Influence of Therapeutic Ceftiofur Treatments of Feedlot Cattle on Fecal and Hide Prevalences of Commensal Escherichia coli Resistant to Expanded-Spectrum Cephalosporins, and Molecular Characterization of Resistant Isolates

Abstract

bIn the United States, the bla CMY-2 gene contained within incompatibility type A/C (IncA/C) plasmids is frequently identified in extended-spectrum-cephalosporin-resistant (ESC r ) Escherichia coli strains from both human and cattle sources. Concerns have been raised that therapeutic use of ceftiofur in cattle may increase the prevalence of ESC r E. coli. We report that herd ESC r E. coli fecal and hide prevalences throughout the residency of cattle at a feedlot, including during the period of greatest ceftiofur use at the feedlot, were either not significantly different (P > 0.05) or significantly less (P < 0.05) than the respective prevalences at arrival. Longitudinal sampling of cattle treated with ceftiofur demonstrated that once the transient increase of ESC r E. coli shedding that follows ceftiofur injection abated, ceftiofur-injected cattle were no more likely than untreated members of the same herd to shed ESC r E. coli. Pulsed-field gel electrophoresis (PFGE) genotyping, antibiotic resistance phenotyping, screening for presence of the bla CMY-2 gene, and plasmid replicon typing were performed on 312 ESC r E. coli isolates obtained during six sampling periods spanning the 10-month residence of cattle at the feedlot. The identification of only 26 unique PFGE genotypes, 12 of which were isolated during multiple sampling periods, suggests that clonal expansion of feedlot-adapted bla CMY-2 E. coli strains contributed more to the persistence of bla CMY-2 than horizontal transfer of IncA/C plasmids between E. coli strains at this feedlot. We conclude that therapeutic use of ceftiofur at this cattle feedlot did not significantly increase the herd prevalence of ESC r E. coli. Extended-spectrum cephalosporins (ESC) are critically important to human medicine and are frequently prescribed for the treatment of invasive Escherichia coli and Salmonella enterica infections (1-3). The bla CMY-2 gene, encoding the AmpC-like ␤-lactamase CMY-2, is frequently harbored by large (Ͼ120-kbp) incompatibility type A/C (IncA/C) plasmids in ESC-resistant (ESC r ) E. coli and ESC r S. enterica strains isolated from human and animal sources in the United States (4-13). IncA/C plasmids are considered broad-host-range plasmids since they have been identified in many bacterial species, including Aeromonas, Escherichia, Klebsiella, Photobacterium, Salmonella, Vibrio, and Yersinia (14,15). IncA/C plasmids possess conserved backbone sequences, but the sequences of genetic element insertions carrying antibiotic resistance genes are often divergent (14,(16)(17)(18). However, the insertions carrying genes conferring resistance to tetracyclines (tetA), phenicols (floR), and streptomycin (aadA2) are generally conserved between bla CMY-2 IncA/C plasmids harbored in E. coli and S. enterica hosts (17, 18). Thus, it has been hypothesized that commensal E. coli populations in the lower gastrointestinal systems of cattle may serve as a reservoir of bla CMY-2 IncA/C plasmids, which could then be transferred to more virulent food-borne pathogens, inclu...

Antimicrobial-Resistant Fecal Bacteria from Ceftiofur-Treated and Nonantimicrobial-Treated Comingled Beef Cows at a Cow–Calf Operation

“…3GC r , 3rd-generation cephalosporin-resistant; TET r , tetracycline resistant E. coli; COT r , trimethoprim-sulfamethoxazole-resistant. 6 AGGA ET AL.…”

“…Studies have shown that the level of AMR associated with feedlot cattle returns to a baseline level 2-5 weeks after withdrawal of treatment. [3][4][5][6] Of 729 NTS E. coli isolates tested, 17.7% (95% CI = 11.4-23.8%) were TET r . The proportions of TET r isolates did not significantly ( p > 0.05) differ between ceftiofur-treated and nonantimicrobial-treated cows, nor among the birth cohorts.…”

“…The concentration of cefotaxime was based on recommendations by European Food Safety Authority 26 and this concentration previously has been used to isolate 3GC r fecal E. coli from animals. 6,18 The concentrations of trimethoprim and sulfamethoxazole were based on resistance breakpoints described in the NARMS retail meat report. 27 The concentration of tetracycline was used at one dilution higher than the recommended resistance breakpoint 27 based on prior pilot study in our laboratory.…”

“…Cull beef cows and dairy cows together contribute a substantial proportion of meat destined for ground beef production in the United States. 7 Multiple studies have investigated AMR in feedlot cattle 4,6,8,9 and dairy cows. 3,[10][11][12][13] However, there is a paucity of information regarding AMR in beef cows.…”

“…1,2 Previous studies have shown that following antimicrobial treatment in fed cattle, there is an increase in the antimicrobial-resistant bacterial (ARB) population, which then returns to pretreatment levels *14-36 days after cessation of treatment. [3][4][5][6] Breeding beef cows (hereafter referred to as beef cows), typically pasture raised, make up one-third of the cattle inventory in the United States. Cull beef cows and dairy cows together contribute a substantial proportion of meat destined for ground beef production in the United States.…”