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

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The aim of this study was to determine both the occurrence and the genetic basis of resistance to erythromycin among 1 235 <I>Staphylococcus</I> 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 (iMLS<sub>B</sub>). Forty isolates were found to be either resistant, or intermediate, to erythromycin (3.2% of isolates); of these, more than 50% were identified as <I>S. epidermidis</I>. A total of 15 (1.2%) resistant isolates of staphylococci originated from foodstuffs. Resistance mediated by methylation – i.e. iMLS<sub>B</sub>-resistance (10 isolates with the <I>erm</I>(A) or <I>erm</I> (C) gene) and constitutive MLS<sub>B</sub>-resistance (one isolate with the <I>erm</I> (B) and <I>erm</I> (C) genes) – exhibited a significantly high level of resistance to erythromycin with minimum inhibitory concentrations (MIC) of 64 – >128 mg/l (MIC<sub>mode</sub> = >128 mg/l). In contrast, the efflux mechanism encoded by the <I>msr</I>(A) gene (13 isolates; MIC<sub>range</sub> = 4–128, MIC<sub>mode</sub> = 128 mg/l), the inactivation mechanisms of resistance encoded by the <I>mph</I>(C) gene (three isolates; MIC<sub>range</sub> = 8–32 mg/l), and/or their combination (13 isolates; MIC<sub>range</sub> = 4–128, MIC<sub>mode</sub> = 64 mg/l) led to lower MIC values. The efflux gene <iomsr</I>(A) dominated among the erythromycin-resistant isolates (65% of resistant isolates). This first report on the resistance of <I>Staphylococcus</I> spp. to erythromycin in the Czech Republic illustrates that, while occurrence was low, isolates from food were nevertheless carriers of <I>erm</I> (A), <I>erm</I> (B), <I>erm</I> (C),<I> msr</I>(A) and <I>mph</I>(C) genes for resistance to erythromycin and, therefore, represent a potential thread to humans.