A quantitative analysis of clincal nasal swab samples testing positive for methicillin-resistant Staphylococcus aureus

Abstract number: 1733_826

Mermel L., Lonks J., Gordon S., Perl T., Fadem S., Acharya M., Dacus D., Eells S., Maxey G., Cartony J., Morse D.J., Wood M.E., Mach P.A., Jacobson C.J.

Objective: Approximately 25–30% of the population is asymptomatically colonised with S. aureus, which is an important pathogen that accounts for approximately 20% of surgical-site infections. Over the past couple of decades, antibiotic treatment of infections due to S. aureus has become more difficult because of increasing methicillin resistance. The objective of this study was to quantify methicillin-resistant Staphylococcus aureus (MRSA) on clinical nasal swab samples, which were obtained from patients known to be positive for MRSA, or suspected to be at increased risk for MRSA carriage.

Methods: This is a prospective, multi-centre US clinical study. Swab samples were collected from in-patient and out-patient populations, using a sterilised rayon swab. Samples were obtained from both nares of patients at 11 sites. Samples were analysed at a central microbiology laboratory by a standard quantitative method using a selective medium and enrichment broth. Primary isolate slants were generated from MRSA samples for strain analysis via pulse field gel electrophoresis.

Results: A total of 771 subjects were enrolled, of which 219 (31%) were positive for MRSA. Fourteen of the 219, (6%), yielded MRSA growth only by broth enrichment. Quantification of MRSA samples resulted in a mean of 2.9 logs, range 0.4–6.8 logs. MRSA strain analysis of 115 isolates identified 7 unique MRSA strains, with 72% percent identified as USA100. Sites with active surveillance yielded 81% of cultures with MRSA, due to the fact that potential carriers were flagged upon hospital admittance. For sites without active surveillance yielded 15%

of cultures yielded MRSA, from potential carriers in general high risk populations.

Conclusion: Based on swab samples from both nares, there was an average of 103 logs of MRSA colony forming units (cfu's) found in patients from multiple regions of the USA. The USA100 strain predominated in patients from multiple sites. To increase confidence in the diagnoses of MRSA, it may be beneficial to include a broth enrichment step. The yield of MRSA cultures was greater in sites using active surveillance, compared to sites obtaining samples from a general high risk population.

Session Details

Date: 31/03/2007
Time: 00:00-00:00
Session name: European Society of Clinical Microbiology and Infectious Diseases
Location: ICC, Munich, Germany
Presentation type:
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