Medical and Clinical Microbiology

Statement of the problem:  In recent years, clonal lineages of Staphylococcus aureus from animals have emerged. Subclinical mastitis, a financially important disease of dairy sheep, can contribute to dissemination of strains with zoonotic potential to the general population through dairy product consumption and to farm workers through direct contact with animals. Objective of the study was the molecular characterization of S. aureus isolates from milk samples of ewes with subclinical mastitis in Greece.

Methodology: In total, 28 S. aureus strains were tested, which had been isolated from ewes with subclinical mastitis (defined as lack of clinically evident systemic or mammary abnormalities, but with simultaneous bacterial isolation and increased somatic cell counts). 

They had been identified by use of catalase and coagulase production test, combined with detection of nuc gene. Susceptibility to various antimicrobial agents was performed by the automated VITEK® 2 system. 

The detection of the genes (mecA, ermA, ermC, tetL, tetM, tetO, tetK) was assessed by PCR, while, clonality of isolates was defined by Multi Locus Sequence Typing (MLST).

Findings: Rates of resistance to penicillin, ampicillin, cefoxitin, erythromycin, clindamycin, tetracycline and fucidic acid were 10%, 10%, 0%, 7%, 7%, 7% and 0%, respectively. The ermC and tetK genes were found to be associated with erythromycin and tetracycline resistance. 

Molecular typing revealed that 19 isolates (68%) belonged to ST133; the remaining isolates were distributed among ST5 (3 strains), ST2011 (2 strains), ST7 (1 strain), ST30 (1 strain), ST130 (1 strain) and ST581 (1 strain).

Conclusions & significance : This is the first report of predominant S. aureus clones from ewes in Greece. The findings indicate that ST133 S. aureus predominates in sheep milk in Greece, a clone considered predominant in small ruminants.  Although lack of extensive antibiotic resistance in the tested strains is indicative of correct therapeutic management of S. aureus-associated infections, continuous surveillance is important to monitor development of the situation, especially as these clones might be of zoonotic concern.