Quantifying factors associated with changes in antibiotic susceptibility patterns of enterococci isolated from dairy cattle

Abstract number: P1726

Lang K.S., Singer R.

Objective: The objective of this study was to quantify factors associated with changes in antibiotic susceptibility patterns in enterococci isolated from bovine faeces over a 2.5-year longitudinal study. We hypothesised that increasing levels of resistance would be positively associated with prior antibiotic treatment and younger animals. We also hypothesised that farms with the highest overall use of antibiotics would have the highest levels of resistance, but that the specific resistance patterns would vary among species of enterococci.

Methods: Four dairy farms were chosen in central Illinois, USA, and a cohort of approximately 50 animals of different ages was randomly selected on each farm. Faecal samples were collected from each animal every three months. Multiplex PCR was used to speciate the isolates into E. faecium, E. faecalis, E. hirae or Enterococcus spp. For each isolate, minimum inhibitory concentrations (MICs) for 17 antimicrobial agents were determined using microbroth dilution and CLSI-recommended control strains.

Results:E. faecium (n = 1481) and E. hirae (n = 1355) were the predominant species isolated. Resistance was found more commonly in E. faecium isolates compared to E. hirae isolates for bacitracin (78%, 5%), ciprofloxacin (16%, 1%), erythromycin (14%, 1%), and quinupristin/dalfopristin (24%, 11%). Decreased susceptibility to flavomycin was observed in 88% of all E. faecium and 84% of E. hirae isolates. Tetracycline resistance was more common in E. hirae isolates than E. faecium, occurring in 54% and 31% of isolates respectively. Various resistances, including tetracycline and ciprofloxacin, were positively associated with the amount of antibiotic used on the farm. Calves were more likely to have resistance enterococci than older animals.

Conclusions: At the farm level, the prevalence and diversity of resistance was greatest in those farms using the most antibiotic. During the study, resistance levels did not change significantly within the farm. As seen in other studies, young animals had bacterial populations with more resistance, and this appeared to be independent of antibiotic treatment. Although E. faecium and E. hirae were exposed to the same selection pressures, there were noticeable differences in the resistance patterns between them. With many isolates having multidrug resistance, the choice of antibiotic on the farm becomes more challenging knowing that a single antibiotic can co-select for multiple resistances.

Session Details

Date: 19/04/2008
Time: 00:00-00:00
Session name: 18th European Congress of Clinical Microbiology and Infectious Diseases
Location: Barcelona, Spain
Presentation type:
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