Numerous studies have documented an increase in community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) infections in patients lacking traditional methicillin-resistant Staphylococcus aureus (MRSA) risk factors.1-5 A high rate of CA-MRSA infection in HIV-infected individuals has also been observed recently.6-11 As with non-HIV-infected patients, most of the CA-MRSA infections have involved skin and/or soft tissue.7-10
The primary reservoir of S. aureus is the anterior nares. Approximately 22%-38% of healthy individuals are colonized with methicillin-susceptible Staphylococcus aureus (MSSA).12-16 Colonization frequently precedes infection with MSSA and/or health care-associated MRSA.16,17 The rate of colonization with MRSA has been relatively low: 0.8%-3% in prior studies in the general population,12-15 although recent studies indicated that it may be increasing in certain locales among children (9% in Nashville, TN).18
The prevalence of nasal colonization with S. aureus in HIV infection seems to be in the same range as the general population,19-23 with some studies showing a somewhat higher rate.16,24-27 Some have suggested that HIV may be an independent risk for S. aureus nasal colonization.22,24,25,27 Prior studies have indicated that the prevalence of MRSA colonization in HIV-infected patients is between 0% and 17%.19-25,28 However, these studies were largely conducted before the explosion of CA-MRSA in the community.
As the role of nasal colonization with CA-MRSA is not well established, we decided to undertake a prospective study. Our objectives were to determine the prevalence of MRSA colonization in HIV-infected individuals and to identify demographic and clinical factors that correlate with MRSA nasal colonization in this population.
The population for this prospective study was drawn from the HIV clinic of Parkland Hospital in Dallas, Dallas, TX. The clinic provides comprehensive HIV primary care to approximately 4000 patients. Eligible patients were HIV seropositive, active clinic patients who presented to the HIV clinic in July or August 2005 for a routine clinic visit and who were willing to participate in the study including completion of a questionnaire and consent to cultures. Exclusions from the study included non-English-speaking patients (if no interpreter was available at that time).
Nasal samples were obtained from both anterior nares and both axilla using a culturette swab (BBL CultureSwab, BBL Microbiology Systems; Becton Dickinson, Sparks, MD). One swab was used for both anterior nares, and a separate swab was used to culture both axillae. Swabs were not premoistened. Swabs were plated on “BBL CHROMagar MRSA” media (Becton Dickinson). This media is a selective and differential medium for qualitative direct detection of nasal colonization with MRSA and has been previously validated for this purpose (BD BBL CHROMagar MRSA package insert; Becton Dickinson, Sparks, MD). Culture plates were incubated at 36°C for 48 hours. Growth of bacterial colonies consistent with MRSA on the CHROMagar plates was then confirmed by the Staphaurex test (Murex Biotech Limited), which tests for the presence of coagulase and/or protein A.
DNA of the MRSA strains was digested with Sma 1 and separated by pulse-field gel electrophoresis using the CHEF-DR II system (Bio-Rad, Hercules, CA) as previously described.29 Characterization of the staphylococcal cassette chromosome (SCC)mec type of each isolate was determined using multiplex polymerase chain reaction methods30; type IV subtypes were determined according to methods described by Okuma.31
A questionnaire was conducted to ascertain risks associated with MRSA colonization. Medical records were reviewed to document laboratory and clinical data. The study was approved by the Institutional Review Board of the University of Texas Southwestern Medical Center, and informed consent was obtained from all patients. Statistical analysis was performed with SPSS for windows. Adjusted odds ratios and 95% confidence intervals were computed in a forward stepwise logistic regression model incorporating variables that were significant (at <0.2 level) on χ2 test.
One hundred forty-six patients were enrolled: median age 42 years (range 21-71). There were 122 males: 45% were African American, 42% white, and 12% Hispanic. The primary HIV risk category was male-male sex in 91 (62%), injecting drug use in 29 (20%), and other or not specified in 26 (18%). The median CD4 count was 328 cells/mm3. The mean HIV viral load was 56,213 copies/mL (median < 400). Twenty-nine (20%) patients were receiving trimethoprim-sulfamethoxazole (T/S).
Fifteen of 146 (10.3%) patients had a positive nasal swab for MRSA. One of 146 (0.7%) had a positive axillary swab for MRSA; this patient also had nasal colonization. Patients colonized with MRSA had lower CD4 cell counts and were less likely to be receiving antibiotics currently or in the past 6 months (recent antibiotics). Among patients receiving recent antibiotics, 38 of 43 received an antibiotic with potential activity versus MRSA including T/S in 29. Colonized patients were more likely to have had prior MRSA or MSSA infection (Table 1). Current use of T/S was protective for colonization: 0 of 29 T/S recipients were colonized with MRSA versus 15 of 117 patients not receiving T/S, P = 0.04. There was a trend toward association with nasal colonization for hospitalization or emergency department visit in the past year (P = 0.064). Most recent HIV viral load, use of nasal corticosteroids, documented skin disease (past or present) (eg, folliculitis, psoriasis), current receipt of highly active antiretroviral therapy (HAART), HIV risk factor, or incarceration (past or present) were not associated with MRSA nasal colonization. In a multivariate logistic regression model, prior infection with MSSA or MRSA and lower CD4 count were each associated with MRSA colonization whereas recent antibiotics were negatively associated with MRSA nasal colonization (Table 1).
Twelve of 15 MRSA isolates tested were SCCmec type IV whereas 1 isolate was type I and 2 were type II (Table 2). Pulse-field gel electrophoresis profiles were conducted on 13 of 15 isolates, and the patterns were consistent with the USA100 strain in 1 isolate, USA300 in 7 isolates, USA400 in 1 isolate, USA500 in 3 isolates, and USA800 in 1 isolate. Of the 5 patients with USA100, USA500, or USA800, 4 had been hospitalized in the past 12 months whereas 8 of 9 patients with USA300 or USA400 were not hospitalized in the past 12 months.
We determined the prevalence of MRSA nasal colonization among ambulatory HIV-infected patients to be 10%. This rate of nasal colonization is higher than previous studies conducted before the rapid increase in CA-MRSA.16,19-23,25,26 HIV infection is an independent risk for MRSA nasal colonization.24,27,32 A prevalence of MRSA nasal colonization of 10% in this population with a high rate of MRSA skin and/or soft tissue infections (SSTIs)33 suggests a poor correlation between nasal colonization and SSTI in CA-MRSA.
As expected in this outpatient population, the majority of strains contained SCCmec type IV, which is most common in CA-MRSA strains and is associated with less antibiotic resistance versus other SCCmec types. The majority of MRSA colonization was with community-associated strains, for example, USA300 or USA400. However, several isolates were health care-associated strains, for example, USA100, USA500, and USA800 clones. As expected, the non-community-associated strains were those associated with health care exposure; all but one of these patients had recently been hospitalized.
Although the role of health care-associated MRSA nasal colonization in subsequent infections is accepted, the role of colonization with CA-MRSA in subsequent infections is not. The pathogenesis of infection with CA-MRSA may be different compared with infection with MSSA or health care-associated MRSA. It is not known if the CA-MRSA strains harbored in the anterior nares of individuals are the same strains causing infection or whether other sites are colonized (eg, vagina, rectum, skin).34 CA-MRSA strains may be more likely to result in infection compared with MSSA strains.35,36 Recent studies of CA-MRSA infection have suggested a role of direct contact with patients or materials colonized with CA-MRSA.9,37 Alternatively, patients colonized with MRSA may be more prone to subsequent MRSA infection (compared with those colonized with MSSA).35,38
Our study found a very low prevalence of MRSA axillary colonization (0.7%). It is unlikely that axillary colonization with MRSA is important in the pathogenesis of CA-MRSA infections in HIV-infected individuals. A recent study found that 12% of patients had S. aureus throat colonization in the absence of nose colonization.39 The role of colonization of the throat and other body sites should be investigated in future studies.
Lower CD4 count was associated with higher MRSA colonization risk in multivariate analysis. Lower CD4 count has also been previously demonstrated to be a risk for MRSA colonization.20,23 It is not clear if lower CD4 count independently correlates with an increased risk for nasal colonization or if it is associated with other factors. For example, patients with lower CD4 cell counts may be more likely to have had prior staphylococcal infections.
Prior infection with either MRSA or MSSA was associated with MRSA nasal colonization in the multivariate model. Although it seems straightforward why prior MRSA infection might increase risk of subsequent colonization, for example, it is known that some patients are persistent carriers and it is not clear why prior MSSA was associated with colonization. It is possible that persons predisposed to persistent carriage with S. aureus may have MRSA replace the niche formerly occupied by MSSA. Recurrence of MRSA SSTIs is common6 and may be partially explained by persistent colonization (either nasal or other site).38,40 A previous study that included both HIV-infected and noninfected patients demonstrated prior MRSA colonization or infection to be a predictor of nasal colonization.24 Whether eradication of MRSA colonization is beneficial in prevention of future infections is not clear.
Receipt of current or recent antibiotics was associated with a low rate of MRSA colonization. Eighty-eight percent of these patients received antibiotics with potential MRSA activity including T/S in the majority. Prior studies have shown a protective effect of antistaphylococcal antibiotics on staphylococcal nasal colonization.19,25 Previous studies have confirmed the protective effect of T/S on CA-MRSA infections.9,10 CA-MRSA isolates are almost universally susceptible to T/S. Thus, the protective effect of T/S on MRSA nasal colonization is not unexpected and represents an additional benefit to T/S prophylaxis in addition to the well-established benefit for prevention of Pneumocystis jirovecii pneumonia.
Limitations of our study are that we do not have longitudinal data on the subsequent clinical course of our cohort to ascertain the rate of MRSA infection. It would also be of interest to look at colonization over time to determine if persistent colonization results in a higher infection rate than transient colonization. In addition, we may have potentially excluded some Spanish-speaking patients. This could have affected the ethnic makeup of the cohort. However, the ethnicity of the population we studied was reflective of the overall HIV clinic population. Finally, it would have been of interest to know the overall S. aureus (MSSA and MRSA) nasal colonization rate in this cohort.
In conclusion, we determined the prevalence of nasal colonization with MRSA in an ambulatory HIV-infected population to be 10%. Prior staphylococcal infection (MSSA or MRSA), more advanced immunodeficiency, and not receiving current antibiotics (primarily T/S) correlated with current MRSA colonization. Future studies should address the role of nasal colonization and other body sites on subsequent infection and the utility of decolonization.
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