Oropharyngeal and fecal carriage of Pseudomonas aeruginosa in hospital patients

Systemic infection facilitates transmission of Pseudomonas aeruginosa in mice

Health care-associated infections such as Pseudomonas aeruginosa bacteremia pose a major clinical risk for hospitalized patients. However, these systemic infections are presumed to be a "dead-end" for P. aeruginosa and to have no impact on transmission. Here, we use a mouse infection model to show that P. aeruginosa can spread from the bloodstream to the gallbladder, where it replicates to extremely high numbers. Bacteria in the gallbladder can then seed the intestines and feces, leading to transmission to uninfected cage-mate mice. Our work shows that the gallbladder is crucial for spread of P. aeruginosa from the bloodstream to the feces during bacteremia, a process that promotes transmission in this experimental system. Further research is needed to test to what extent these findings are relevant to infections in patients.

Surveillance of surgical site infections by Pseudomonas aeruginosa and strain characterization in Tanzanian hospitals does not provide proof for a role of hospital water plumbing systems in transmission

Background

The role of hospital water systems in the development of Pseudomonas aeruginosa (P. aeruginosa) surgical site infections (SSIs) in low-income countries is barely studied. This study characterized P. aeruginosa isolates from patients and water in order to establish possible epidemiological links.

Methods

Between December 2014 and September 2015, rectal and wound swabs, and water samples were collected in the frame of active surveillance for SSIs in the two Tanzanian hospitals. Typing of P. aeruginosa was done by multi-locus sequence typing.

Results

Of 930 enrolled patients, 536 were followed up, of whom 78 (14.6%, 95% CI; 11.6–17.5) developed SSIs. P. aeruginosa was found in eight (14%) of 57 investigated wounds. Of the 43 water sampling points, 29 were positive for P. aeruginosa. However, epidemiological links to wound infections were not confirmed. The P. aeruginosa carriage rate on admission was 0.9% (8/930). Of the 363 patients re-screened upon discharge, four (1.1%) possibly acquired P. aeruginosa during hospitalization. Wound infections of the three of the eight P. aeruginosa SSIs were caused by a strain of the same sequence type (ST) as the one from intestinal carriage. Isolates from patients were more resistant to antibiotics than water isolates.

Conclusions

The P. aeruginosa SSI rate was low. There was no evidence for transmission from tap water. Not all P. aeruginosa SSI were proven to be endogenous, pointing to other routes of transmission.

Effectiveness of oral hygiene interventions against oral and oropharyngeal reservoirs of aerobic and facultatively anaerobic gram-negative bacilli

BACKGROUND

Aerobic and facultatively anaerobic gram-negative bacilli (AGNB) are opportunistic pathogens and continue to cause a large number of hospital-acquired infections. AGNB residing in the oral cavity and oropharynx have been linked to nosocomial pneumonia and septicemia. Although AGNB are not considered members of the normal oral and oropharyngeal flora, medically compromised patients have been demonstrated to be susceptible to AGNB colonization.

METHODS

A literature search was conducted to retrieve articles that evaluated the effectiveness of oral hygiene interventions in reducing the oral and oropharyngeal carriage of AGNB in medically compromised patients.

RESULTS

Few studies have documented the use of mechanical oral hygiene interventions alone against AGNB. Although a number of studies have employed oral hygiene interventions complemented by antiseptic agents such as chlorhexidine and povidone iodine, there appears to be a discrepancy between their in vitro and in vivo effectiveness.

CONCLUSION

With the recognition of the oral cavity and oropharynx as a reservoir of AGNB and the recent emergence of multidrug and pandrug resistance in hospital settings, there is a pressing need for additional high-quality randomized controlled trials to determine which oral hygiene interventions or combination of interventions are most effective in eliminating or reducing AGNB carriage.

Identification of blaOXA-128 and blaOXA-129, two novel OXA-type extended-spectrum-β-lactamases in Pseudomonas aeruginosa, in Hunan Province, China

We collected 97 non-repetitive carbapenemases-sensitive clinical isolates of Pseudomonas aeruginosa in Human Province, China, during the period of October 2006 to January 2007. From these isolates, we identified two novel oxacillin-hydrolysing (OXA) type extended-spectrum-β-lactamases (ESBLs): bla OXA-128 and bla OXA-129, which contain the mutations of I89V from bla OXA-56 and K134N from bla OXA-10, respectively. Clinical isolates containing either bla OXA-128 or bla OXA-129 show resistance to cephamycin-class antibiotics but sensitive to carbapenem-class antibiotics. The occurrence of novel OXA-type lactamases suggests a regional prevalent pattern of ESBLs Pseudomonas aeruginosa in this area.

Multilevel modelling of the prevalence of hospitalized patients infected with Pseudomonas aeruginosa

Pseudomonas aeruginosa is one of the leading nosocomial pathogens. The question of the respective contribution of endogenous and exogenous sources remains controversial. In this study, we shed new light on this issue by means of a multilevel logistic regression analysis which allowed a simultaneous investigation of factors associated with prevalence of patients infected with P. aeruginosa at two levels: patient and healthcare facility (HCF) in the eastern regions of France. A total of 25 533 in-patients from 51 HCFs were included in the analysis. The overall prevalence was 0·37% (range 0-1·65%). Multilevel modelling estimated that <14% of total variability of the outcome variable was explained by differences between HCFs and that after adjusting for patient-level variables, which explained 52% of HCF-level variance, the latter became non-significantly different from zero. A compositional effect (patient factors), rather than a contextual effect (ecological factors), explains heterogeneity of the prevalence of patients infected with P. aeruginosa in the eastern HCFs of France.

Effects of carbapenem exposure on the risk for digestive tract carriage of intensive care unit-endemic carbapenem-resistant Pseudomonas aeruginosa strains in critically ill patients

To determine the epidemiology and risk factors for carbapenem-resistant Pseudomonas aeruginosa (CR-PA) digestive tract colonization, weekly rectal and pharyngeal swabs were obtained in two serial incidence surveys (266 patients). Forty-two (16%) patients were CR-PA colonized (12 [29%] on admission and 30 [71%] in intensive care units). Pulsed-field gel electrophoresis showed extensive clonal diversity, although one specific clone (type B) was isolated from 11 patients. The presence of similar genotypes of CR-PA colonizing 30% of the CR-PA-colonized patients suggests the occurrence of cross-colonization; in addition, 10 pairs of carbapenem-susceptible P. aeruginosa (CS-PA) and subsequent CR-PA strains isolated from the same patients were found to be clonally identical and were considered to have been endogenously acquired (33%). All endogenously acquired CR-PA strains were isolated after exposure to a carbapenem, and 80% showed a phenotype of imipenem resistance (IR pattern) alone, while 67% of the CR-PA strains acquired by cross-transmission exhibited a multiresistant (MR) phenotype, with previous carbapenem exposure in 44%. Logistic regression analysis identified severity of acute illness (odds ratio [OR], 1.0; 95% confidence interval [CI], 1.0 to 1.1), prior carbapenem use (OR, 7.8; 95% CI, 1.7 to 35.3), and prior use of fluoroquinolones (OR, 11.0; 95% CI, 1.7 to 67.9) as independent risk factors for CR-PA digestive tract colonization. Overall, the local epidemiology of CR-PA digestive tract colonization was characterized by polyclonal endemicity with phenotype patterns of IR and MR divided evenly between patients. Restricting the use of particular agents, such as carbapenems and fluoroquinolones, should be considered advisable to minimize the problem of this antibiotic resistance. However, the possible risk for development of collateral unexpected bacterial resistance patterns should be accurately monitored.

Role of hospital stay and antibiotic use on Pseudomonas aeruginosa gastrointestinal colonization in hospitalized patients

The prospective cohort study presented here assessed the risk factors associated with Pseudomonas aeruginosa gastrointestinal colonization (PAGIC) in 933 patients hospitalized in five different wards in a French university hospital. A total of 195 patients were colonized. By logistic regression, hospitalization in an intensive care unit and length of hospital stay were independent risk factors. A significant association was observed between fluoroquinolone use and PAGIC caused by an ofloxacin-resistant strain (p < 0.0001), imipenem use and PAGIC caused by an imipenem-resistant strain (p < 0.0002) and ceftazidime use and PAGIC caused by a ceftazidime-resistant strain (p < 0.02). The ecological impact of antibiotic use is of great clinical relevance and clinicians should consider antimicrobial resistance in order to limit the development and dissemination of resistant microorganisms.

Molecular epidemiology of Pseudomonas aeruginosa colonization in a burn unit: persistence of a multidrug-resistant clone and a silver sulfadiazine-resistant clone

To study the epidemiology of Pseudomonas aeruginosa colonization in a 32-bed burn wound center (BWC), 321 clinical and 45 environmental P. aeruginosa isolates were collected by prospective surveillance culture over a 1-year period and analyzed by serotyping, drug susceptibility testing, and amplified fragment length polymorphism (AFLP) analysis. Among 441 patients treated at the center, 70 (16%) were colonized with P. aeruginosa, including 12 (17%) patients who were colonized on admission and 58 (83%) patients who acquired the organism during their stay. Of the 48 distinct AFLP genotypes found, 21 were found exclusively in the environment, 15 were isolated from individual patients only, and 12 were responsible for the colonization of 57 patients, of which 2 were also isolated from the environment, but secondary to patient carriage. Polyclonal P. aeruginosa colonization with strains of two to four genotypes, often with different antibiotic susceptibility patterns, was observed in 19 patients (27%). Two predominant genotypes were responsible for recurrent outbreaks and the colonization of 42 patients (60% of all colonized patients). The strain with one of those genotypes appeared to be endemic to the BWC and developed multidrug resistance (MDR) at the end of the study period, whereas the strain with the other genotype was antibiotic susceptible but resistant to silver sulfadiazine (SSD(r)). The MDR strain was found at a higher frequency in sputum samples than the SSD(r) strain, which showed a higher prevalence in burn wound samples, suggesting that anatomic habitat selection was associated with adaptive resistance to antimicrobial drugs. Repeated and thorough surveys of the hospital environment failed to detect a primary reservoir for any of those genotypes. Cross-acquisition, resulting from insufficient compliance with infection control measures, was the major route of colonization in our BWC. In addition to the AFLP pattern and serotype, analysis of the nucleotide sequences of three (lipo)protein genes (oprI, oprL, and oprD) and the pyoverdine type revealed that all predominant strains except the SSD(r) strain belonged to recently identified clonal complexes. These successful clones are widespread in nature and therefore predominate in the patient population, in whom variants accumulate drug resistance mechanisms that allow their transmission and persistence in the BWC.

Beta-1,2-mannosylation of Candida albicans mannoproteins and glycolipids differs with growth temperature and serotype

Increasing the growth temperature from 28 to 37 degrees C reduced the expression of beta-1,2-oligomannoside epitopes on mannoproteins of Candida albicans serotypes A and B. In contrast, beta-1,2-mannosylation of phospholipomannan (PLM) remained constant despite a slight decrease in the relative molecular weight (M(r)) of this compound. At all growth temperatures investigated, serotype A PLM displayed an M(r) and an antigenicity different from those of serotype B PLM when they were tested with a panel of monoclonal antibodies.

Nosocomial pneumonia. Diagnostic and therapeutic considerations

Many patients with presumed nosocomial pneumonia probably have infiltrates on the chest radiograph, fever, and leukocytosis resulting from noninfectious causes. Because of the high mortality and morbidity associated with nosocomial pneumonias, however, most clinicians treat such patients with a 2-week empiric trial of antibiotics. Before therapy is initiated, the clinician should rule out other causes of pulmonary infiltrates, fever, and leukocytosis that mimic a nosocomial pneumonia (e.g., pre-existing interstitial lung disease, primary or metastatic lung carcinomas, pulmonary emboli, pulmonary drug reactions, pulmonary hemorrhage, collagen vascular disease affecting the lungs, or congestive heart failure). If these disorders can be eliminated from diagnostic consideration, a 2-week trial of empiric monotherapy is indicated. The clinician should treat cases of presumed nosocomial pneumonia as if P. aeruginosa were the pathogen. Although P. aeruginosa is not the most common cause of nosocomial pneumonia, it is the most virulent pulmonary pathogen associated with nosocomial pneumonia. Coverage directed against P. aeruginosa is effective against all other aerobic gram-negative bacillary pathogens causing hospital-acquired pneumonia. The clinician should select an antibiotic for empiric monotherapy that is highly effective against P. aeruginosa, has a good side-effect profile, has a low resistance potential, and is relatively inexpensive in terms of its cost to the institution. The preferred agents for empiric monotherapy for nosocomial pneumonia are cefepime, meropenem, and piperacillin. Single organisms are responsible for nosocomial pneumonia, not multiple pathogens. S. aureus rarely, if ever, causes nosocomial pneumonia but is mentioned frequently in studies based on cultures of respiratory tract secretions. S. aureus, unless accompanied by a necrotizing pneumonia with rapid cavitation within 72 hours, in the sputum indicates colonization rather than infection and should not be addressed therapeutically. Antibiotics associated with a high resistance potential should not be used as monotherapy or included in combination therapy regimens (i.e., ceftazidime, ciprofloxacin, imipenem, or gentamicin). Combination therapy is more expensive than monotherapy and is indicated only when P. aeruginosa is extremely likely, based on its characteristic clinical presentation, or is proved by tissue biopsy. Therapy should not be based on respiratory secretion cultures regardless of technique. Optimal combination regimens include cefepime or meropenem plus levofloxacin or piperacillin or aztreonam or amikacin. Nosocomial pneumonias usually are treated for 14 days. Lack of radiographic or clinical response to appropriate empiric nosocomial pneumonia monotherapy after 14 days suggests an alternate diagnosis. In these patients, a tissue biopsy specimen should be obtained to determine the cause of the persistence of pulmonary infiltrates unresponsive to appropriate antimicrobial therapy.

Differences in skin flora between inpatients and chronically ill outpatients

BACKGROUND

Changes in skin flora have been reported among hospitalized and critically ill patients, but little is known about whether these changes are associated with hospitalization or with chronic, serious illness. The purpose of this survey was to compare skin flora of chronically ill outpatients and inpatients.

METHODS

Aerobic skin flora of forearm and midsternum of 250 patients in an intensive care unit and 251 outpatients was sampled by contact plates.

RESULTS

Mean colony-forming units were 160.6, forearm; 229. 4, sternum (P <.000). In logistic regression analysis, patients in the medical intensive care unit were significantly more likely to have high counts on the arm (odds ratio, 2.48; 95% confidence interval: 1.34-4.43; P =.004), and blacks were significantly more likely to have higher counts on the sternum when compared with other ethnic groups (odds ratio, 1.92; confidence interval: 1.18-3.11; P =. 009). No differences were noted between inpatients or outpatients in prevalence of methicillin-sensitive Staphylococcus aureus, but inpatients were more likely to carry methicillin-resistant Staphylococcus aureus (arm, P =.007; sternum, P =.02). Outpatients had a higher prevalence of micrococci and gram-negative bacteria at both skin sites (all P <.01) and yeast at the sternal site (P =.007).

CONCLUSIONS

This comparison provides data to differentiate between effects of hospitalization and effects of chronic illness on skin flora.

Cross-colonisation with Pseudomonas aeruginosa of patients in an intensive care unit

BACKGROUND

Ventilator-associated pneumonia (VAP) caused by Pseudomonas aeruginosa is usually preceded by colonisation of the respiratory tract. During outbreaks, colonisation with P aeruginosa is mainly derived from exogenous sources. The relative importance of different pathways of colonisation of P aeruginosa has rarely been determined in non-epidemic settings.

METHODS

In order to determine the importance of exogenous colonisation, all isolates of P aeruginosa obtained by surveillance and clinical cultures from two identical intensive care units (ICUs) were genotyped with pulsed field gel electrophoresis.

RESULTS

A total of 100 patients were studied, 44 in ICU 1 and 56 in ICU 2. Twenty three patients were colonised with P aeruginosa, seven at the start of the study or on admission and 16 of the remaining 93 patients became colonised during the study. Eight patients developed VAP due to P aeruginosa. The incidence of respiratory tract colonisation and VAP with P aeruginosa in our ICU was similar to that before and after the study period, and therefore represents an endemic situation. Genotyping of 118 isolates yielded 11 strain types: eight in one patient each, two in three patients each, and one type in eight patients. Based on chronological evaluation and genotypical identity of isolates, eight cases of cross-colonisation were identified. Eight (50%) of 16 episodes of acquired colonisation and two (25%) of eight cases of VAP due to P aeruginosa seemed to be the result of cross-colonisation.

CONCLUSIONS

Even in non-epidemic settings cross-colonisation seems to play an important part in the epidemiology of colonisation and infection with P aeruginosa.

Prevention of nosocomial pneumonia

Prevention of nosocomial pneumonia becomes an achievable goal only to the extent that the mechanisms of infection are known. It is likely that there is variation among microorganisms, patients, type of care and intensive care, and practices that determines the relative efficacy of preventive measures. Certain procedures appear to be universally required and include adequate reprocessing of ventilation equipment and infection control measures in patient care. In contrast, many factors may affect the role of the stomach as a cause of nosocomial pneumonia, including enteral feeding procedures and gastric acidity. Differences in results between studies of preventive measures may reflect uncontrolled practice factors, which make the measures either more or less important. Selective decontamination of the digestive tract is a measure with potentially serious consequences, primarily cost and microbial resistance, and should be undertaken with care probably in selected high-risk patients. Other recommendations such as the use of sucralfate, which is effective in prevention of bleeding without clear side effects, are probably worth instituting based on existing evidence. The primary need may be for better reprocessing of equipment or hand washing and sterile gloves, or it may be several changes at once. Certainly an array of preventive measures is necessary, and there is probably no single procedure likely to solve such a complex problem. Use of the collective findings of many investigators needs to be made and strategies applied to each patient and setting. There has been a marked increase in our knowledge of nosocomial pneumonia, and effective measures for prevention are available. Application of these measures widely should reduce the frequency of respiratory complications. The microbiologic tools to compare bacterial isolates have been developed, and the course of events preceding infection of the patient can be demonstrated. The role of equipment, environment, other patients, personnel, colonization sites, and other factors can now be examined. Future studies should control for the many known factors that may predispose to nosocomial pneumonia to make the results meaningful. This would include definition of infection, patient risk factors, identification of microorganisms, details of enteral nutrition, type of stress ulcer prophylaxis, exposure to antimicrobial agents, and institutional resistance patterns. In some studies, surveillance cultures and molecular epidemiology techniques would be required. Large controlled multicenter studies are necessary to determine the significance of the results of promising smaller studies.(ABSTRACT TRUNCATED AT 400 WORDS)

Pseudomonas aeruginosa cytotoxin as a pathogenicity factor in a systemic infection of leukopenic mice

The effect of Pseudomonas aeruginosa cytotoxin was assessed in leukopenic outbred Swiss male mice (20 g) using a high cytotoxin-producing strain (PA158) and its cytotoxin-deficient isogenic mutant (PA114F5) generated by Tn7::Tn5 transposon mutagenesis of PA158. Leukopenia was induced by intraperitoneal (i.p.) administration of cyclophosphamide (150 micrograms/g). Anesthetized mice were infected via a 4 mm incision on the shaved back with 300 CFU/mouse (9 LD50; expected death rate 85%). Precleared mouse cytotoxin-specific heat inactivated rabbit polyclonal antibody (RPA) was administered i.p. (0.2 ml) 24 hr before challenge. Controls received i.p. normal rabbit serum, RPA, cyclophosphamide alone, or a sham procedure. Challenge with the high cytotoxin-producing strain PA158 caused earlier and a significantly greater mortality than that observed with a cytotoxin-deficient strain PA114F5 (P < 0.01). Cytotoxin-specific polyclonal antibody was protective. Pretreatment with antibody decreased the mortality rate following challenge with PA158 from 88.9% to 27.8% (P < 0.01). Pretreatment with antibody decreased the mortality rate following challenge with PA114F5 from 27.8% to 5.6% (P < 0.05). These results demonstrate that P. aeruginosa cytotoxin contributes to the pathogenicity of the organism and that cytotoxin antibody is protective in a systemic P. aeruginosa infection in leukopenic mice.

Nosocomial outbreak of severe Pseudomonas aeruginosa infections in haematological patients

From June to September 1988, an outbreak of Pseudomonas aeruginosa infections in neutropenic patients admitted to the Haematological Wards of "Ospedali Riuniti" in Bergamo, Italy, was detected. Out of 11 cases of P. aeruginosa infections, 8 were bacteremic. Of these, 7 died within few days of onset (mortality rate: 87.5%). Consequently, possible sources of infection were investigated, and moist areas of the hospital environment were shown to be highly contaminated by P. aeruginosa. A clinical and microbiological follow-up of patients admitted to the Haematological Wards was performed for a 10 month period following the outbreak. Adequate measures for cleaning and disinfection were shown to reduce the frequency of P. aeruginosa hospital infections.

Molecular epidemiology of Pseudomonas aeruginosa in an intensive care unit

Genotyping was used to analyse Pseudomonas aeruginosa isolates from sink drains and 15 intubated patients as part of a 3-month prospective study of strain transmission in a medical-surgical intensive care unit. Ninety percent of all washbasin drains were persistently contaminated with several P. aeruginosa genotypes. In 60% (9/15) of the patients, P. aeruginosa colonization or infection was hospital-acquired: P. aeruginosa strains isolated from these patients were present in hospital sinks or in other patients before their admission. Since all patients were immobile, personnel were the probable route of transmission of P. aeruginosa in the hospital. The mechanism of strain transmission from sinks to hands during hand washing was investigated in a children's hospital. When P. aeruginosa was present at densities of > 10(5)/c.f.u. per ml in sink drains, hand washing resulted in hand contamination with P. aeruginosa via aerosol generation in the majority of experiments or P. aeruginosa was detected using an air sampler above the washing basin. High P. aeruginosa cfu were present at 4.30 h in the eight sinks (5.4 x 10(5)-7.0 x 10(10) c.f.u./ml), whereas at 13.00 h P. aeruginosa c.f.u. were significantly lower (3.1 x 10(2)-8.0 x 10(5) c.f.u./ml). These data reveal that the danger of bacterial contamination of hands during hand washing is highest in the morning. The identified transmission routes demand more effective hygienic measures in hospital settings particularly concerning personnel hands and sink drains.

Epidemiological studies of nosocomial infections with Pseudomonas aeruginosa using a DNA probe

A DNA probe encoding the Pseudomonas aeruginosa pilin gene has been developed in the authors' laboratory and has been shown to be a useful epidemiological tool. In the present study this technology, together with other typing methods, has been used to define relatedness and possible transmission routes of P aeruginosa strains isolated in several hospital wards. Clusters of P aeruginosa infections, suspected to be the result of nosocomial transmission, developed in a general intensive care unit (ICU) and a neurosurgical ward/ICU, as well as in a burn unit, were studied using antibiograms, lipopolysaccharide-serotyping, and gene probe analysis. Results of these studies demonstrated that each of the general and neurosurgical ICU isolates were different, making nosocomial transmission very unlikely. However, within the burn unit, patient isolates had identical profiles, suggesting that spread between patients was occurring or that a common source of infection was present. Changes in infection control measures within the unit were introduced and may have contributed to eradication of the outbreak. DNA probe studies were valuable in clarifying epidemiological relatedness of isolates that was not evident with the other typing strategies and identified a possible burn-associated strain.

Isolation and characterization of a transposon-induced cytotoxin-deficient mutant of Pseudomonas aeruginosa

In order to provide a better system for investigating the role of cytotoxin in pathogenesis, we mutated wild-type Pseudomonas aeruginosa PA158 by introducing a transposon. The resulting pool of mutants was screened for cytotoxin-deficient strains. One mutant strain, PA114F5, was compared with PA158. Except for cytotoxin production and antibiotic resistance (specified by the transposon), the two strains appear isogenic. This mutant strain should be useful in further clarifying the role of cytotoxin in pathogenesis.

Molecular epidemiological study of Pseudomonas aeruginosa isolates from patients with acute leukemia

In an attempt to determine the genetic relationship between strains of Pseudomonas aeruginosa isolated from patients with acute leukemia, a recently described restriction fragment from the region upstream of the exotoxin A structural gene was used as a probe in Southern hybridization. The overall rate of cultures positive for Pseudomonas aeruginosa during 169 admissions (119 patients) was 17%. Twelve genotypically distinct strains were found among 18 colonized and/or infected individuals. Three of these strains were recovered from more than one patient, suggesting a certain risk of nosocomial transmission of Pseudomonas aeruginosa and cross-infection. Genotypic comparison showed identical restriction patterns in multiple isolates from single patients, and also in colonizing and subsequently infecting strains. Genotyping distinguished isolates with similar O serotypes and established the identity between isolates with differing susceptibility to agents used for antibacterial prophylaxis.

Antimicrobial susceptibility in gram-negative bacteremia: are nosocomial isolates really more resistant?

Bloodstream isolates of gram-negative aerobic bacilli from nosocomial infections are more likely to be resistant to antimicrobial agents than isolates from community-acquired cases are. It is not clear, however, how much this is due to the markedly different distribution of organisms in the two groups. We compared the susceptibilities of organisms of a given species which caused community-acquired bacteremia with the susceptibilities of isolates from nosocomial cases. Nine antimicrobial agents were tested against 1,077 isolates which were obtained during a 4-year nonepidemic period. Marked differences in crude rates of resistance were noted for all isolates from nosocomial cases versus all isolates from cases acquired in the community. When results were adjusted for the different organism distributions in the two groups, statistically significant differences were found for only six drug-organism pairs; in each of these, resistance rates were higher in nosocomial isolates. However, when results were further adjusted for the effect of multiple analyses, no significant differences were seen. The major factor leading to the greater prevalence of antimicrobial resistance in our hospital organisms was the markedly different distribution of organisms in the nosocomial and community-acquired groups. For individual organisms, greater resistance in nosocomial strains was confined to certain drugs. Factors that influence differences in organism distribution may not be solely the result of antimicrobial use.