Infectious Diseases: November 30, 2004

John Bartlett, MD


January 03, 2005

In This Article

Methicillin-Resistant Staphylococcus aureus

Ellis MW, Hospenthal DR, Dooley DP, Gray PJ, Murray CK. Natural history of community-acquired methicillin-resistant Staphylococcus aureus colonization and infection in soldiers. Clin Infect Dis. 2004;39:971-979. The report is from Brooke Army Medical Center at Fort Sam Houston of a prospective, observational study to assess community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) in US Army soldiers. Participants were military personnel enrolled in the Health Care Specialists Course in 2003 who had nasal cultures performed at entry and at 8-10 weeks later. There were 812 volunteers with a mean age of 21 years. Initial samples showed that 229 (28%) were colonized with methicillin-sensitive S aureus (MSSA) and 24 (3%) were colonized by CA-MRSA. At the second sampling, the yield of CA-MRSA decreased to 12 (1.6%). The participants were observed during the 8- to 10-week interval. During this time, 9 of the 24 participants colonized with CA-MRSA developed soft-tissue infections compared with 8 of 229 colonized with MSSA. The majority of these infections were skin abscesses. The difference of 38% vs 3% was highly significant statistically. These results are summarized in Table 1 .

In total there were 45 isolates of CA-MRSA, including 36 from nasal cultures and 9 from infection sites. Molecular analysis by pulsed-field gel electrophoresis showed 8 genotypes; the predominant strain was USA-300 that accounted for 25 (56%) of the 45 total CA-MRSA isolates and 8 of 9 from the infections. Antibiotic sensitivity testing showed that all 45 strains of CA-MRSA were sensitive to trimethoprim-sulfamethoxazole (TMP-SMX) and tetracycline. Analysis for the Panton-Valentine leukocidin (PVL) showed that this was present in 30 of the 45 CA-MRSA strains, including all 9 associated with infections. These results are summarized in Table 2 .

The study authors conclude that colonization with CA-MRSA with PVL-positive strains is associated with a high risk of soft-tissue infection.

Comment: This appears to be one of the few prospective studies of the natural history of colonization with CA-MRSA, and the conclusion is that it is associated with a high risk of soft-tissue abscess and furunculosis, which suggests that CA-MRSA is more virulent than MSSA. Of particular interest is the observation that 5 of the 6 subjects who were hospitalized due to the severity of soft-tissue infections had PVL-positive strains of CA-MRSA. The major risk for colonization was antibiotic use within the prior 6 months, which applied to 17 of 24 (71%) of the 24 colonized patients. This report adds to a gathering database on CA-MRSA that now shows rather consistently that this is clonal; the predominant strain in the United States is "USA 300" by molecular typing; the mechanism of methicillin resistance is mec IV; most strains are sensitive to multiple antibiotics other than beta-lactams, including TMP-SMX and tetracycline; most possess genes for production of PVL, which is a possible virulence factor; and the characteristic infections are predominantly soft-tissue abscesses and to a lesser extent necrotizing pneumonia superimposed on influenza. The importance of this report is that it supports the concept that nasal colonization is infrequent but poses a significant risk for subsequent soft-tissue infection that is often severe, requiring hospitalization and incisional drainage.

Davis KA, Stewart JJ, Crouch HK, Florez CE, Hospenthal DR. Methicillin-resistant Staphylococcus aureus (MRSA) nares colonization at hospital admission and its effect on subsequent MRSA infection. Clin Infect Dis. 2004;39:776-782. This is another report from Brooke Army Medical Center examining the consequences of nasal colonization with CA-MRSA. In this report, patients admitted to 5 hospital units were evaluated on admission with nasal cultures, which were repeated during hospital stays exceeding 7 days and at discharge. They were subsequently followed for 1 year to determine the frequency of infection. The results showed that 26 of the 758 study subjects (3.4%) were colonized on admission with CA-MRSA and 137 (21%) were colonized with MSSA. Of the 26 subjects, 5 (19%) subsequently developed MRSA infection; this frequency is substantially greater than the 2% rate of infection in those colonized with MSSA or with negative nasal cultures, with an odds ratio of approximately 10. These results are summarized in Table 3 .

With regard to colonization after admission, follow-up nasal cultures were obtained in 394 patients; 12 (3%) subsequently acquired colonization with MRSA and 4 (25%) subsequently developed MRSA infections. The odds ratio for this group as compared with those who were not colonized was 12. The study authors conclude that colonization with MRSA at the time of hospitalization or after hospitalization is associated with an increased risk of MRSA infection.

Comment: This study, unlike the one that precedes it, did not provide information on molecular typing, analysis of PVL, or detailed information about the types of infections. Nevertheless, an inference can be made on the basis of the reported sensitivity profiles of the 56 colonizing isolates and the 30 strains recovered from infections. This shows that 96% were sensitive to trimethoprim and 95% were sensitive to tetracycline.


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