Although the prevalence of MRSA colonization among healthy persons in the U.S. population is low (0.8 to 1.5%) (18
), some reports suggest that it is increasing (18
). The prevalence of MRSA colonization in our HIV-infected study population was higher at all three study visits (range, 13 to 15%) than the reported rates in the community; the cumulative MRSA prevalence was 21%. The reported prevalence of MRSA colonization in HIV-infected adults varies widely and has been reported to be as low as 2 to 4% in Boston, MA, and Nebraska (34
) and as high as 17% in Atlanta and New York City (21
). This variation may be due to factors associated with HIV status or local MRSA colonization pressure, but is also likely to reflect the number and source of the swab specimens taken and the method used for culture, as well as the natural dynamics of human carriage of S. aureus
Numerous clinical laboratory studies have compared chromogenic or selective media for the recovery of MRSA, but most used isolates from culture collections or cultures of specimens taken from clinical infections or from populations with high MRSA colonization rates (19
). It is difficult to compare the results of these studies to MRSA recovery from surveillance samples, which are often from populations with low MRSA prevalence.
We found that broth enrichment was more sensitive than direct plating to MSA or CM for detection of MRSA and MSSA colonization. Overall, broth enrichment increased sensitivity 19% compared to the use of CM and 24% compared to the use of MSA for recovery of MRSA; the MSSA recovery rate increased 29% with broth enrichment compared to direct plating on MSA. Safdar et al. evaluated 32 different microbiological techniques, including broth enrichment, for detection of MRSA nasal carriage in hospitalized patients and also found an increase of 7 to 14% in sensitivity when broth enrichment was compared to direct plating (48
). Other studies comparing broth enrichment to direct plating for MRSA recovery have reported a wide variation in sensitivity, ranging from a decrease of 4% to an increase of 20% (10
). The dramatic differences in recovery from broth enrichment that we observed may reflect low numbers of MRSA present in groin swab samples which could be missed by direct plating, as the increase in sensitivity between direct plating and broth enrichment was most noticeable from groin swab samples. Among the 19 MRSA isolates identified with CM but not detected with broth enrichment in our study, 14 (74%) occurred when patients were cocolonized with MRSA and MSSA. We hypothesize that in these instances, small amounts of MRSA could be if missed if abundant MSSA is present in the original sample.
Although MSA requires up to 48 h of incubation and some experience for optimal use, it allows one to detect both MSSA and MRSA, and variations in the color and intensity produced by different S. aureus strains on the same plate can be distinguished. CM is approximately four times the cost of MSA, but the result is easy to interpret and can be read at 24 h. A limitation of our study is the lack of specificity data for the recovery of MRSA from the three methods that we employed. The primary goal of our study was to identify all S. aureus carriage. Because of this, we erred on the side of oversampling by subculturing all suspicious isolates grown on CM and MSA and using a latex agglutination test to rule out coagulase-negative staphylococci. We found that the most sensitive method for recovery of MRSA and MSSA was broth enrichment and then plating to MSA, but broth enrichment and plating to CM or use of MRSA-specific broth enrichment might be the most sensitive for recovery of MRSA only. Only one swab did not yield any Staphylococcus species when plated directly to MSA (data not shown), and we observed no decreased S. aureus yield when swabs were stored under refrigeration for up to 7 days. Thus, weekly batch culturing of swabs might be a practical approach for detecting S. aureus colonization in large-scale or multisite surveillance studies.
The addition of a groin swab specimen culture in our study dramatically enhanced recovery of both MRSA and MSSA. Other studies have reported increased sensitivity when multiple body sites were sampled. Several studies have demonstrated that culture of samples from additional body sites, such as the throat, axilla, perineum, or groin, in addition to the nares, increased the sensitivity of S. aureus
detection, with improvements ranging from 5 to 25% (6
). However, others have found that the addition of a specimen from a second body site had little impact on the overall sensitivity of MRSA recovery (8
). Although other colonization studies have demonstrated a dramatic increase in MRSA detection when throat swabs were cultured in addition to the nasal swabs, we specifically sought to address the impact of carriage site on MRSA strain types and thought that strains carried in the nose and throat were likely to be the same. Inclusion of a groin swab specimen culture in our population not only increased the overall recovery of MRSA but also specifically increased recovery of PVL-positive USA300 strains. Overall, 41.7% of MRSA strains recovered from the groin were USA300, compared to 29.0% from the nose. Similar findings have been reported; Lautenbach et al. found that a larger percentage of CA-MRSA was isolated from the groin and perineum than the nares, throat, and axilla (28
), and a recent study by Faden and colleagues found an association between rectal, but not nasal, MRSA carriage and S. aureus
skin and soft tissue infection in children (15
). At 11.5% prevalence, the USA500/Iberian type was the most frequently isolated strain in our population, representing 48.0% of the MRSA isolates recovered from the groin and 57.4% from the nares. This is higher than the reported prevalence of USA500 among HIV-infected patients in Dallas, TX, and New York City (range, 4.8 to 5.6%) (8
) but lower than the reported prevalence (19%) among general patients screened at admission to another Atlanta-area hospital (21
), and both USA300 and USA500 colonization has been associated with HIV infection (36
In summary, the rate of MRSA colonization among our cohort of HIV-infected outpatients was higher than that which has been reported for the healthy U.S. population and for other groups of HIV-infected individuals. Broth enrichment significantly increased detection of MRSA and MSSA compared to direct plating, and we saw no difference in sensitivity of direct plating to CM or MSA. Inclusion of a groin swab specimen also increased MRSA detection and specifically increased detection of PVL-positive USA300 strains. Thus, the impact on both strain diversity and overall sensitivity should be considered when selecting body sites for MRSA sampling.