Antimicrobial resistance determinants in antibiotic and biocide-resistant gram-negative bacteria from organic foods

In the present study, a collection of 29 Gram-negative bacteria selected from organic foods according to antibiotics and biocide resistance were screened for multidrug efflux pumps and specific antibiotic resistance genes. The acrB pump gene of the AcrAB–TolC system was detected in all isolates. EfrAB determinants efrA and efrB were frequently detected (Enterobacter, Pantoea, Salmonella), while other efflux pump genes (norE, mepA mdeA, norC) were detected only in some Enterobacter and Pantoea isolates. Genes encoding resistance to quaternary ammonium compounds qacE and qacJ were detected only in Enterobacter isolates, while sugE was detected in Enterobacter and in one Pantoea isolate. The aminoglycoside-resistance ant(4_)-Ia gene was detected in Enterobacter, Pantoea agglomerans and Klebsiella oxytoca. The antibiotic resistance determinant most frequently found was lsa gene (Enterobacter, Pantoea and Salmonella isolates). Erythromycin resistance genes (ereA, ereB) were detected among Enterobacter, Klebsiella or Salmonella. The macrolide efflux pump msrA and the phosphotransferase type I mphA genes were detected in Salmonella. Other resistance determinants analyzed (mdfA, yhiUV, evgA, 455emeA, yvcC, qacA/B, qacC, qacG, qacH, norA, norB, norC, sdrM, sepA, mecA, emrE, smr, bla, cat, aac(6_)-Ie-aph(2_)-Ia, aph(2_)-Ib, aph(2_)-Ic, aph(2_)-Id, aph(3_)-IIIa, ermA, ermB, ermC, and mefA genes) were not detected in any isolate. Results from the present study suggest that the main mechanisms of antibiotic and biocide resistance in Gram-negative bacteria from organic foods rely on different kinds of efflux pumps of broad substrate specificity. The indiscriminate use of biocides in the food chain should be revised in order to minimize persistence of reservoirs of antimicrobial resistance in the food chain.