High diversity of genetic lineages and virulence genes of Staphylococcus aureus isolated from dairy products in Tunisia

Staphylococcus aureus can produce staphylococcal enterotoxins (SEs) in foods and cause staphylococcal food poisoning. Milk and dairy products are foodstuffs among the most frequently associated with staphylococcal food poisoning. It is also relevant to consider that the transmission of S. aureus is possible either by direct contact with animals or through contaminated food as milk or cheese. The aim of this study was to determine the genetic lineages and the genotypes of antibiotic resistance and virulence factors of S. aureus isolates recovered of different dairy products in Tunisia. We collected 136 samples of various traditional and commercial dairy products in different areas in Tunisia. Bacterial isolates were identified by conventional methods (Gram staining, catalase test, oxidase test, DNase production, and ability to coagulate rabbit plasma). The antimicrobial resistance profile and the virulence gene content were investigated. Molecular typing was performed by spa-typing, agr-typing and pulse-field gel electrophoresis (PFGE). Twenty six (19.2%) of the total analyzed samples (N = 136) were contaminated by methicillin-sensitive S. aureus (MSSA). Antimicrobial sensitivity assays have shown that MSSA isolates exhibit a resistance rate of 73.1% to penicillin, 23.1% to tetracyclin, 19.2% to kanamycin, 15.4% to amikacin, and 3.9% to gentamicin. Furthermore, the following toxin encoding genes have been identified among MSSA: luk ED (24 isolates), luk M (2), see (3), sel (3), sea (2), sej (1), sec (1), seb (1), and tst (1). Results of molecular characterization and phylogenetic relations analysis revealed a considerable genetic diversity. Thirteen different spa-types were detected among the 26 isolated MSSA. S. aureus was attributed to the agr type I (16 isolates), II (2), III (2), and non-typables (6). Sixteen different electrophoretic profiles were revealed. Dairy products could constitute a reservoir of virulent and antibiotic-resistant Staphylococcus aureus.