Antimicrobial Drug Resistance Genes Do Not Convey a Secondary Fitness Advantage to Calf-Adapted Escherichia coli

Abstract

Maintenance of antimicrobial drug resistance in bacteria can be influenced by factors unrelated to direct selection pressure such as close linkage to other selectively advantageous genes and secondary advantage conveyed by antimicrobial resistance genes in the absence of drug selection. Our previous trials at a dairy showed that the maintenance of the antimicrobial resistance genes is not influenced by specific antimicrobial selection and that the most prevalent antimicrobial resistance phenotype of Escherichia coli is specifically selected for in young calves. In this paper we examine the role of secondary advantages conveyed by antimicrobial resistance genes. We tested antimicrobial-susceptible null mutant strains for their ability to compete with their progenitor strains in vitro and in vivo. The null mutant strains were generated by selection for spontaneous loss of resistance genes in broth supplemented with fusaric acid or nickel chloride. On average, the null mutant strains were as competitive as the progenitor strains in vitro and in newborn calves (in vivo). Inoculation of newborn calves at the dairy with antimicrobial-susceptible strains of E. coli did not impact the prevalence of antimicrobial-resistant E. coli. Our results demonstrate that the antimicrobial resistance genes are not responsible for the greater fitness advantage of antimicrobial-resistant E. coli in calves, but the farm environment and the diet clearly exert critical selective pressures responsible for the maintenance of antimicrobial resistance genes. Our current hypothesis is that the antimicrobial resistance genes are linked to other genes responsible for differential fitness in dairy calves.Antimicrobial drug-resistant bacteria present a significant risk to public health, and consequently there is great interest in reducing the prevalence of antimicrobial resistance (AR) genes in both commensal and pathogenic bacteria. One strategy to decrease the prevalence of AR bacteria is to discontinue using antimicrobial drugs for growth promotion and prophylaxis in food animals (4,11,22,43,44). Unfortunately, this strategy has produced mixed results that are dependent on the duration and amount of antimicrobial drug use (10,16,24,25,37,38,41). While pathogenic bacteria are the primary concern, commensal bacteria are an important reservoir for AR genes. It is hypothesized that, because AR genes can easily transfer among diverse bacterial species (6,29,32,35,39,45), then they can also move easily from commensal to pathogenic bacteria. Consequently abundance and ease of acquisition of the antimicrobial drug resistance genes ultimately complicate the orthodox treatment of diverse bacterial diseases (13,18).It is generally accepted that the prevalence of AR bacteria is directly correlated with antimicrobial drug use (19,20,40), but mechanisms unrelated to specific antimicrobial drug selection have also been implicated in the wide distribution and maintenance of AR genes. Some of these mechanisms include plasmid addiction (14,17) and close linkage...

Changes in Tetracycline Susceptibility of Enteric Bacteria following Switching to Nonmedicated Milk Replacer for Dairy Calves

“…This suggests that the intervention had a relatively quick effect on Tet susceptibility but that these changes were not permanent. It is possible that, in the intervention herds, the lack of long-term selective pressure by the use of medicated milk replacers allowed nonsusceptible bacteria surviving in the calf population to compete more successfully against susceptible bacteria for reasons beyond selection pressure (23).…”

The Streptomycin-Sulfadiazine-Tetracycline Antimicrobial Resistance Element of Calf-Adapted Escherichia coli Is Widely Distributed among Isolates from Washington State Cattle

“…The two other most common resistance patterns at the WSU dairy were ASSuT and ASSuTCh (13). None of the tested isolates with these two phenotypes harbored the SSuT element described herein.…”

“…In a separate study we have shown that the antimicrobial drug resistance genes themselves do not convey obvious selective advantage in the absence of antimicrobial use (13). The aim of the present study was to identify the nucleotide sequence of the DNA of the SSuT antimicrobial resistance genes and to examine the distribution of this genetic element among strains of E. coli and among cattle farms in Washington State.…”

Erratum: Can landscape ecology untangle the complexity of antibiotic resistance?

“…Much of this assumption is based on the expected fitness cost of maintaining the resistance gene(s). Recent studies indicate that the maintenance of resistance might not impose a significant fitness cost [23][24][25][26] or that this cost can be overcome [19][20][21]27 . However, there is also a large background pool of resistance, probably a result of the fact that many resistance genes protect the bacterial cell in such a way that they are likely to be functional against other compounds in the environment.…”

“…This assumption has been questioned [19][20][21] , and recent evidence demonstrates that, in the absence of selection pressure due to antibiotic use, resistance levels in bacterial populations are often slow to decline, if at all 5,7,22 . In fact, some organisms with antibiotic-resistant phenotypes and genotypes seem to be fitter than their susceptible counterparts [23][24][25][26] , and compensatory mutations have been observed that allow the resistant organism to retain its fitness level 27 . Many potential selection forces for resistance have been described, including antibiotics, metals and other compounds such as disinfectants.…”