial coagulase-negative staphylococcal infections in bone marrow transplantation recipients with central vein catheter. A 5-year prospective study

The purpose of this study was to examine coagulase-negative staphylococcal infections in bone marrow transplantation (BMT) patients with central vein catheters by investigating incidence, clinical relevance, risk factors, methicillin resistance, clinical impact of initial empiric antimicrobial therapy without vancomycin, and management of documented catheter-related infections. A 5-year prospective study was conducted with daily evaluation of 242 BMT patients during hospitalization, including clinical assessment and blood culture via the Hickman/Broviac catheter. If fever or infected appearance occurred, peripheral blood cultures or exit site cultures, respectively, were done. Results showed a septicemia incidence of 7.0%, including in 6 patients following colonization, in 1 patient with tunnel infection, in 1 patient with thrombophlebitis, in 1 patient with exit site infection, and in 8 patients with septicemia of unknown origin. Total colonization incidence was 7%, with colonization only in 11 patients who had 16 episodes; incidence of exit site infection was 3.7%. Age > or = 18 years was the only identified risk factor for developing staphylococcal infection (P = 0.03). Despite a methicillin resistance rate of 45% and omission of vancomycin from the routine initial empiric antimicrobial regimen, the clinical course of coagulase-negative staphylococcal infections was relatively benign. A single patient, who experienced marrow rejection, died on day +31 with septicemia and only one patient experienced microbiological failure with recurrent colonization. Bacteria grown in both aerobic and anaerobic bottles were more likely true bacteremia than contaminant (P = 0.03). We conclude that the hazard of coagulase-negative staphylococcal infection does not mandate inclusion of a glycopeptide in the initial empiric antimicrobial regimen in BMT patients, even during febrile neutropenia. Hickman/Broviac-related staphylococcal infections, except for tunnel infection or thrombophlebitis, can usually be treated successfully without removing the catheter.

Evaluation of staphylococci isolated from a blood culture system (Colorbact)

During a one-year period, all blood cultures positive for staphylococci from two of the smaller Danish counties with non-university hospitals only were evaluated. The isolates were speciated and coagulase-negative staphylococci (CoNS) were biotyped. Furthermore, antibiogram, phage typing and lectin typing were performed for all isolates. Clinical information was obtained by telephone, and before any bacteriological identification was performed, a preliminary judgment was made as to whether the positive blood culture was of clinical significance. A total of 3,500 blood cultures were evaluated and 426 (12.2%) were positive. One hundred and sixty blood cultures from 137 patients contained staphylococci; 36 of these patients had a pure culture of Staphylococcus aureus. One hundred and twenty-four CoNS were found and identified as S. epidermidis (81), S. hominis (19), S. haemolyticus (8), S. simulans (1), and Micrococcus species (3), and another 12 staphylococcal isolates which could not be identified to species level. A total of 35 patients had mixed cultures, including 6 with S. aureus as one of the isolates and 15 with mixed CoNS. Clinical relevance was estimated in 90% of cases where the later bacterial identification showed S. aureus, whereas clinical relevance was absent in the majority of cases including CoNS. Methicillin and gentamicin resistance was absent among S. aureus, but frequent among CoNS, especially S. epidermidis, where 40% were resistant to methicillin and 30% to gentamicin.

Comparison of genotypic and phenotypic methods for species identification of coagulase-negative staphylococcal isolates from blood cultures

Coagulase-negative staphylococci are often isolated from blood cultures. Simple methods are needed for correct identification of those species most frequently found. In this study, PCR methods were developed for the identification of S. epidermidis and S. haemolyticus, based on DNA sequences in their 16S rDNA. The results obtained by these methods were compared with those obtained using a number of phenotypic methods, including two commercial kits API Staph and Staphzyme. Fifteen type collection strains and 133 blood culture isolates were tested. The sensitivity of the PCR for identification of S. epidermidis was 99% compared with API Staph, but the specificity was lower, 94%, because of positive results also for S. capitis. The results by the PCR for identification of S. haemolyticus correlated closely with the Staphzyme results, 13 isolates being identified by Staphzyme and 16 by the PCR. API Staph, however, identified only four clinical isolates as S. haemolyticus, probably too few. Among the individual phenotypic tests performed, a trehalose-mannitol agar method and a desferrioxamine disc diffusion test for the identification of S. epidermidis were found to be very accurate. Anaerobic growth after overnight incubation could be used to distinguish S. epidermidis from S. hominis. The conclusion is that a majority of all Gram-positive, catalase-positive and coagulase-negative blood culture isolates can be typed as regards species level using only a few genotypic and/or phenotypic tests.

Coagulase-negative staphylococcal bacteraemia with special reference to septic shock: experience in an intensive care unit

During a period of 4.5 years, 48 patients with bacteraemia due to coagulase-negative staphylococci were studied prospectively in order to evaluate their clinical profile, management and outcome. There were 25 males and 23 females with ages ranging between 13 and 100 years. Over 60% of patients belonged to the age group 30 to 69 years. Shock was recorded in 23 (48%) of the 48 patients. Of the shocked patients, 16 were immunocompromised and also had abnormal coagulation. Their mortality was 44%. By contrast, none of the immunocompromised patients without shock died. Abnormal coagulation was found in 17 patients without septic shock. Their mortality was 5.9%. The commonest underlying disease was respiratory failure especially in shocked patients. The source of infection was identified in the majority of cases. In addition, most patients had an indwelling intravascular catheter especially an arterial one. The overall mortality was 16.7% (8/48). It was significantly higher in patients with shock than in those without shock (30.4% vs. 4.0%, P < 0.05).

Infections due to antibiotic-resistant gram-positive cocci

Gram-positive cocci are becoming increasingly resistant to traditionally used antimicrobial agents. Staphylococcus aureus, coagulase-negative staphylococci, the enterococcus, and Streptococcus pneumoniae are the most commonly encountered of such pathogens in clinical practice. Clinicians should be keenly aware of the usual types of infections that are caused by these organisms and the importance of documenting susceptibilities of infecting strains. The basic mechanisms of resistance should be familiar to clinicians so that an inappropriate empiric regimen will not be selected (e.g., addition of a beta-lactamase inhibitor for penicillin-resistant pneumococci). Vancomycin remains the agent of choice, sometimes in combination with gentamicin and/or rifampin, for most cases of infection due to these resistant gram-positive organisms. Last, increased efforts toward prevention, such as strict adherence to infection control measures, selective use of broad-spectrum antibiotics, and increased use of pneumococcal vaccine, may be useful to help stem the rising tide of infections due to resistant gram-positive cocci.

Epidemiological markers of coagulase-negative staphylococci

Several different epidemiological typing methods have been used in studies of coagulase-negative staphylococci; these include biotyping, antibiotic susceptibility pattern analysis, serological typing, phage typing, slime production detection, protein profile analysis, immunoblot fingerprinting and DNA typing. The most frequently used tests are antibiotic susceptibility and extrachromosomal DNA banding patterns. Comparison of chromosomal DNA restriction patterns is facilitated when the fragments are separated by pulsed-field electrophoresis. Typing variations occur quite frequently even in isolates of demonstrated clinical significance. Caution should therefore be exercised in denying the clinical relevance of coagulase-negative staphylococci, even when successive isolates show distinct typing differences.

Species distribution of coagulase-negative staphylococcal isolates at a community hospital and implications for selection of staphylococcal identification procedures

A total of 499 coagulase-negative staphylococci (CoNS) were isolated from a variety of clinical specimens at a community hospital. Ten different species and many strains of CoNS were identified. Staphylococcus epidermidis was the most common isolate. The species distribution suggests that S. saprophyticus and, to a lesser extent, S. haemolyticus may be important in urinary tract infections. S. lugdunensis may be a significant isolate from wound infections. Frequently, mixed cultures were found with either multiple species or multiple strains of the same species of CoNS. These mixed cultures could not be detected by colony morphology upon initial overnight incubation of the cultures but could be distinguished following colony development for several days. In addition, sequential positive cultures from an individual patient often yielded different species or different strains of the same species which again could not be detected upon initial observations of colony morphology. Procedures for the identification of the CoNS need to be improved, and microbiology laboratories should consider the use of more definitive identification procedures for the CoNS.

A cluster of coagulase-negative staphylococcal bacteremias associated with peripheral vascular catheter colonization in a neonatal intensive care unit

BACKGROUND

A cluster of six neonatal cases of coagulase-negative staphylococcal bacteremias occurred in a Los Angeles County neonatal intensive care unit in March 1989.

METHODS

A retrospective cohort study assessed the impact of host-and delivery-related variables, length of hospitalization, duration of antibiotic treatment, performance or duration of invasive procedures, and staffing variables on risk of coagulase-negative staphylococcal bacteremia.

RESULTS

Unstratified analyses yielded eight risk factors with risk ratios greater than 2. After stratification by gestational age (less than 29 weeks) and low birth weight (less than 1500 gm), frequency of blood transfusions, duration of respiratory therapy, heparin lock and central vascular line placement, and hyperalimentation remained associated with elevated risk. Two species were identified, arguing against a common source of infection. Of four cohort months with more than 15 very low birth weight infants in the neonatal intensive care unit, an elevation of coagulase-negative staphylococcus-positive blood cultures and diagnosed bacteremias occurred in only two.

CONCLUSIONS

This cluster of coagulase-negative staphylococcal bacteremia was probably caused by frequent manipulation of catheters in neonates who were at heightened risk because of low birth weight and prematurity.

Prospective multicentric study of the etiology of 1051 bacteremic episodes in 782 cancer patients. CEMIC (French-Belgian Study Club of Infectious Diseases in Cancer)

A total of 1051 bacteremic episodes (782 patients) were prospectively recorded in 10 cancer centers (9 French, 1 Belgian), with: patient's age (mean 53, range 1-89 years), underlying cancer, neutropenia (< 1000 neutrophils/microliters; 233), signs and symptoms, type of i.v. line (percutaneous central: 534; peripheral: 228; central implanted: 304), treatment, blood culture system, number of positive blood culture bottles/total obtained, time to growth. Of all episodes, 23.2% occurred within 48 h of admission. The patients were receiving systemic antibiotics at sampling (on AB) in 34.6% of cases. The 1147 pathogens isolated (86 polymicrobial) were: E. coli (10.7%), Klebsiella-Enterobacter-Serratia (6.1%), other enterobacteriaceae (2.2%), Pseudomonas aeruginosa (4.8%), other nonfermenters (4.7%), coagulase-negative staphylococci (CNS, 40.8%), Staphylococcus aureus (9.9%), streptococci (5.4%), enterococci (2.2%), anaerobes (3.4%), yeasts (3.5%), and other bacteria (6.9%). The CDC (Centers for Disease Control) criteria (1988) were used to assess significance: group 1: pathogenic species (616 episodes; 59%); group 2: clinical signs and isolation of a "contaminant" species (47; 4.5%); group 3: as in group 2 with an i.v. line and empiric antibiotic treatment (181 episodes including 176 CNS; 17%); group 4: non-significant (207 episodes including 203 CNS; 20%). Groups 1-3, in which the episodes were considered to be significant (844 episodes; 80%) were compared with non-significant episodes (Fisher). Significant differences (P < or = 0.05) were seen in time to growth (median growth within 24 h vs 48 h), fever (86% vs 54%), chills (40% vs 3%), hypotension (10% vs 2%), septic shock (9% vs 1%), polymicrobial etiology (10% vs 0.5%), and initiation of empiric antibiotic treatment (71% vs 4%). Bacteremic episodes of CDC groups 1, 3 and 4 were further studied in episodes with a single isolate as a doubtful clinical significance (482 episodes) and episodes with > or = 2 bottles positive of probable clinical significance (569 episodes; 54%). In group 1 (218 doubtful, 398 probably significant episodes) significant differences were seen in chills (36% vs 52%), shock (7% vs 13%), polymicrobial (8% vs 17%), initiation of empiric antibiotic treatment (60% vs 72%); in group 3 (87 doubtful, 94 probably significant) in time to growth delay; in group 4 (177 doubtful, 30 probably significant) in proportion with implanted catheter (26% vs 52%), fever (62% vs 10%), and time to growth. This study confirms the predominant role of Gram-positive cocci in bacteremia occurring in cancer patients.

Hemagglutination and adherence to plastic by Staphylococcus epidermidis

Staphylococcus epidermidis is an important nosocomial pathogen responsible for intravenous catheter-related bacteremia and infections of other prosthetic medical devices. We found that the ability of S. epidermidis to hemagglutinate erythrocytes correlated with the adherence of bacteria to plastic and to intravenous catheters. S. epidermidis isolates responsible for prosthetic-valve endocarditis (n = 61) and isolates from intravenous catheters (n = 59) were significantly more likely to cause hemagglutination than isolates from the skin of preoperative cardiac surgery patients (n = 19) (P = 0.027). S. epidermidis isolates (n = 23) recovered from the skin of patients 7 to 10 days after cardiac surgery were significantly more likely to exhibit hemagglutination than the preoperative isolates (P = 0.015). By a quantitative adherence assay, we also observed that the hemagglutination titer and number of species of erythrocytes agglutinated correlated directly with adherence to polystyrene (P less than 0.001). In addition, hemagglutinating isolates were significantly more likely to be recovered in high number from intravenous catheters when semiquantitative catheter culture techniques were used (P less than 0.001). We speculate that hemagglutinin(s) either plays a direct role in adherence to polymers and thus prosthetic-device infection or serves as an easily demonstrable marker for adherence-prone isolates.

Clinical significance of Staphylococcus warneri bacteremia

Twenty-seven episodes of bacteremia caused by Staphylococcus warneri were identified at Long Island Jewish Medical Center in New York between 1984 and 1989. Fourteen of these were thought to represent true bacteremias and 13 to represent contaminants. Of the 14 true bacteremias, 5 were in pediatric and 9 were in adult patients. Eight of 14 patients (57%) had catheter-related bacteremia and 5 of 14 had bacteremia of unknown source. There was one case of fulminant native valve S. warneri endocarditis. All cases of catheter-related bacteremia, except one, were nosocomially acquired, and 75% of these patients had an underlying immunosuppressive condition. Only 40% of patients with bacteremias of unknown source were immunocompromised, and S. warneri appeared to be noninvasive in this group. Interestingly, all five of the pediatric isolates were oxacillin susceptible, although four of five were resistant to penicillin, despite the fact these patients were hospitalized an average of 29 days. In contrast, seven of nine adult isolates were resistant to both oxacillin and penicillin. The only case of native valve S. warneri endocarditis occurred in a patient who had no known underlying valvular heart disease, but had an underlying immunosuppressive condition. Identification to species level of coagulase-negative staphylococci may lead to appreciation of the importance of bacteria such as S. warneri as human pathogens.

Comparison of blood cultures with corresponding venipuncture site cultures of specimens from hospitalized premature neonates

We compared the presence and identities of isolates from blood culture samples obtained by percutaneous venipuncture with those of commensal skin organisms cultured from respective venipuncture sites after skin cleansing; 677 blood and skin site culture pairs from 488 infants were compared. Organisms grew in 58 blood cultures; nine of these cultures had corresponding venipuncture site cultures that also grew organisms. Forty-two blood culture isolates were coagulase-negative staphylococci; five of these were associated with similar venipuncture site cultures. According to restriction-endonuclease fingerprinting of chromosomal DNA and plasmid analysis, three pairs of blood and venipuncture site cultures were identical and two pairs were different. Thus only 7% (3/42) of coagulase-negative staphylococcal blood isolates were associated with identical contamination at the venipuncture site. We conclude that, if the venipuncture site has been carefully cleansed, the growth of coagulase-negative staphylococci in blood cultures of specimens from premature neonates indicates bacteremia rather than skin contamination in the vast majority of cases.

The impact of converting to a biphasic blood-culture system on the overall cost and the incidence of pseudobacteremia

After converting from a conventional broth (CB) system to a biphasic (BP) agar-slide blood-culture system (Septi-Chek), our laboratory noted an increase in positive blood cultures in general, and in coagulase-negative staphylococci (CNS) in particular. To investigate these findings, we compared all blood cultures collected over a 21-month period using CB and then BP systems, totaling 28,199 blood cultures. The frequency of positive blood cultures increased from 9.2% to 12.7% (p less than 0.0001), whereas CNS isolation increased from 2.6% to 5.2% (p less than 0.0001). There was no significant change in the incidence of true primary or secondary bacteremia due to CNS (p = 0.9). The isolation of other pathogens, including Staphylococcus aureus, Candida albicans, Bacteroides species, and Gram-negative bacilli increased from 6.5% to 7.1% (p less than 0.05). We estimated the cost of processing 28,000 blood cultures by both CB and BP systems, using positivity rates of 9.2% and 12.7%, respectively, and standards provided by the College of American Pathologists (CAP, 1991) for workload hours of technologist time. We calculated a higher overall cost for the BP system. However, the use of this system eliminated the use of needles and syringes for subculture of bottles showing no growth, thus decreasing the risk of technologist exposure to body fluids. Despite the increased cost and more frequent occurrence of pseudobacteremia, the enhanced sensitivity and increased safety of the BP system justified its use in the prompt identification of patients with true bacteremia.

Contamination of blood cultures during venepuncture: fact or myth?

Contamination of blood cultures is believed to occur mainly during the venepuncture procedure. Consequently, meticulous preparation of the venepuncture site is widely recommended. To determine whether the contamination rate is indeed affected by the quality of the antiseptic procedure at the venepuncture site, 181 paired cultures were collected from 176 patients during a 6-month period after either strict antiseptic cleansing of skin with alcohol followed by povidone-iodine, or after brief disinfection with alcohol alone. The contamination rate was not influenced by the antiseptic procedure, and corresponded to the accepted percentage reported in most other studies. Eight false positive cultures (4.4%) were obtained after strict antisepsis of the skin and 6 (3.3%) after short simple cleansing with alcohol (P = 0.39). Our results suggest that contamination of blood cultures may not be related to the venepuncture procedure--regardless of the antiseptic technique used--but may be due to later stages of laboratory handling and processing of the specimens. Review of the literature has provided further indirect evidence to support this conclusion.

Clinical implications of positive blood cultures

Positive blood cultures can be classified according to their veracity (true-positive or false-positive culture), clinical severity (inconsequential or life threatening), place of origin (community acquired or nosocomial), source (primary or secondary), duration (transient, intermittent, or continuous), pattern of occurrence (single episode, persistent, or recurrent), or intensity (high or low grade). In general, however, positive blood cultures identify a patient population at high risk of death. In my studies, patients with positive blood cultures were 12 times more likely to die during hospitalization than patients without positive blood cultures. Many bacteremias and fungemias occur in complicated clinical settings, and it appears that only about one-half of the deaths among affected patients are due directly to infection. Hence, it is appropriate to speak of "crude mortality" and "attributable mortality." Among hospitalized patients, recent trends include rising incidences of Staphylococcus aureus and coagulase-negative staphylococcal and enterococcal bacteremias and a dramatic increase in the incidence of fungemias. The diagnostic and therapeutic implications of blood cultures positive for specific microorganisms continue to evolve and are the subject of a large and growing medical literature.

Use of cellular hydrophobicity, slime production, and species identification markers for the clinical significance of coagulase-negative staphylococcal isolates

Determining the clinical relevance of coagulase-negative staphylococci isolated from cultures of clinical specimens remains a common dilemma. One hundred eighteen strains of coagulase-negative staphylococci isolated from patients with and without indwelling foreign bodies were characterized with regard to cell-surface hydrophobicity, slime production, and species to determine the predictive value of these phenotypic markers in distinguishing clinically significant from insignificant isolates. The single test with the highest positive predictive value was hydrophobicity (79%). Hydrophobicity and speciation had the greatest combined predictive value of any two tests (89%), and this increased to only 90% when determination of slime production was added. These tests provide additional clinical information when coagulase-negative staphylococci are isolated in culture.

Construction of a database to identify Staphylococcus species

A database was constructed for the routine identification of Staphylococcus species, isolated from man. The method comprised 15 conventional characterisation tests using substrates incorporated into agar plates and a multipoint inoculation system. The database was constructed from results of 125 reference strains and 1567 clinical isolates. In an evaluation trial, using a probability profile index generated from the database, 529 of 559 (94.6%) further clinical isolates were identified to species level. A further 20 (3.6%) gave low discrimination between two species. The proposed scheme was rapid, reliable, and inexpensive.

Fifteen-year experience with bloodstream isolates of coagulase-negative staphylococci in neonatal intensive care

Investigators worldwide, as well as the neonatologists and infection control team at our hospital, have reported that the incidence of coagulase-negative staphylococcal bacteremia in critically ill neonates has increased dramatically in recent years. To investigate these claims, we examined the results of all blood cultures obtained from 1970 to 1984 in our neonatal intensive care unit. Throughout this study period, coagulase-negative staphylococci were prominent blood culture isolates (crude overall incidence of 4.4 positive bacteremia workups per 100 neonates admitted; range, 2.5 to 6.7), representing 26.3 to 69.6% of all positive cultures. There was no significant increase in incidence over time by analysis of linear trend. Detailed analysis of data from 1976 and 1982 (two selected years for which complete information concerning culturing practices and patient characteristics was available) revealed that these observations were not explained by changes in the frequency of blood culturing. In both 1976 and 1982, the probability that a blood culture would grow coagulase-negative staphylococci increased steadily from 2 to 3% shortly after admission to reach a level of about 12% in week 3 of hospitalization, before declining to an intermediate level thereafter. This pattern is more consistent with nosocomial bacteremia than with contamination of blood cultures. Contrary to clinical reports, coagulase-negative staphylococci have been the principal pathogens isolated from blood cultures in our neonatal intensive care unit since at least 1970, with no measurable increase over the subsequent 14 years.

Antimicrobic susceptibility and plasmid profile analysis as identity tests for multiple blood isolates of coagulase-negative staphylococci

We compared a disk diffusion antimicrobic susceptibility panel with plasmid DNA profiles as tests for identity of 106 isolates of coagulase-negative staphylococci cultured from the blood of 45 patients on multiple occasions. The antimicrobic panel included penicillin, oxacillin, clindamycin, trimethoprim-sulfamethoxazole, chloramphenicol, tetracycline, tobramycin, kanamycin, and gentamicin. Nineteen patterns of antimicrobic susceptibility were found. The most common pattern was present in 25% of the isolates, and at least one isolate from 31% of the patients had this pattern. Forty-seven distinct plasmid DNA profiles were found. The most common plasmid profile was present in 8.5% of the isolates, and at least one isolate from 15% of the patients had this profile. Twenty-eight patients had multiple isolates that were identical by plasmid profile analysis. Twenty-seven (96%) of these patients had isolates that were also identical by antimicrobic susceptibility. Nineteen patients had multiple isolates that were different by plasmid profile analysis. In 18 (95%) of these patients, the isolates were also different by antimicrobic susceptibility. Although plasmid DNA profile analysis is a more discriminating tool, these data confirm that a selected disk diffusion antimicrobic susceptibility panel may be used to screen multiple blood isolates of coagulase-negative staphylococci for identity or differences.

Staphylococcus epidermidis: a significant nosocomial pathogen

Staphylococcus epidermidis is an organism formerly believed to be nonpathogenic. It is now recognized as a pathogen, causing infections on implanted devices and among immunosuppressed patients. Further, it has been involved in the development of resistance to a number of antibiotics. The epidemiology of this organism, its pathogenesis, and its treatment are important to infection control practitioners.