Contamination Rates and Antimicrobial Resistance in Three Types of Bacteria Isolated from Chicken Products Labelled 'No Antibiotics Added'

Chicken meat from birds not fed antimicrobials had more Salmonella but meat from those reared with antimicrobials was more likely to have antimicrobial-resistant E. coli and Enterococcus spp., according to research from Purdue University.

13 August 2011

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Contamination rates and antimicrobial resistance in Enterococcus spp., Escherichia coli, and Salmonella isolated from chicken products bearing the label 'no antibiotics added' were investigated by Jiayi Zhang of Purdue University and co-authors there and at the University of Illinois at Urbana—Champaign. Their paper has been published online in the journal, Foodborne Pathogens and Disease.

In the United States, products from chickens that were not administered antimicrobial medications during grow-out can carry labels stating 'no antibiotics added'. Zhang and co-authors explain that they compared microbial profiles of chicken products labelled as coming from birds raised without antimicrobial medications (N=201; NON) with chicken products carrying conventional labels (N=201; CONV).

They found no differences in percentages of samples positive for Enterococcus spp. (CONV: 17.4 per cent; NON: 21.3 per cent) or E. coli (CONV: 25.9 per cent; NON: 22.3 per cent).

The number of samples positive for Salmonella was low in both groups but statistically higher in the NON samples (5.0 per cent) than CONV samples (1.5 per cent; p<0.05).

Conversely, CONV samples contained higher concentrations of coliforms (CONV: 3.0 log₁₀CFU/mL; NON: 2.5 log₁₀CFU/mL; p<0.05).

E. coli (N=190) and Enterococcus spp. isolates (N=113) were tested for resistance to common antimicrobials. E. coli isolates from CONV samples were more frequently resistant to at least one antimicrobial (CONV: 61.3 per cent; NON: 41.2 per cent; p<0.05). Enterococcus spp. isolates from the two groups were equally likely to be resistant to at least one antimicrobial but Enterococcus spp. isolates from CONV samples were more likely to be resistant to erythromycin, kanamycin and gentamicin (p<0.05).

Taken together, Zhang and co-authors say that the data suggest that NON samples may more frequently carry Salmonella; however, E. coli and Enterococcus spp. found on CONV are more likely to be resistant to some antimicrobials.

Reference

Zhang J., A. Massow, M. Stanley, M. Papariella, X. Chen, B. Kraft and P. Ebner. 2011. Contamination rates and antimicrobial resistance in Enterococcus spp., Escherichia coli, and Salmonella isolated from 'no antibiotics added'-labeled chicken products. Foodborne Pathogens and Disease. ahead of print. doi:10.1089/fpd.2011.0852