Molecular Characterization and Clonal Diversity of Methicillin-Resistant and -Susceptible Staphylococcus aureus Isolates of Milk of Cows with Clinical Mastitis in Tunisia

The aim of this study was to determine the genetic lineages, and the frequency of antibiotic resistance and virulence determinants in methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-susceptible S. aureus (MSSA) isolates recovered from milk of cows with clinical mastitis. Three hundred milk samples from bovine with clinical mastitis were obtained from 30 dairy farms in different regions of Tunisia. Fifteen of the 300 tested samples contained S. aureus (5%), in three cases were MRSA. Isolates (one/sample) were typed (S. aureus protein A [spa], multilocus sequence typing and accessory gene regulator [agr]). The presence of resistance and virulence genes was analyzed by PCR. The three MRSA isolates contained mecA and blaZ genes (one of them also the msr(A) gene), and carried the enterotoxin gene sen; they were typed as t10381-ST4114 or t267-ST4120, and corresponded to agr type-I. Twelve MSSA isolates were recovered and harbored the blaZ (7 strains) or erm(C) genes (1 strain). The MSSA isolates presented seven different spa-types, associated to new sequence types (STs): t426-ST4118, t267-ST4120, t1773-ST4115, t509-ST4119, t529-ST4117, t2844-ST4113, and t2802-ST4112; most isolates (8/12) were typed as t267/ST4120. All S. aureus isolates were scn-negative, except one MSSA of lineage ST4119 that exhibited the immune evasion cluster type D, and harbored the seg, sei, sem, seo, and seu enterotoxin genes. Four MSSA isolates carried the toxic shock syndrome toxin 1 gene (tst). S. aureus (including MRSA) is an important cause of bovine mastitis, showing isolates with high genetic diversity and high content in virulence genes.