In vitro susceptibility of Staphylococcus aureus strains isolated from cows with subclinical mastitis to different antimicrobial agents

The sensitivity to commercial teat dips (nonoxinol-9 iodine complex and chlorhexidine digluconate) of 56 Staphylococcus aureus strains isolated from quarter milk samples from various German dairy herds treated with different teat dipping schemes was investigated in this study. The minimum inhibitory concentration was determined using a broth macrodilution method according to the German Veterinary Association guidelines. The main objective of the current study was to induce in vitro resistance of Staphylococcus aureus to chemical disinfectants. Ten different strains were repeatedly passed ten times in growth media with sub-lethal concentrations of disinfectants. Nine strains showed a significant reduction in susceptibility to the nonoxinol-9 iodine complex but only one strain developed resistance to chlorhexidine digluconate. Stability of the acquired resistance was observed in allStaphylococcus aureus strains adapted to the nonoxinol-9 iodine complex and chlorhexidine digluconate. In contrast, simultaneous resistance to different antibiotics was not observed in any of the ten investigated Staphylococcus aureus strains. However, the isolates exhibited a high degree of resistance to penicillin G. Based on these results, resistance ofStaphylococcus aureus to chemical disinfectants may be more likely to develop if the chemicals are used at concentrations lower than is required for an optimal biocidal effect.

Source: El Behiry et al. – Journal of Veterinary Science, 13, 153-163, 2012

Complete abstract: www.m²-magazine/org > 028-25.02.2013