Molecular epidemiology of the nasal colonization by methicillin-susceptible staphylococcus aureus in Swiss children

Abstract Nasal carriage of Staphylococcus aureus contributes to an increased risk of developing an infection with the same bacterial strain. Genetic regulatory elements and toxin-expressing genes are virulence factors associated with the pathogenic potential of S. aureus. Our study aimed at performing an extensive molecular characterization of methicillin-susceptible S. aureus (MSSA) carried by children. MSSA were recovered from the nostrils of children. Panton-Valentine leukocidin (PVL), exfoliatins A and B (exfoA and exfoB), as well as the toxic-shock staphylococcal toxin (TSST-1) and agr group typing were determined by qPCR. Multiple-locus variable-number of tandem repeat analysis (MLVA) assay was also performed for genotyping. Five hundred seventy two strains of MSSA were analyzed. Overall, 30.1% of S. aureus were positive for toxin-expressing genes. 28.5% contained one toxin and 1.6% two toxins. The most commonly detected toxin gene was tst, present in 145 (25,4%) strains. TSST-1 gene was significantly associated with the agr group 3 (OR=56.8, 95%CI=32.0-100.8). MLVA analysis revealed a large diversity of genetic content and no clonal relationship was demonstrated between analyzed MSSA strains. MLST confirmed this observation of diversity and identified ST45 as a frequent colonizer. Molecular characterization of MSSA revealed an extraordinary diversity in terms of genomic content and important virulence factors. This broad diversity in MSSA carriage strains suggests a limited selection pressure in our geographic area.