Staphylococci Isolated from Raw Milk of Yak and Cattle in Mongolia : studies on the occurrence, characterization, detection of enterotoxin and antimicrobial susceptibility profile of the isolates

Sammanfattning: Occurrence, characterization, detection of enterotoxin and antimicrobial susceptibility profile in staphylococci isolated from yak and cattle in Mongolia were investigated. Staphylococci were isolated from 72 (74%) of the 97 raw milk samples investigated. Of the samples containing staphylococci, 69% (50/72) were from yak, whereas 31% (22/72) were from cattle. Of the samples containing staphylococci, S. aureus was detected in 14% (7/50) of yak milk samples and in 68% (15/22) of cattle samples. Staphylococcal enterotoxin C (SEC) was detected in 19% (5/26) of the S. aureus strains investigated. Three of the enterotoxigenic strains were from yak and two from cattle. None of the S. aureus strains tested produced SEA, SEB, or SED. The MICs of 12 antimicrobial agents for 45 isolates of Staphylococcus spp. from yak and cattle were determined. Broth microdilution was used for the susceptibility testing and because of high oxacillin MICs all isolates were also subjected to oxacillin agar screening and PCR for the mecA gene. Nitrocefin test was used to determine β-lactamase production. The proportion of resistance to β-lactamse based on β-lactamase production was high (37-84%). However, no mecA gene was detected. Resistance to erythromycin, clindamycin, chloramphenicol, tetracycline, trimethoprim and fusidic acid was recorded among isolates from both yak and cattle. Cephalothin resistance was found only among coagulase-negative staphylococci from yak.