Fifteen-year study of the changing epidemiology of methicillin-resistant Staphylococcus aureus

Thiadiazolidinones: a new class of alanine racemase inhibitors with antimicrobial activity against methicillin-resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) is a human pathogen and a major cause of hospital-acquired infections. New antibacterial agents that have not been compromised by bacterial resistance are needed to treat MRSA-related infections. We chose the S. aureus cell wall synthesis enzyme, alanine racemase (Alr) as the target for a high-throughput screening effort to obtain novel enzyme inhibitors, which inhibit bacterial growth. Among the 'hits' identified was a thiadiazolidinone with chemical properties attractive for lead development. This study evaluated the mode of action, antimicrobial activities, and mammalian cell cytotoxicity of the thiadiazolidinone family in order to assess its potential for development as a therapeutic agent against MRSA. The thiadiazolidones inhibited Alr activity with 50% inhibitory concentrations (IC₅₀) ranging from 0.36 to 6.4 μM, and they appear to inhibit the enzyme irreversibly. The series inhibited the growth of S. aureus, including MRSA strains, with minimal inhibitory concentrations (MICs) ranging from 6.25 to 100 μg/ml. The antimicrobial activity showed selectivity against Gram-positive bacteria and fungi, but not Gram-negative bacteria. The series inhibited human HeLa cell proliferation. Lead development centering on the thiadiazolidinone series would require additional medicinal chemistry efforts to enhance the antibacterial activity and minimize mammalian cell toxicity.

Neutrophils in innate host defense against Staphylococcus aureus infections

Staphylococcus aureus has been an important human pathogen throughout history and is currently a leading cause of bacterial infections worldwide. S. aureus has the unique ability to cause a continuum of diseases, ranging from minor skin infections to fatal necrotizing pneumonia. Moreover, the emergence of highly virulent, drug-resistant strains such as methicillin-resistant S. aureus in both healthcare and community settings is a major therapeutic concern. Neutrophils are the most prominent cellular component of the innate immune system and provide an essential primary defense against bacterial pathogens such as S. aureus. Neutrophils are rapidly recruited to sites of infection where they bind and ingest invading S. aureus, and this process triggers potent oxidative and non-oxidative antimicrobial killing mechanisms that serve to limit pathogen survival and dissemination. S. aureus has evolved numerous mechanisms to evade host defense strategies employed by neutrophils, including the ability to modulate normal neutrophil turnover, a process critical to the resolution of acute inflammation. Here we provide an overview of the role of neutrophils in host defense against bacterial pathogens and discuss strategies employed by S. aureus to circumvent neutrophil function.

Outcomes and costs associated with a history of vancomycin exposure in patients with MRSA-related complicated bacteremia and infective endocarditis

BACKGROUND

Methicillin-resistant Staphylococcus aureus (MRSA) is the primary cause of complicated bacteremia (CB) and infective endocarditis (IE). Studies have compared the costs of treatment with vancomycin to those of other agents, as well as the efficacy and tolerability of these treatments. However, a literature search found no published studies of the effects of vancomycin exposure on outcomes and hospital costs in patients with CB or IE due to MRSA.

OBJECTIVE

The aim of this study was to determine whether there is a quantitative relationship between the duration of vancomycin treatment or cumulative vancomycin exposure and outcomes or costs in patient with CB or IE due to MRSA.

METHODS

Electronic medical records of confirmed cases of MRSA-related CB or IE from July 1, 2006, to June 30, 2008, were retrospectively reviewed to identify patients with a history of vancomycin exposure or no vancomycin exposure. Those who received vancomycin were stratified by the amount of drug administered or the duration of treatment to determine the relationship between treatment and outcomes. Data collected included demographic information, treatment information, attributable mortality, MIC data, and hospital costs. Classification and regression tree analysis (CART) was used to determine whether a history of vancomycin exposure was associated with treatment failure, attributable mortality, or both. The Mann-Whitney U test and the Fisher exact test were used for univariate analyses, and logistic regression was used for multivariate modeling.

RESULTS

Data from 50 patients were evaluated (CB, 32; IE, 18). Overall rates of failure and attributable mortality were 32% and 16%, respectively. No significant differences were observed between the variables and costs. The CART break points for failure were ≥18.75 g and ≥14 days of vancomycin treatment in the previous 3 years; for attributable mortality, the CART break points were ≥45 g and ≥31 days. In the final multivariate model for failure, ≥18.75 g and ≥14 days of vancomycin treatment in the previous 3 years were predictors of failure (both, P = 0.002). Acute Physiology and Chronic Health Evaluation (APACHE) II score (P = 0.04), ≥45 g (P = 0.002), and ≥31 days of treatment (P = 0.002) in the previous 3 years were predictors of attributable mortality after adjustment for all covariates.

CONCLUSIONS

Using the present model, cumulative vancomycin amount and duration were associated with attributable mortality and clinical failure but not with costs.

Self-reported incidence of skin and soft tissue infections among deployed US military

The incidence of skin and soft tissue infections has steadily increased over the past decade, and military populations, particularly recruits, have been affected. However, the epidemiology of skin and soft tissue infections in deployed personnel has not previously been described. We conducted a cross-sectional study of United States military personnel in mid-deployment using self-reported questionnaire data containing 11 demographic questions and 20 questions related to skin and soft tissue infections. The primary outcome was self-reported incident SSTI. Descriptive analyses were conducted and incidence estimates calculated. Multivariable regression models were developed to evaluate the association between SSTI and important covariates. Self-reported treatment modalities and effect on work performance were also assessed. The study was approved by the Institutional Review Board. 2125 questionnaires were completed over 12 months using convenience sampling. 110 personnel (5%) reported one or more skin and soft tissue infection during their most recent employment, for an incidence of 52 cases per 100,000 person-days. The majority reported a single infection. A higher proportion of individuals reporting skin and soft tissue infection were female, reported antibiotic use in the 6 months prior to completing the survey, had a family member in the healthcare occupation, and were senior enlisted or officers. 40 (36%) were treated with antibiotics and 24 (22%) underwent incision and drainage. Less than 5% (3 patients) required admission. Eighty eight respondents (81%), reported no days of lost job performance. There is a higher than expected incidence of skin and soft tissue infections in deployed military personnel. Although fewer than 20% of patients report missing at least one day of work, this can have a significant impact on the military mission. Further study should be conducted into how to prevent skin and soft tissue infections in military populations.

Nasopharynx as a microbiologic reservoir in chronic suppurative otitis media: preliminary study

Objectives

The present study was designed to identify the correlations of bacterial strains of the middle ear and the nasopharynx in chronic suppurative otitis media (CSOM) patients who were scheduled for operations.

Methods

Sixty-three patients with CSOM were enrolled in the study. Culture specimens were collected from the middle ear and nasopharynx of patients who were admitted for operation. Samples collections were performed 3 times; from the middle ear and nasophaynx at the admission day, from the middle ear during the operation, and from the external auditory canal post-operatively. Bacteria were identified by gram staining and biochemical tests. The correspondence rate of organisms which simultaneously exist in the middle ear and the nasopharynx was measured.

Results

Sixty-eight organisms were isolated from the middle ear and 57 organisms from the nasopharynx among 63 patients. Of 68 bacteria identified in middle ear, 26.52% (18 bacteria) corresponded with those of nasopharynx. MRSA had the high correspondence rate, and of 18 methicillin-resistant Staphylococcus aureus (MRSA) isolated from middle ear, 33.3% (6 bacteria) corresponded with nasophaynx. Meanwhile, 3 organisms of MRSA were detected from the external auditory canal post-operatively, although they were only found in nasopharynx pre-operatively.

Conclusion

The current trend of middle ear swab alone for bacterial detection would be insufficient to identify the potent MRSA and impede early antibiotic intervention for the effective middle ear surgery. Therefore, it is necessary to perform nasopharynx cultures together with conventional middle ear culture to control potent risk for infection pre-operatively.

The environment as an unrecognized reservoir for community-associated methicillin resistant Staphylococcus aureus USA300: a case-control study

Background

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) infections are spreading, but the source of infections in non-epidemic settings remains poorly defined.

Methods

We carried out a community-based, case-control study investigating socio-demographic risk factors and infectious reservoirs associated with MRSA infections. Case patients presented with CA-MRSA infections to a New York hospital. Age-matched controls without infections were randomly selected from the hospital's Dental Clinic patient population. During a home visit, case and control subjects completed a questionnaire, nasal swabs were collected from index respondents and household members and standardized environmental surfaces were swabbed. Genotyping was performed on S. aureus isolates.

Results

We enrolled 95 case and 95 control subjects. Cases more frequently reported diabetes mellitus and a higher number of skin infections among household members. Among case households, 53 (56%) were environmentally contaminated with S. aureus, compared to 36 (38%) control households (p = .02). MRSA was detected on fomites in 30 (32%) case households and 5 (5%; p<.001) control households. More case patients, 20 (21%) were nasally colonized with MRSA than were control indexes, 2 (2%; p<.001). In a subgroup analysis, the clinical isolate (predominantly USA300), was more commonly detected on environmental surfaces in case households with recurrent MRSA infections (16/36, 44%) than those without (14/58, 24%, p = .04).

Conclusions

The higher frequency of environmental contamination of case households with S. aureus in general and MRSA in particular implicates this as a potential reservoir for recolonization and increased risk of infection. Environmental colonization may contribute to the community spread of epidemic strains such as USA300.

Staphylococcus aureus infections in US veterans, Maryland, USA, 1999-2008

Trends in Staphylococcus aureus infections are not well described. To calculate incidence in overall S. aureus infection and invasive and noninvasive infections according to methicillin susceptibility and location, we conducted a 10-year population-based retrospective cohort study (1999-2008) using patient-level data in the Veterans Affairs Maryland Health Care System. We found 3,674 S. aureus infections: 2,816 (77%) were noninvasive; 2,256 (61%) were methicillin-resistant S. aureus (MRSA); 2,517 (69%) were community onset, and 1,157 (31%) were hospital onset. Sixty-one percent of noninvasive infections were skin and soft tissue infections; 1,112 (65%) of these were MRSA. Ten-year averaged incidence per 100,000 veterans was 749 (± 132 SD, range 549-954) overall, 178 (± 41 SD, range 114-259) invasive, and 571 (± 152 SD, range 364-801) noninvasive S. aureus infections. Incidence of all S. aureus infections significantly increased (p<0.001), driven by noninvasive, MRSA, and community-onset infections (p<0.001); incidence of invasive S. aureus infection significantly decreased (p<0.001).

Endogenous methicillin-resistant Staphylococcus aureus endophthalmitis

PURPOSE

To report on the presentation, treatment, and outcomes of the largest series to date for patients with endogenous methicillin-resistant Staphylococcus aureus endophthalmitis at a single institution and compare the results from previously published reports of this disease entity.

METHODS

Retrospective, noncomparative, consecutive case series of patients who were treated for endogenous methicillin-resistant S. aureus endophthalmitis from a single tertiary referral institution were reviewed.

RESULTS

Eight eyes of seven consecutive patients, with a mean age of 58.1 years, were diagnosed with culture-proven endogenous methicillin-resistant S. aureus endophthalmitis. Five patients were male, and six patients had a unilateral presentation. Five of eight eyes were treated with initial vitreous tap and injection of antibiotics, and six eyes showed a retinal detachment during the course of treatment. Final visual acuity ranged from 20/40 to no light perception, and 5 of 8 eyes showed improvement in visual acuity from initial presentation. Only one eye became blind and painful and required enucleation.

CONCLUSION

This series represents the largest single-institution report on endogenous methicillin-resistant S. aureus endophthalmitis and associated outcomes. Although previously reported cases cited poor visual outcomes and high enucleation rates, the authors' current series shows that retinal detachments are common and enucleation rates are low.

Appropriateness is critical

Inappropriate empirical antibiotic therapy for severe infections in the intensive care unit is a modifiable prognostic factor that has a great effect on patient outcome and health care resources. Inappropriate treatment is usually associated with microorganisms resistant to the common antibiotics, which must be empirically targeted when risk factors are present. Previous antibiotic exposure, prolonged length of hospital stay, admission category, local susceptibilities, colonization pressure, and the presence of invasive devices increase the likelihood of infection by resistant pathogens. Consideration of issues beyond in vitro susceptibility, such as antibiotic physicochemistry, tissue penetration, and pharmacokinetic/pharmacodynamic-driven dosing, is mandatory for the optimization of antibiotic use.

Incidence of community-acquired meticillin resistant Staphylococcus aureus hand infections in Tayside, Scotland: a guide to appropriate antimicrobial prescribing

The prevalence of community-acquired meticillin resistant Staphylococcus aureus (ca-MRSA) as an important cause of hand infections has gained much recognition. The clinical impact of this is less well documented in the United Kingdom. This study looks at the incidence of hand infections in an urban Scottish teaching hospital over a 12 month period.

Methicillin-resistant Staphylococcus aureus: a community health threat

Methicillin-resistant Staphylococcus aureus (MRSA), one of the most common causes of infections, has been traditionally recognized as a nosocomial pathogen. However, in recent years, its epidemiology has radically changed, being now observed even more frequently in the community, and accounting for > 50% of staphylococcal infections in the US outpatient setting. Community-acquired (CA)-MRSA strains typically cause infections among otherwise healthy individuals, with risk factors differing from those of nosocomial MRSA. The clinical manifestations may range from a furuncle to life-threatening infections, such as necrotizing fasciitis and pneumonia. The antibiotic treatment of these infections may also differ because CA-MRSA strains often retain susceptibility to antimicrobials other than glycopeptides and newer agents. Moreover, the production of toxins, such as the Panton-Valentine leukocidin (PVL), should influence the antibiotic choice because in these cases the use of a combination therapy with antimicrobial agents able to decrease toxin production is suggested. There are still many unanswered key questions regarding the epidemiology, prevention, and treatment of CA-MRSA infections. This article reviews current knowledge of CA-MRSA.

Emerging drugs for complicated skin and skin-structure infections

IMPORTANCE OF THE FILED: With the worldwide surge of MRSA, skin and skin-structure infection (SSTI) treatment has become a challenge for physicians. Cultures and antibiotic susceptibility tests for SSTIs are the rule due to the implication in morbidity and mortality rates associated with MRSA infections. The need for new antibiotics is evident and the effort to decrease antibiotic resistance is a world priority.

AREAS COVERED IN THIS REVIEW

This manuscript accesses the actual treatments and the developing of antibiotics for MRSA SSTIs.

WHAT THE READER WILL GAIN

This is a review of the data on the available and emerging treatments for MRSA SSTIs.

TAKE HOME MESSAGE

There is an unmet medical need for new antibiotics in the new millennium. As physicians, we must assure all appropriate procedures are completed in order to reduce the bacterial resistance, especially for MRSA.

Health care-associated and community-associated methicillin-resistant Staphylococcus aureus infections: A comparison of definitions

BACKGROUND

Different approaches are used to classify methicillin-resistant Staphylococcus aureus (MRSA) infections as either community-acquired (CA-MRSA) or health care-associated MRSA (HA-MRSA).

METHODS

We collected information on patients seen at the Atlanta Veterans Affairs Medical Center with MRSA infections from June 2007 through May 2008. We classified MRSA infections as either HA or CA using an epidemiologic definition and an antibiotic susceptibility phenotype rule. We used multivariate logistic regression to describe factors significantly associated with HA-MRSA infections compared with CA-MRSA infections.

RESULTS

Using the epidemiologic definition to classify infections, we found white race (odds ratio [OR], 3.2; 95% confidence interval [CI]: 2.0-5.2), oral antibiotics in the 3 months prior (OR, 4.0; 95% CI: 1.5-10.4), and endoscopy in the past year (OR, 3.8; 95% CI: 1.8-8.0) were significantly associated with health care-associated infections. When classifying by the resistance phenotype rule, we found hospitalization in the past year (OR: 1.8; 95% CI: 1.1-3.1) and an indwelling device in the past year (OR: 6.3; 95% CI: 2.5-15.8) were significantly associated with health care-associated infections.

CONCLUSION

We found few differences between CA- and HA-MRSA infections, regardless of how health care-association was defined. We believe that the migration of CA-MRSA into health care settings and the recent increasing antibiotic resistance of CA-MRSA strains contribute to the lack of factors associated with HA (vs CA) MRSA.

Community-associated methicillin-resistant Staphylococcus aureus: epidemiology and clinical consequences of an emerging epidemic

Staphylococcus aureus is an important cause of skin and soft-tissue infections (SSTIs), endovascular infections, pneumonia, septic arthritis, endocarditis, osteomyelitis, foreign-body infections, and sepsis. Methicillin-resistant S. aureus (MRSA) isolates were once confined largely to hospitals, other health care environments, and patients frequenting these facilities. Since the mid-1990s, however, there has been an explosion in the number of MRSA infections reported in populations lacking risk factors for exposure to the health care system. This increase in the incidence of MRSA infection has been associated with the recognition of new MRSA clones known as community-associated MRSA (CA-MRSA). CA-MRSA strains differ from the older, health care-associated MRSA strains; they infect a different group of patients, they cause different clinical syndromes, they differ in antimicrobial susceptibility patterns, they spread rapidly among healthy people in the community, and they frequently cause infections in health care environments as well. This review details what is known about the epidemiology of CA-MRSA strains and the clinical spectrum of infectious syndromes associated with them that ranges from a commensal state to severe, overwhelming infection. It also addresses the therapy of these infections and strategies for their prevention.

Stratification of the risk factors of community-acquired methicillin-resistant Staphylococcus aureus hand infection

PURPOSE

Several recent studies showed an increase in methicillin-resistant Staphylococcus aureus (MRSA) hand infections. The purpose of this study was to determine the prevalence of community-acquired MRSA hand infections in an urban setting and to determine independent risk factors for such infections.

METHODS

A retrospective chart review of patients with hand infections was performed from 2002 to 2009. Those with community-acquired hand infections who had surgical irrigation and debridement and intraoperative culture were entered into the study. Patient demographics-including age and gender; mechanism of injury; infection risk factors (diabetes, chronic hepatitis, intravenous intravenousdrug use, and immune-compromised conditions); place of residence/housing status; history of hospitalization, prior antibiotics use and surgery; and culture results, erythrocyte sedimentation rate, C-reactive protein, and white blood cell count-were extracted from the medical records. Regression analyses were performed to identify significant risk factors for MRSA infection.

RESULTS

A total of 102 patients met our inclusion criteria. The MRSA organism was identified in 32 patients. In the analysis of all the potential risk factors, only intravenous drug use showed significant correlation with MRSA infection.

CONCLUSIONS

In our patients, only intravenous drug use correlated with community-acquired MRSA hand infections. Patient education about intravenous drug use and empiric treatment with MRSA-appropriate antibiotics for intravenous drug users presenting with hand infections are recommended.

TYPE OF STUDY/LEVEL OF EVIDENCE

Prognostic IV.

Sustained antimicrobial activity of tigecycline against methicillin-resistant Staphylococcus aureus (MRSA) from United States Medical Centers from 2004 through 2008

Tigecycline, a glycylcycline, has been approved by the United States Food and Drug Administration (USA-FDA) for the treatment of complicated skin and skin structure infections, intra-abdominal infections and community-acquired bacterial pneumonia. based on broth microdilution minimum inhibitory concentration (MIC) testing, tigecycline demonstrated sustained high activity (MIC(50/90), 0.12/0.25 mg/L) against a contemporary collection (10,242) of methicillin (oxacillin)-resistant Staphylococcus aureus (MRSA) collected from 32 USA hospitals over a 5-year period (2004-2008). Tigecycline MIC distribution did not vary significantly during the study period and only three isolates (0.03%) were non-susceptible at USA-FDA breakpoints. Vancomycin (MIC(90), 1 mg/L), trimethoprim/sulfamethoxazole (MIC( 90), <0.5 mg/L) and linezolid (MIC(90), 2 mg/L) were also very active. The results of this study indicate that tigecycline potency and spectrum against MRSA have not changed since its initial regulatory approval by the USA-FDA.

A veterinary perspective on methicillin-resistant staphylococci

OBJECTIVE

To familiarize the reader with the epidemiology, diagnosis, and infectious and zoonotic potential of methicillin-resistant staphylococci.

DATA SOURCES

Original research publications, scientific reviews and abstracts, case reports, and conference proceedings.

HUMAN DATA SYNTHESIS

Staphylococcus aureus is a common human commensal organism; acquisition of genes encoding an altered penicillin-binding protein confers resistance to beta-lactam antimicrobial drugs. Methicillin-resistant S. aureus (MRSA) are often resistant to non-beta-lactam antimicrobial drugs as well. Originally described as an important cause of nosocomial infection, MRSA colonization and infection are now often identified in humans outside healthcare settings. Like other S. aureus, MRSA may be present without clinical illness. However, when they do cause infection the consequences can be extremely serious.

VETERINARY DATA SYNTHESIS

The major domestic animal species, including pets and livestock, may become contaminated, colonized, or infected with methicillin-resistant staphylococci, including MRSA. Dogs and cats are more likely to be colonized/infected with Staphylococcus pseudintermedius than S. aureus, but this pathogen can acquire genes encoding methicillin resistance (ie, MRSP). Diagnosis of MRSA or MRSP has implications not only for treatment of infected animals, but for potential zoonotic transmission.

CONCLUSIONS

MRSA infection is an important cause of morbidity and mortality in humans. Animals may be contaminated, colonized, or infected with MRSA, with implications for the animal's health and as a potential reservoir for human infection. Staphylococci other than S. aureus may also acquire genes for methicillin resistance, and these species can also result in animal and occasionally human morbidity or mortality.

Community-associated meticillin-resistant Staphylococcus aureus

Meticillin-resistant Staphylococcus aureus (MRSA) is endemic in hospitals worldwide, and causes substantial morbidity and mortality. Health-care-associated MRSA infections arise in individuals with predisposing risk factors, such as surgery or presence of an indwelling medical device. By contrast, many community-associated MRSA (CA-MRSA) infections arise in otherwise healthy individuals who do not have such risk factors. Additionally, CA-MRSA infections are epidemic in some countries. These features suggest that CA-MRSA strains are more virulent and transmissible than are traditional hospital-associated MRSA strains. The restricted treatment options for CA-MRSA infections compound the effect of enhanced virulence and transmission. Although progress has been made towards understanding emergence of CA-MRSA, virulence, and treatment of infections, our knowledge remains incomplete. Here we review the most up-to-date knowledge and provide a perspective for the future prophylaxis or new treatments for CA-MRSA infections.

Skin and soft tissue infections in hospitalised patients with diabetes: culture isolates and risk factors associated with mortality, length of stay and cost

AIMS/HYPOTHESIS

Skin and soft tissue infections (SSTIs) cause substantial morbidity in persons with diabetes. There are few data on pathogens or risk factors associated with important outcomes in diabetic patients hospitalised with SSTIs.

METHODS

Using a clinical research database from CareFusion, we identified 3,030 hospitalised diabetic patients with positive culture isolates and a diagnosis of SSTI in 97 US hospitals between 2003 and 2007. We classified the culture isolates and analysed their association with the anatomic location of infection, mortality, length of stay and hospital costs.

RESULTS

The only culture isolate with a significantly increased prevalence was methicillin-resistant Staphylococcus aureus (MRSA); prevalence for infection of the foot was increased from 11.6 to 21.9% (p < 0.0001) and for non-foot locations from 14.0% to 24.6% (p = 0.006). Patients with non-foot (vs foot) infections were more severely ill at presentation and had higher mortality rates (2.2% vs 1.0%, p < 0.05). Significant independent risk factors associated with higher mortality rates included having a polymicrobial culture with Pseudomonas aeruginosa (OR 3.1), a monomicrobial culture with other gram-negatives (OR 8.9), greater illness severity (OR 1.9) and being transferred from another hospital (OR 5.1). These factors and need for major surgery were also independently associated with longer length of stay and higher costs.

CONCLUSIONS/INTERPRETATION

Among diabetic patients hospitalised with SSTI from 2003 to 2007, only MRSA increased in prevalence. Patients with non-foot (vs foot) infections were more severely ill. Independent risk factors for increased mortality rates, length of stay and costs included more severe illness, transfer from another hospital and wound cultures with Pseudomonas or other gram-negatives.

Skin and soft tissue infections caused by community-associated methicillin-resistant staphylococcus aureus among children in China

AIM

To investigate the characteristic of community-associated methicillin-resistant staphylococcus aureus (CA-MRSA) skin and soft tissue infections (SSTIs) among children in China.

METHODS

Forty-seven children with CA-MRSA SSTIs were enrolled in this study. Clinical information was collected and analysed. The strains from the children were analysed by multilocus sequence typing (MLST), staphylococcus cassette chromosome mec (SCCmec) typing and spa typing. The Panton-Valentine leukocidin (PVL) gene was also detected.

RESULTS

The majority of the 47 cases were impetigo (20; 42.6%) and abscesses (14; 29.8%). The rest was cellulites, infected wounds, omphalitis, paronychia and conjunctivitis combined folliculitis. Thirty-two of the isolates (68.1%) were PVL-positive, and the abscesses infected with PVL-positive strains usually required incision and drainage (87.5% vs. 16.7%, p = 0.026). Most of the isolates belonged to ST type 59, which accounted for 46.8%, followed by ST1 (7/47, 14.9%) and ST910 (5/47, 10.6%). The clone of ST59-MRSA-IV with t437 was the most prevalent one. The multiresistant rate of these strains was 93.6%.

CONCLUSION

The most common disease of CA-MRSA SSTIs was impetigo, and PVL-positive abscess was associated with incision and drainage. ST59-MRSA-IV with t437 was the most prevalent clone, and the multiresistant rate was high in Chinese children.

Epidemiological typing of methicillin-resistant Staphylococcus aureus (MRSA): spa typing versus pulsed-field gel electrophoresis

Molecular methods based on sequencing, such as spa typing, have facilitated epidemiological typing of bacterial isolates compared to the gold standard pulsed-field gel electrophoresis (PFGE), a technically more demanding method. We studied methicillin-resistant Staphylococcus aureus (MRSA) in 4 Swedish counties from 2003 through 2005, and compared spa typing and PFGE results to epidemiological data. Of 280 MRSA isolates, 91 were from sporadic cases and 189 were associated with 35 outbreaks. A total of 50 spa types and 74 PFGE patterns were detected. 60 (21%) of the MRSA isolates carried the Panton-Valentine leukocidin (PVL) genes. 12 of the PVL-positive MRSA were healthcare associated. 25 of the spa types and 31 of the PFGE patterns were associated with outbreaks. In 1 of the outbreaks we found isolates with different but closely related spa types, and in 6 of the outbreaks we observed isolates with different but related PFGE patterns. In this low-endemic setting, with outbreaks limited in time and place, we found spa typing to be a useful tool for epidemiological typing of MRSA, due to its rapidity, accessibility, ease of use, and standardized nomenclature.

Risk factors associated with methicillin resistance among Staphylococcus aureus infections in veterans

BACKGROUND

Methicillin-resistant Staphylococcus aureus (MRSA) is an emerging concern in infectious disease practice. Although MRSA infections occur in a wide variety of anatomic sites, the majority of studies considering the risk factors for methicillin resistance among S. aureus infections have focused on MRSA bacteremia.

OBJECTIVE

To describe risk factors associated with methicillin resistance among S. aureus infections at different anatomic sites.

METHODS

We collected information on the demographic and clinical characteristics of patients examined at the Atlanta Veterans Affairs Medical Center with S. aureus infections during the period from June 2007 through May 2008. We used multivariate logistic regression to describe factors significantly associated with methicillin resistance.

RESULTS

There were 568 cases of S. aureus infection among 528 patients. We identified 352 cases (62%) of MRSA infection and 216 cases (38%) of methicillin-sensitive S. aureus infection. The adjusted odds of methicillin resistance were higher among infections that occurred among patients who had a prior history of MRSA infection (odds ratio [OR], 3.9 [95% confidence interval {CI}, 2.3-6.4]) or resided in a long-term care facility during the past 12 months (OR, 2.0 [95% CI, 1.0-4.0]) but were lower for infections that occurred among patients who had undergone a biopsy procedure during the past 12 months (OR, 0.7 [95% CI, 0.6-0.9]). Most cases of infection were community-onset infections (523 [92%] of 568 cases), and about one-half (278 [49%]) were not healthcare associated.

CONCLUSIONS

Compared with previous studies of methicillin resistance among patients with S. aureus bacteremia, we found similar factors to be associated with methicillin resistance among S. aureus isolates recovered from more diverse anatomic sites of infection. Of note, nearly one-half of our cases of MRSA infection were not healthcare associated.

Community-associated methicillin-resistant Staphylococcus aureus in outpatients, United States, 1999-2006

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) has become a major problem in US hospitals already dealing with high levels of hospital-associated MRSA (HA-MRSA). Using antimicrobial drug susceptibility data for 1999-2006 from The Surveillance Network, we characterized the relationship between outpatient and inpatient levels of CA-MRSA nationally. In outpatients, the frequency of CA-MRSA isolates has increased >7 x during 1999-2006, which suggests that outpatients have become a major reservoir for CA-MRSA. However, contrary to results in other reports, although CA-MRSA increases are associated with decreases in the frequency of HA-MRSA in hospitals, the decreases are only modest. This finding suggests that instead of replacing HA-MRSA in the hospital, CA-MRSA is adding to the overall presence of MRSA already found within the hospital population.

Community-associated methicillin-resistant Staphylococcus aureus infections at an Army training installation

To assess the burden of community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) in a high-risk population, the monthly incidence of laboratory-confirmed MRSA in service members/trainees stationed at Fort Benning, Georgia, USA without hospitalization or surgery documented 30 days prior to infection was calculated for calendar years 2002–2007. Clinical management and antibiotic susceptibility patterns were also evaluated. By 2007, ~67% of S. aureus strains were MRSA, and ~82% of these were community-associated, primarily in trainees. In total, 3531 CA-MRSA infections were identified. Rates appeared to be seasonal, peaking at 42 cases/1000 soldiers in 2005, with rates remaining above 35/1000 soldiers thereafter. Increased prescription of effective antibiotics was documented. Susceptibility to clindamycin, ciprofloxacin, and levofloxacin decreased from 2002 to 2007 by 6%, 17%, and 14%, respectively. The sustained high prevalence of CA-MRSA observed highlights the need for more vigilant population-based counter-measures at military training installations.

Impact of rapid methicillin-resistant Staphylococcus aureus polymerase chain reaction testing on mortality and cost effectiveness in hospitalized patients with bacteraemia: a decision model

BACKGROUND

Patients hospitalized with Staphylococcus aureus bacteraemia have an unacceptably high mortality rate. Literature available to date has shown that timely selection of the most appropriate antibacterial may reduce mortality. One tool that may help with this selection is a polymerase chain reaction (PCR) assay that distinguishes methicillin (meticillin)-resistant S. aureus (MRSA) from methicillin-susceptible S. aureus (MSSA) in less than 1 hour. To date, no information is available evaluating the impact of this PCR technique on clinical or economic outcomes.

OBJECTIVE

To evaluate the effect of a rapid PCR assay on mortality and economics compared with traditional empiric therapy, using a literature-derived model.

METHODS

A literature search for peer-reviewed European (EU) and US publications regarding treatment regimens, outcomes and costs was conducted. Information detailing the rates of infection, as well as the specificity and sensitivity of a rapid PCR assay (Xpert MRSA/SA Blood Culture PCR) were obtained from the peer-reviewed literature. Sensitivity analysis varied the prevalence rate of MRSA from 5% to 80%, while threshold analysis was applied to the cost of the PCR test. Hospital and testing resource consumption were valued with direct medical costs, adjusted to year 2009 values. Adjusted life-years were determined using US and WHO life tables. The cost-effectiveness ratio was defined as the cost per life-year saved. Incremental cost-effectiveness ratios (ICERs) were calculated to determine the additional cost necessary to produce additional effectiveness. All analyses were performed using TreeAge Software (2008).

RESULTS

The mean mortality rates were 23% for patients receiving empiric vancomycin subsequently switched to semi-synthetic penicillin (SSP) for MSSA, 36% for patients receiving empiric vancomycin treatment for MRSA, 59% for patients receiving empiric SSP subsequently switched to vancomycin for MRSA and 12% for patients receiving empiric SSP for MSSA. Furthermore, with an MRSA prevalence of 30%, the numbers of patients needed to test in order to save one life were 14 and 16 compared with empiric vancomycin and SSP, respectively. The absolute mortality difference for MRSA prevalence rates of 80% and 5% favoured the PCR testing group at 2% and 10%, respectively, compared with empiric vancomycin and 18% and 1%, respectively, compared with empiric SSP. In the EU, the cost-effectiveness ratios for empiric vancomycin- and SSP-treated patients were Euro 695 and Euro 687 per life-year saved, respectively, compared with Euro 636 per life-year saved for rapid PCR testing. In the US, the cost-effectiveness ratio was $US 898 per life-year saved for empiric vancomycin and $US 820 per life-year saved for rapid PCR testing. ICERs demonstrated dominance of the PCR test in all instances. Threshold analysis revealed that PCR testing would be less costly overall, even at greatly inflated assay prices.

CONCLUSIONS

Rapid PCR testing for MRSA appears to have the potential to reduce mortality rates while being less costly than empiric therapy in the EU and US, across a wide range of MRSA prevalence rates and PCR test costs.

Epidemiology and susceptibilities of methicillin-resistant Staphylococcus aureus in Northeastern Ohio

A retrospective survey was performed on all staphylococcal infections diagnosed by the Ashtabula County Medical Center (Ashtabula, OH) during 2006 and 2007. Of the 1,612 Staphylococcus aureus isolates evaluated for their antibiotic resistances, 947 were methicillin-resistant S aureus (MRSA). In 2007, MRSA infections reached 589 cases per 100,000 inhabitants, a 77% increase compared with 2006. The increase in MRSA infections was noticeable among youth (6-25 years old), middle-aged people (45-50 years old), and elderly people (86-90 years old). MRSA infections increased among inpatients by 58%, among outpatients by 43%, and among nursing home residents by 183%. More than 66% of MRSA infections were found among healthy people in the community with no apparent risk factors. More than 88.7% of the infections belong to only 9 profiles of antibiotic resistance indiscriminately distributed among inpatients, outpatients, and nursing home residents. This report sheds further light on the rapid spread of MRSA across Northeastern Ohio, stressing the need for better education in preventive measures and infection control at the level of community and health care settings.

Presence of genes encoding the panton-valentine leukocidin exotoxin is not the primary determinant of outcome in patients with complicated skin and skin structure infections due to methicillin-resistant Staphylococcus aureus: results of a multinational trial

The role of Panton-Valentine leukocidin (PVL) in determining the severity and outcome of complicated skin and skin structure infections (cSSSI) caused by methicillin (meticillin)-resistant Staphylococcus aureus (MRSA) is controversial. We evaluated potential associations between clinical outcome and PVL status by using MRSA isolates from patients enrolled in two large, multinational phase three clinical trials assessing telavancin for the treatment of cSSSI (the ATLAS program). MRSA isolates from microbiologically evaluable patients were genotyped by pulsed-field gel electrophoresis (PFGE) and PCR for pvl and 31 other putative virulence determinants. A single baseline pathogen of MRSA was isolated from 522 microbiologically evaluable patients (25.1%) among 2,079 randomized patients. Of these MRSA isolates, 83.2% (432/519) exhibited the USA300 PFGE genotype and 89.1% (465/522) were pvl positive. Patients with pvl-positive MRSA were more likely than those with pvl-negative MRSA to be young, to be North American, and to present with major abscesses (P < 0.001 for each). Patients were significantly more likely to be cured if they were infected with pvl-positive MRSA than if they were infected with pvl-negative MRSA (91.6% versus 80.7%; P = 0.015). This observation remained statistically significant after adjustment for presence of abscess, fever, or leukocytosis; infection size; diabetes; patient age; and study medication received. The fnbA, cna, sdrC, map-eap, sed, seg, sei, sej, SCCmec type IV, and agr group II genes were also associated with clinical response (P < 0.05). This contemporary, international study demonstrates that pvl was not the primary determinant of outcome in patients with MRSA cSSSI.

Severe soft tissue infections

Severe skin and soft tissue infections (SSTIs) frequently require management in the ICU, in part related to associated septic shock or toxic shock syndrome or associated organ failure. Four fundamental management principles are key to a successful outcome in caring for patients who have severe SSTIs, including (1) early diagnosis and differentiation of necrotizing versus nonnecrotizing SSTI, (2) early initiation of appropriate empiric broad-spectrum antimicrobial therapy with consideration of risk factors for specific pathogens and mandatory coverage for methicillin-resistant Staphylococcus aureus (MRSA), (3) source control (ie, early aggressive surgical intervention for drainage of abscesses and debridement of necrotizing soft tissue infections), and (4) pathogen identification and appropriate de-escalation of antimicrobial therapy. MRSA has emerged as the most common identifiable cause of severe SSTIs; therefore, initiation of empiric anti-MRSA antimicrobials is warranted in all cases of severe SSTIs. In addition, appropriate critical care management-including fluid resuscitation, organ support and nutritional support-is a necessary component in treating severe SSTIs.

Reemergence of antibiotic-resistant Staphylococcus aureus in the genomics era

Staphylococcus aureus is the leading cause of bacterial infections in developed countries and produces a wide spectrum of diseases, ranging from minor skin infections to fatal necrotizing pneumonia. Although S. aureus infections were historically treatable with common antibiotics, emergence of drug-resistant organisms is now a major concern. Methicillin-resistant S. aureus (MRSA) was endemic in hospitals by the late 1960s, but it appeared rapidly and unexpectedly in communities in the 1990s and is now prevalent worldwide. This Review focuses on progress made toward understanding the success of community-associated MRSA as a human pathogen, with an emphasis on genome-wide approaches and virulence determinants.

Waves of resistance: Staphylococcus aureus in the antibiotic era

Staphylococcus aureus is notorious for its ability to become resistant to antibiotics. Infections that are caused by antibiotic-resistant strains often occur in epidemic waves that are initiated by one or a few successful clones. Methicillin-resistant S. aureus (MRSA) features prominently in these epidemics. Historically associated with hospitals and other health care settings, MRSA has now emerged as a widespread cause of community infections. Community or community-associated MRSA (CA-MRSA) can spread rapidly among healthy individuals. Outbreaks of CA-MRSA infections have been reported worldwide, and CA-MRSA strains are now epidemic in the United States. Here, we review the molecular epidemiology of the epidemic waves of penicillin- and methicillin-resistant strains of S. aureus that have occurred since 1940, with a focus on the clinical and molecular epidemiology of CA-MRSA.

Contamination of equipment in emergency settings: an exploratory study with a targeted automated intervention

BACKGROUND

Despite standard manual decontamination, hospital equipment remains contaminated with microorganisms, contributing to nosocomial transmission and hospital acquired infections. This has the potential to negate the effects of healthcare workers' hand-washing protocols. In order to decrease the likelihood of equipment contamination, there has been a rise in the use of disposable pieces of equipment, especially non-critical disposables. However, these carry a significant cost, both a direct financial cost (running into billions of dollars), as well as a cost to the environment. This is important because we hope to contain the cost of healthcare, one way to do that, is to look to the hospitals themselves, for innovative solutions that maintain the standard of care.

OBJECTIVE

To develop and evaluate the effectiveness of an simple decontamination device for use with portable hospital equipment, by comparing rates of residual contamination after use of the novel device versus those seen with standard manual decontamination methods.

METHODS

The Self-cleaning Unit for the Decontamination of Small instruments (SUDS) is a user-friendly, automated instrument developed via multi-disciplinary collaboration for decontamination in the clinical area. Pre- and post- utilization of portable medical equipment in an emergency department (ED) setting were cultured. To evaluate durability of the decrease in antimicrobial contamination, objects were re-cultured 48 hours after SUDS cleaning and following re-introduction into the clinical setting.

RESULTS

After manual decontamination, 25% (23/91) of the tested objects in the ED were found to be culture positive with clinically significant microorganisms(CSO). Fifteen percent (ED) of non-critical equipment tested had multiple organisms. Following the use of SUDS, the colonization rate decreased to 0%. Following SUDS treatment and re-introduction into the clinical settings, after 48 hours the contamination rates as reflected by the cultures remained 0%.

CONCLUSION

Standard non-critical equipment is contaminated with clinically significant microorganisms. The SUDS device allows for effective and durable decontamination of hospital equipment of varying sizes in the clinical area without disrupting patient care.

Variable-number tandem repeat analysis and multilocus sequence typing data confirm the epidemiological changes observed with Staphylococcus aureus strains isolated from bloodstream infections

Since 2000, our geographical region in France systematically surveys bloodstream infections (BSI) due to Staphylococcus aureus. This survey involves 39 health care institutions (HCIs) encompassing 6,888 short-stay beds and was performed during two 3-month periods during 2007 and 2008. The study periods of this survey identified 292 S. aureus isolates causing BSI. Extensive molecular characterization, including genotyping as well as toxin, agr, and staphylococcal cassette chromosome content determinations, allowed us to describe epidemiological evolution in comparison to that discussed in our previous study. Our main epidemiological observation shows that the incidence of BSI remained constant but that methicillin (meticillin)-resistant S. aureus strains with a wider variety of genetic backgrounds now harbor pyl, as has already been reported in different European countries. We noticed stable numbers of BSI episodes involving community-acquired methicillin-sensitive S. aureus (MSSA), whereas a drastic increase in the number of strains harboring the tst gene was recorded. The increase in the number of tst gene-harboring strains is related to known hospital-acquired MSSA isolates and appears related to epidemic episodes in specific HCIs. Monitoring the increase in prevalence of specific strains helps us understand where the standard precautions are not satisfactorily applied or do not efficiently prevent the spread of epidemic MSSA strains in these HCIs. The recent increases in incidence of these strains call for particular vigilance to avoid the spread of potentially virulent MSSA strains harboring the tst gene and for continuance of this strategy of BSI surveillance.

Trimethoprim-sulfamethoxazole for methicillin-resistant Staphylococcus aureus: a forgotten alternative?

Methicillin-resistant Staphylococcus aureus (MRSA) is a growing infectious concern, mainly in the context of its rapid adaptation to novel antibiotic options for its treatment and the growing morbidity, mortality, and healthcare costs associated with its emergence. the authors sought to investigate whether an older antibiotic, such as trimethoprim-sulfamethoxazole (SXT), may have a role in treating MRSA-related infections, according to the available literature on the subject. The authors reviewed literature data on: resistance of MRSA to SXT worldwide in recent years, efficacy of SXT for MRSA decolonization or prophylaxis from MRSA infections, and clinical therapeutic efficacy of SXT in treating mild or severe community-acquired or hospital-acquired MRSA infections. Resistance varies worldwide, in general being low in the industrialized world and higher in developing countries. SXT is one of the numerous understudied options for MRSA decolonization and is growingly recognized as potentially effective in preventing MRSA infections in certain settings. Limited data on its therapeutic efficacy are encouraging, at least for mild, community-acquired infections. SXT may represent a cost-effective alternative weapon against MRSA. Its utility against this increasingly threatening pathogen need clarification through further clinical trials.

Recurrent community-associated methicillin-resistant Staphylococcus aureus infections among HIV-infected persons: incidence and risk factors

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) infections are well described among the general population, but little is known regarding the incidence of and predictors for recurrent CA-MRSA infections among HIV-infected persons. We retrospectively evaluated HIV-infected patients seen at the Naval Medical Center San Diego from January 1, 2000 to June 30, 2007 for wound culture-proven MRSA infections defined as community-associated based on Centers for Disease Control and Prevention (CDC) criteria. Data on skin/soft tissue infections (SSTIs) following an initial CA-MRSA infection were collected by review of medical records and culture results. Patients with or without recurrent infections were compared for predictors of recurrence using multivariate logistic regression models. Thirty-one (6.8%) of 458 patients with HIV had wound culture-proven CA-MRSA SSTIs for an incidence rate of 12.3 infections per 1000 person-years. Those who developed a MRSA infection had a mean age of 40 years, 97% were male, 58% were Caucasian, 23% were Hispanic, 16% were African American, and 3% were other; demographics were similar to the overall study population. Fourteen (41%) HIV patients with an initial MRSA infection had recurrent SSTIs; of these, seven (21%) had culture-confirmed recurrent CA-MRSA. The median time between infection recurrences was 4 months (range, 1-20 months). Suppressed HIV-1 RNA levels of less than 1000 copies per milliliter (odds ratio [OR] 0.14, p = 0.03) was associated with a lower rate of SSTI recurrence. In summary, HIV-infected persons have a high incidence of CA-MRSA skin/soft tissue infections and a high rate of recurrence. HIV control may be associated with a reduced risk of recurrent skin/soft tissue infections.

Evidence based approach to the treatment of community-associated methicillin-resistant Staphylococcus aureus

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) infections have increased dramatically over the last two decades. The types of infections can range from complicated skin and skin structure infections (cSSSI) to pneumonia and endocarditis. Oral antimicrobial therapy, such as trimethoprim-sulfamethoxazole, clindamycin, long-acting tetracyclines, or linezolid may provide enhanced benefit to those with uncomplicated cutaneous lesions when used in conjunction with incision and drainage in an outpatient setting. However, resistance, susceptibilities, patient-specific circumstances, and adverse effects can impact a healthcare professional's choice of antibiotics. In patients with complicated infections requiring hospitalization or parenteral treatment, vancomycin remains the drug of choice, even though increased resistance and decreased efficacy have crept into clinical practice. Linezolid, quinupristin/dalfopristin, daptomycin, and tigecycline are alternative intravenous agents for the treatment of CA-MRSA. Investigational agents such as dalbavancin, telavancin, oritivancin, iclaprim, ceftobiprole, ceftaroline, and others may expand our therapeutic armamentarium for the treatment of infections caused by CA-MRSA in the future.

Antibacterial properties and mode of action of a short acyl-lysyl oligomer

We investigated the potency, selectivity, and mode of action of the oligo-acyl-lysine (OAK) NC(12)-2 beta(12), which was recently suggested to represent the shortest OAK sequence that retains nonhemolytic antibacterial properties. A growth inhibition assay against a panel of 48 bacterial strains confirmed that NC(12)-2 beta(12) exerted potent activity against gram-positive bacteria while exhibiting negligible hemolysis up to at least 100 times the MIC. Interestingly, NC(12)-2 beta(12) demonstrated a bacteriostatic mode of action, unlike previously described OAKs that were bactericidal and essentially active against gram-negative bacteria only. The results of various experiments with binding to model phospholipid membranes correlated well with those of the cytotoxicity experiments and provided a plausible explanation for the observed activity profile. Thus, surface plasmon resonance experiments performed with model bilayers revealed high binding affinity to a membrane composition that mimicked the plasma membrane of staphylococci (global affinity constant [K(app)], 3.7 x 10(6) M(-1)) and significantly lower affinities to mimics of Escherichia coli or red blood cell cytoplasmic membranes. Additional insertion isotherms and epifluorescence microscopy experiments performed with model Langmuir monolayers mimicking the outer leaflet of plasma membranes demonstrated the preferential insertion of NC(12)-2 beta(12) into highly anionic membranes. Finally, we provide mechanistic studies in support of the view that the bacteriostatic effect resulted from a relatively slow process of plasma membrane permeabilization involving discrete leakage of small solutes, such as intracellular ATP. Collectively, the data point to short OAKs as a potential source for new antibacterial compounds that can selectively affect the growth of gram-positive bacteria while circumventing potential adverse effects linked to lytic compounds.

Comparative severity of pediatric osteomyelitis attributable to methicillin-resistant versus methicillin-sensitive Staphylococcus aureus

BACKGROUND

Staphylococcus aureus remains the most common etiology of hematogenous osteomyelitis in children. Recently emerged virulent strains of methicillin-resistant S. aureus (MRSA) strains now predominate. It remains uncertain whether these pathogens cause a measurably more severe illness than methicillin-sensitive strains.

METHODS

We reviewed records of 97 pediatric patients with hematogenous osteomyelitis of varying etiologies to determine whether there were significant differences among the groups in terms of predetermined measures of disease severity. These measures included degree and duration of elevated temperature, acute-phase reactants, length of hospitalization, the number of surgical procedures required, and long-term sequelae.

RESULTS

Differences in the clinical course of illness were observed. Patients with MRSA had a significant increase in degree and duration of elevated temperature (P = 0.0001 for both), acute-phase reactant measures (white blood cell count, P = 0.0001; C-reactive protein, P = 0.0058; erythrocyte sedimentation rate, P = 0.0046), initial length of hospital stay (P = 0.0001), and surgical interventions (all procedures, P = 0.0001; therapeutic procedures, only P = 0.0002) as compared with patients with osteomyelitis caused by other bacterial pathogens and culture-negative osteomyelitis. A similar general pattern was observed when the analysis was restricted to culture-positive staphylococcal cases only.

CONCLUSIONS

Our results support the hypothesis that MRSA produces more severe bone infection and is likely to require more aggressive surgical and medical management.

Microbial interactions during upper respiratory tract infections

Competitive interactions between bacteria differ by number and species present; thus, vaccination and treatment strategies may alter nasopharyngeal flora and disease susceptibility.

Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, and Staphylococcus aureus often colonize the nasopharynx. Children are susceptible to bacterial infections during or soon after upper respiratory tract infection (URI). We describe colonization with these 4 bacteria species alone or in combination during URI. Data were from a prospective cohort of healthy children 6 to 36 months of age followed up for 1 year. Analyses of 968 swabs from 212 children indicated that S. pneumoniae colonization is negatively associated with colonization by H. influenzae. Competitive interactions shifted when H. influenzae and M. catarrhalis colonized together. In this situation, the likelihood of colonization with all 3 species is higher. Negative associations were identified between S. pneumoniae and S. aureus and between H. influenzae and S. aureus. Polymicrobial interactions differed by number and species of bacteria present. Antimicrobial therapy and vaccination strategies targeting specific bacterial species may alter the flora in unforeseen ways.