Role of Ceftiofur in Selection and Dissemination of bla _CMY-2 -Mediated Cephalosporin Resistance in Salmonella enterica and Commensal Escherichia coli Isolates from Cattle

Abstract

Third-generation cephalosporin resistance of Salmonella and commensal Escherichia coli isolates from cattle in the United States is predominantly conferred by the cephamycinase CMY-2, which inactivates ␤-lactam antimicrobial drugs used to treat a wide variety of infections, including pediatric salmonellosis. The emergence and dissemination of bla CMY-2--bearing plasmids followed and may in part be the result of selection pressure imposed by the widespread utilization of ceftiofur, a third-generation veterinary cephalosporin. This study assessed the potential effects of ceftiofur on bla CMY-2 transfer and dissemination by (i) an in vivo experimental study in which calves were inoculated with competent bla CMY-2 -bearing plasmid donors and susceptible recipients and then subjected to ceftiofur selection and (ii) an observational study to determine whether ceftiofur use in dairy herds is associated with the occurrence and frequency of cephalosporin resistance in Salmonella and commensal E. coli. The first study revealed bla CMY-2 plasmid transfer in both ceftiofur-treated and untreated calves but detected no enhancement of plasmid transfer associated with ceftiofur treatment. The second study detected no association (P ‫؍‬ 0.22) between ceftiofur use and either the occurrence of ceftiofurresistant salmonellosis or the frequency of cephalosporin resistance in commensal E. coli. However, herds with a history of salmonellosis (including both ceftiofur-resistant and ceftiofur-susceptible Salmonella isolates) used more ceftiofur than herds with no history of salmonellosis (P ‫؍‬ 0.03) These findings fail to support a major role for ceftiofur use in the maintenance and dissemination of bla CMY-2 -bearing plasmid mediated cephalosporin resistance in commensal E. coli and in pathogenic Salmonella in these dairy cattle populations.

Selection Pressure Required for Long-Term Persistence of bla CMY-2 -Positive IncA/C Plasmids

“…In fact, it is surprising that the bla CMY-2 plasmids were retained in the populations for several hundred generations. Overall, the long-term passage culture experiments in the absence of selection suggest that the plasmids are very stable but that random mutations accumulate in the plasmid, leading to plasmid decay, and eventually the entire plasmids were lost from the populations (2,9,13,24,29). We observed some variance in the loss of different resistance phenotypes during the long-term passage trials.…”

“…Resistance to expanded-spectrum cephalosporins is considered the most likely trait, and the use of an expanded-spectrum cephalosporin (ceftiofur) in livestock has been implicated as an important selective pressure responsible for the expansion of bla CMY-2 plasmids in the enteric flora of livestock (40,43,48). Nevertheless, when (juvenile and adult) cattle are treated with ceftiofur, the results have not been consistent, suggesting that ceftiofur does not present a strong selective pressure in vivo (9,24,43,48). bla CMY-2 plasmids have also been detected in places where ceftiofur and ceftriaxone are not used, such as in aquaculture farms (Edwardsiella ictaluri isolated from catfish) and in organic cattle farms in the United States (31,50).…”

“…In vivo experiments included neonatal calves (2 to 7 days old; n ϭ 5). Both strains retained trimethoprim resistance (i.e., pTmpR was present), and the peH4H-free strain was selected for nalidixic acid resistance (Nal r ) (9). Calves (fecal samples) were prescreened for the presence of trimethoprim-and nalidixic acid-or trimethoprim-and ceftiofur-resistant E. coli strains and found to be apparently free of bacteria with these combinations of resistance traits.…”

“…We employed both in vitro and in vivo competition studies to assess the relative fitness cost of bla CMY-2 plasmids. For in vitro studies, equal numbers of CFU (ϳ10 9 ) of bla CMY-2 plasmid-free E. coli DH10B and each bla CMY-2 plasmid-bearing strain [DH10B(peH4H), DH10B(pTmpR), DH10B(pTmpR/peH4H), DH10B(pAR), or DH10B(pAM)] were added into 5 ml LB broth. The different combinations of cultures were passaged (every 24 h, we transferred 5 l of cultures into a fresh 5 ml of LB broth) for 8 days.…”

Modeling the Infection Dynamics of Bacteriophages in Enteric Escherichia coli: Estimating the Contribution of Transduction to Antimicrobial Gene Spread

“…This outcome occurred when we assumed that 90% of the infections resulted in lysogeny of 100 hours and that the newly created lysogens were cross-immune to other enteric phages. From the approximately 1/4 daily population turnover of the enteric E. coli bacteria through ingestion and excretion (23,35), the average E. coli intestinal residency time can be expected to be on the order of 100 hours. Hence, a 100-hour average lysogenic cycle duration was sufficiently short for a large fraction of the lysogens created in the intestine (ϳ2/3) to be lysed while still in the intestine, and the rest were excreted in feces.…”