Is increased Staphylococcus aureus colonization during military service caused by specific genotypes? Molecular examination of long-term carriage in a prospective cohort study.

A 22% increase in Staphylococcus aureus colonization was observed in Norwegian recruits during first year of military service. The aim was to determine whether specific genotypes caused the increase and to examine carriage status based on genotyping. Characterization of S. aureus from nose, throat and perineum sampled at enrolment and discharge included spa typing, MLVA, detection of PVL genes and antimicrobial susceptibility testing. spa typing demonstrated high and stable genetic diversity. The three most frequent spa types were found in 15% of recruits at enrolment and in 29% at discharge. Only t084 increased significantly (p = 0.02). Subtyping revealed that t084, t065 and t002 consisted of 13, 6 and 11 different MLVA types, respectively, at discharge. The military cohort (n = 265) consisted of S. aureus carriers of identical genotype (n = 99, 38%), carriers of non-identical genotype (n = 52, 20%), intermittent carriers (n = 86, 33%) and non-carriers (n = 27, 10%). Carrier status was indefinable for one recruit due to unavailable isolates for genotyping. Antibiotic resistance towards erythromycin, fusidic acid and clindamycin increased significantly and above national surveillance levels. The observed increase in S. aureus colonization during military service was caused by many different genotypes implying many transmission routes. Genotype did not correlate with colonization niche or carriage status.