The Clonality of Staphylococcus aureus Nasal Carriage
1Division of Infectious Diseases, Department of Medicine, 2Department of Pathology, College of Physicians and Surgeons, 3Department of Statistics, and 4Department of Epidemiology, Mailman School of Public Health, Columbia University, 5Panna Technologies and 6Division of Infectious Diseases, Department of Medicine, and the Department of Epidemiology and Population Health, Montefiore Medical Center and the Albert Einstein College of Medicine, New York, New York; 7Public Health Research Institute, Newark, New Jersey
Nasal carriage of Staphylococcus aureus is often a prelude to infection with the same strain. The prevailing assumption has been that colonized individuals carry a single strain. The present study investigated the frequency of simultaneous nasal carriage of multiple strains of S. aureus. Three bacterial colonies from plated samples from colonized subjects were initially compared by pulsed‐field gel electrophoresis. Fourteen of 148 S. aureus–positive samples demonstrated at least a difference of a single band; 7 of these 14 samples contained different strains, and 3 of these 7 also belonged to different accessory gene regulator (agr) types. The remaining 7 samples contained clonally related isolates; 3 of these 7 contained pairs that differed by the presence or absence of the staphylococcal chromosomal cassette mec type IV. A mathematical model that we developed predicted that
6.6% of S. aureus–colonized individuals carry >1 strain. The present study demonstrates that carriage of discordant S. aureus strains in individuals with nasal colonization occurs regularly and suggests that the nares are likely sites for horizontal genetic exchange among strains.
Received 18 May 2004; accepted 24 August 2004; electronically published 22 December 2004.
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. Infection Control and Hospital Epidemiology 26:7, 638-645Online publication date: 1-Jul-2005.
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Financial support: National Institutes of Health (grants DA09656, DA11868, and DA15018 to F.D.L. and grant DA14523 to M.M.); Centers for AIDS Research, National Institute of Allergy and Infectious Diseases (grant AI‐051519 to R.K.).





