Role of Calf-Adapted Escherichia coli in Maintenance of Antimicrobial Drug Resistance in Dairy Calves

Abstract

The prevalence of antimicrobial drug-resistant bacteria is typically highest in younger animals, and prevalence is not necessarily related to recent use of antimicrobial drugs. In dairy cattle, we hypothesize that antimicrobial drug-resistant, neonate-adapted bacteria are responsible for the observed high frequencies of resistant Escherichia coli in calves. To explore this issue, we examined the age distribution of antimicrobial drug-resistant E. coli from Holstein cattle at a local dairy and conducted an experiment to determine if low doses of oxytetracycline affected the prevalence of antimicrobial drug-resistant E. coli. Isolates resistant to tetracycline (>4 g/ml) were more prevalent in <3-month-old calves (79%) compared with lactating cows (14%). In an experimental trial where calves received diets supplemented with or without oxytetracycline, the prevalence of tetracycline-resistant E. coli was slightly higher for the latter group (P ‫؍‬ 0.039), indicating that drug use was not required to maintain a high prevalence of resistant E. coli. The most common resistance pattern among calf E. coli isolates included resistance to streptomycin (>12 g/ml), sulfadiazine (>512 g/ml), and tetracycline (>4 g/ml) (SSuT), and this resistance pattern was most prevalent during the period when calves were on milk diets. To determine if prevalence was a function of differential fitness, we orally inoculated animals with nalidixic acid-resistant strains of SSuT E. coli and susceptible E. coli. Shedding of SSuT E. coli was significantly greater than that of susceptible strains in neonatal calves (P < 0.001), whereas there was no difference in older animals (P ‫؍‬ 0.5). These data support the hypothesis that active selection for traits linked to the SSuT phenotype are responsible for maintaining drug-resistant E. coli in this population of dairy calves.

Effects of Strain Type and Water Quality on Soil-Associated Escherichia coli

“…Selected strains exhibited varying responses to changes in pH, sorbent type, ionic strength, and generational cell age. Uses of antibiotics in agricultural animals could be responsible for the expression of certain resistance characteristics in E. coli strains [27]- [29], but our results found no differences between adhesion patterns in domestic animal strains likely exposed to antibiotics and wild strains that likely were not. Nor were different adhesion patterns found between ruminant and non-ruminant mammals, although this finding does not preclude diet having an effect as has been previously shown for cow, deer and sheep [30]- [32].…”

Assessing the effect of a single dose florfenicol treatment in feedlot cattle on the antimicrobial resistance patterns in faecal Escherichia coli

“…In cattle, factors apart from antimicrobial administration affect antimicrobial resistance, including age, environment, and nutrition [9,[16][17][18]. Long-term use of antimicrobials may select for antimicrobial resistance that persists over time, though duration of antimicrobial resistance in faecal E. coli in feedlot cattle following therapeutic antimicrobial usage appears to be short [20,29].…”

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.…”