Comparison of contamination rates and antibiotic resistance in bacterial isolates from grass-fed beef vs. conventionally raised beef

Jiayi Zhang, Purdue University

Abstract

Grass-fed beef products comprise a small (3-4%) but growing segment of the beef market. There are numerous potential differences between grass-fed beef and conventionally raised beef products. Among these differences, it is largely accepted that grass-fed beef products have different fat profiles with higher amounts of omega-3 and unsaturated fatty acids and lower levels of saturated fatty acids. In terms of food safety, grass-fed beef is often marketed as a safer product. The microbiological profiles of these two types of meat, however, have not been well addressed. The purpose of this study was to assess the comparative safety of grass-fed beef by characterizing contamination rates. The results showed that the E. coli (grass-fed: 27.3% (cuts), 76.5% (ground) vs. conventional: 41.2% (cuts), 50.0% (ground)), Enterococcus (grass-fed: 1.9192 log10CFU/mL (cuts) 1.8923 log10CFU/mL (ground) vs. conventional: 1.4586 log10CFU/mL (cuts), 1.2589 log10CFU/mL (ground)), and total coliforms (grass-fed: 2.7405 log10CFU/mL (cuts), 2.6184 log10CFU/mL (ground) vs. conventional: 2.8032 log10CFU/mL (cuts), 2.1114 log10 CFU/mL (ground)) contamination rates were not different between these two types of beef. However, grass-fed beef were overall (cuts and ground samples analyzed together) more highly contaminated by Enterococcus (grass-fed: 1.9100 log10CFU/mL vs. conventional: 1.4065 log 10CFU/mL, P<.05). E. coli and Enterococcus isolated from conventionally raised beef showed more resistance to antibiotics. The frequency of antibiotic resistance was low in E. coli isolated from both grass-fed and conventional beef. E. coli isolated from conventionally raised beef, however, were more frequently resistant to cefoxitin, chloramphenicol, ceftriaxone and ampicillin. Enterococcus faecalis isolated from conventionally raised beef were more frequently resistant to daptomycin, ciprofloxacin, linezolid, and kanamycin. Possible reasons outside of diet that could account for these differences (e.g., antibiotic histories, differences in processing, etc.) as well as the implications of these results are discussed.

Degree

M.S.

Advisors

Ebner, Purdue University.

Subject Area

Food Science|Microbiology

COinS