Impact of “Raised without Antibiotics” Beef Cattle Production Practices on Occurrences of Antimicrobial Resistance

Abstract

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, β-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.

Antimicrobial Resistance in Members of the Bacterial Bovine Respiratory Disease Complex Isolated from Lung Tissue of Cattle Mortalities Managed with or without the Use of Antimicrobials

“…In MH and PM, complete linkage of aph(3")-Ib, aph(6)-Id, aph(3 )-Ia, and sul2 was found in both WGS and PCR datasets. Other studies have also reported similar linkage of these aminoglycoside and sulfonamide/sulfamethoxazole resistance genes in MH [46]. In contrast, for HS aph(6)-Id, aph(3 )-Ia, and sul2 co-occurred with aph(3")-Ib either present or absent.…”

“…Other studies of calves raised conventionally or without antimicrobials concluded that use of antimicrobials did not increase AMR of fecal Escherichia coli [43], organisms isolated from beef [44], or genes associated with carbapenem resistance in feces [45]. Although Vikram et al [46] found increased detection of some macrolide and tetracycline-related AMR genes in feces of CON as compared to NAT calves, these differences were considered of minor importance. Although only two NAT feedlots were sampled, raising calves without antimicrobials significantly reduced the AMR genes present in PM.…”

Investigation of a Reduction in Tylosin on the Prevalence of Liver Abscesses and Antimicrobial Resistance in Enterococci in Feedlot Cattle

“…However, these researchers did record a higher percentage of Ery R enterococci with increasing days on feed between day 20 and day 118. The beneficial effect of reducing tylosin in-feed on the degree of resistance is difficult to predict because antimicrobial resistant bacteria are present in nearly all environments (40). However, shortening the duration of tylosin administered could help reduce the selection pressure that exacerbates the occurrence of antimicrobial resistance (16).…”

“…Other Ery R genes in enterococci include erm(A) and erm(C) (27), but we did not screen for these genes as they are infrequent in enterococci isolated from beef cattle (16,31,46). Nine isolates were negative for both macrolide resistance genes, suggesting that these isolates contained unknown or other known macrolide resistance genes that were not screened (40,46).…”

Detection, Occurrence and Fate of Emerging Contaminants in Agricultural Environments

“…The authors concluded that toxic metals may exert a stronger selection pressure for resistance to an antibiotic than the specific antibiotic itself. The effect of antibiotic administration in beef production on the emergence of ARGs was investigated by Vikram et al (2017). A total of 719 fecal samples were collected from 36 lots of "raised without antibiotics"…”