Resistance to erythromycin of Staphylococcus spp. isolates from the food chain

The aim of this study was to determine both the occurrence and the genetic basis of resistance to erythromycin among 1 235 Staphylococcus spp. isolates obtained between 2000 and 2006 from (a) raw milk and meat (1 704 samples), (b) foodstuffs produced from these (451 samples), and (c) contact surfaces at processing plants and dairy farms (363 samples) in the Czech Republic. Isolates were screened by broth microdilution method for resistance to erythromycin and further 11 antimicrobial agents. In addition, isolates were screened by agar dilution (erythromycin range 1-128 mg/l) and D-zone test for inducible resistance to macrolides, lincosamides and streptogramin B (iMLSB). Forty isolates were found to be either resistant, or intermediate, to erythromycin (3.2% of isolates); of these, more than 50% were identified as S. epidermidis. A total of 15 (1.2%) resistant isolates of staphylococci originated from foodstuffs. Resistance mediated by methylation - i.e. iMLSB-resistance (10 isolates with the erm(A) or erm (C) gene) and constitutive MLSB-resistance (one isolate with the erm (B) and erm (C) genes) - exhibited a significantly high level of resistance to erythromycin with minimum inhibitory concentrations (MIC) of 64 - >128 mg/l (MICmode = >128 mg/l). In contrast, the efflux mechanism encoded by the msr(A) gene (13 isolates; MICrange = 4-128, MICmode = 128 mg/l), the inactivation mechanisms of resistance encoded by the mph(C) gene (three isolates; MICrange = 8-32 mg/l), and/or their combination (13 isolates; MICrange = 4-128, MICmode = 64 mg/l) led to lower MIC values. The efflux gene