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Impact of "Raised Without Antibiotics" Beef Cattle Production Practices On Occurrences of Antimicrobial Resistance.

Appl Environ Microbiol; 2017 Sep 08.
Article in English | MEDLINE | ID: mdl-28887421
The specific antimicrobial resistance (AMR) decreases that can be expected from reducing antimicrobial (AM) use in United States (US) beef production have not been defined. To address this data gap, feces were recovered from 36 lots of "raised without antibiotics" (RWA) and 36 lots of "conventional" (CONV) beef cattle. Samples (N = 719) were collected during harvest and distributed over a year. AMR was assessed by: i) culture of six AM-resistant bacteria (ARB); ii) qPCR for 10 AMR genes (ARGs); iii) an qPCR array of 84 ARGs; and iv) metagenomic sequencing. Generally, AMR levels were similar, but some were higher in CONV. The prevalence of third-generation cephalosporin-resistant (3GC ) was marginally different between production systems (47.5 % CONV, 34.8% RWA, = 0.04), but the seasonal effect (92.8% summer, 48.3% winter, < 0.01) was greater. Erythromycin-resistant (ERY ) spp. concentrations significantly differed between production systems (CONV = 1.91 log CFU/g, RWA = 0.73 log CFU/g, < 0.01). Levels of , , (A), (B), (C), (F), (Q), (A), (B), (M), and (X) ARGs were higher ( < 0.05) in CONV. Aggregate abundances of all 43 ARGs detected by metagenomic sequencing and the aggregate abundances of ARGs in the aminoglycoside, ß-lactam, macrolide-lincosamide-streptogramin B (MLS ), and tetracycline AM classes did not differ (log fold change < 1.0) between CONV and RWA. These results suggest that further reductions of AM use in US beef cattle production may not yield significant AMR reductions beyond MLS and tetracycline resistance. The majority of AM use in the US is for food-animal production, leading to concerns that typical AM use patterns during CONV beef cattle production in the US contribute broadly to AMR occurrence. In the present study, levels of AMR were generally similar between CONV and RWA cattle. Only a limited number of modest AMR increases were observed in CONV, primarily involving MLS and tetracycline resistance. Macrolides (tylosin) and tetracyclines (chlortetracycline) are administered in-feed for relatively long durations to reduce liver abscesses. To ensure judicious AM use, the animal health, economic, and AMR impacts of shorter duration in-feed administration of these AMs should be examined. However, given the modest AMR reductions observed, further reductions of AM use in US beef cattle production may not yield significant AMR reductions beyond MLS and tetracycline resistance.