Isolation and identification of bacteria from organic foods: Sensitivity to biocides and antibiotics

This study focused on biocide and antibiotic sensitivity in a collection of 378 isolates derived from 36 organic foods. Most isolates were sensitive to low concentrations of biocides, with MIC’s lower than 10 mg/l or 100 mg/l. Low percentages of isolates (0.3–5%) required higher biocide concentrations for inhibition, with MIC’s in the range of 1–10 g/l (as in the case of cetrimide, hexadecylpyridinium chloride, chlorhexidine or triclosan). Highest percentages of antibiotic resistance were detected for cefuroxime, followed by amoxicillin and erythromycin, while most isolates were sensitive to nitrofurantoin and ciprofloxacin. A selection of 64 isolates showing high MIC’s to biocides were subsequently identified by PCR amplification or by 16S rDNA sequencing. Most of the selected isolates belonged to Pantoea sp. and Enterobacter sp., followed by Bacillus cereus, Enterococcus faecium, Enterococcus sp., Bacillus sp., Klebsiella oxytoca and Salmonella. A total of 36 isolates (9.5%) with high MIC’s to at least one biocide were resistant to at least one of the antibiotics tested. Isolates with high MIC’s to multiple biocides were more frequently resistant to antibiotics as well. Results from this study indicate that organic foods may act as reservoirs for antibiotic-resistant bacteria and suggest that high levels of biocide tolerance could facilitate the prevalence of antibiotic-resistant strains.