Abstract:

This study aimed to understand the changes in drug resistance of Staphylococcus aureus (Sa). A total of 203
strains of Sa collected during 2006 to 2011 were tested against 10 antibiotics by broth micro-dilution method for determining
their antibiotic susceptibility. With multiplex polymerase chain reaction (M-PCR) method, the methicillin-resistant gene
(mecA) and beta-lactam drug resistance gene (blaZ) of Sa were analyzed. The results showed that the resistance rates of
Sa isolates to penicillin and ampicillin acid were high. An increasing trend was observed for the resistance rates of Sa
to amoxicillin/clavulanic, erythromycin, and trimethoprim/sulfamethoxazole. There was an uncertain resistance rate to
ciprofloxacin whereas the resistance rates of Sa isolates to clindamycin and tetracycline showed a decreasing trend. All the
strains tested were sensitive to both oxacillin and ceftiofur. The multidrug-drug resistant rates of Sa from different years
ranged from 72.1% to 100.0%, with different drug-resistant spectra, but at the same time, they presented a continuously widened
trend. M-PCR analysis showed that no mecA gene was amplified from Sa strains collected during 2006 to 2011. During different
years, more or less isolates carrying the gene blaZ were detected, and the detection rates of blaZ+ from different years ranged
from 49.1% to 87.5%. There was a certain relationship between the susceptibility test results and the related drug resistance gene
detection results. The detection rates of some drug-resistant strains were inconsistent with their resistance genes suggesting that
there are other resistance mechanisms for Sa. The drug resistance of Sa from fresh milk in Sichuan was not optimistic.

Key words: fresh milk, Staphylococcus aureus, drug resistance, multiplex polymerase chain reaction (M-PCR), β-lactam drug resistance genes