The aim of this study was to assess the impact of three ampicillin dosage regimens on ampicillin resistance among Enterobacteriaceae recovered from swine feces by use of phenotypic and genotypic approaches. Phenotypically, ampicillin resistance was determined from the percentage of resistant Enterobacteriaceae and MICs of Escherichia coli isolates. The pool of ampicillin resistance genes was also monitored by quantification of bla _TEM genes, which code for the most frequently produced β-lactamases in gram-negative bacteria, using a newly developed real-time PCR assay. Ampicillin was administered intramuscularly and orally to fed or fasted pigs for 7 days at 20 mg/kg of body weight. The average percentage of resistant Enterobacteriaceae before treatment was between 2.5% and 12%, and bla _TEM gene quantities were below 10 ⁷ copies/g of feces. By days 4 and 7, the percentage of resistant Enterobacteriaceae exceeded 50% in all treated groups, with some highly resistant strains (MIC of >256 μg/ml). In the control group, bla _TEM gene quantities fluctuated between 10 ⁴ and 10 ⁶ copies/g of feces, whereas they fluctuated between 10 ⁶ to 10 ⁸ and 10 ⁷ to 10 ⁹ copies/g of feces for the intramuscular and oral routes, respectively. Whereas phenotypic evaluations did not discriminate among the three ampicillin dosage regimens, bla _TEM gene quantification was able to differentiate between the effects of two routes of ampicillin administration. Our results suggest that fecal bla _TEM gene quantification provides a sensitive tool to evaluate the impact of ampicillin administration on the selection of ampicillin resistance in the digestive microflora and its dissemination in the environment.

Quantitative data on fecal shedding of antimicrobial-resistant bacteria are crucial to assess the risk of transmission from dogs to humans. Our first objective was to investigate the prevalence of quinolone/fluoroquinolone-resistant and beta-lactam-resistant Enterobacteriaceae in dogs in France and Spain. Due to the particular concern about possible transmission of extended-spectrum cephalosporin (ESC)-resistant isolates from dogs to their owners, we characterized the ESBL/pAmpC producers collected from dogs. Rectal swabs from 188 dogs, without signs of diarrhea and that had not received antimicrobials for 4 weeks before the study, were quantified for total and resistant Enterobacteriaceae on selective media alone or containing relevant antibiotic concentrations. Information that might explain antibiotic resistance was collected for each dog. Extended-spectrum cephalosporin-resistant isolates were subjected to bacterial species identification (API20E), genetic lineage characterization (MLST), ESBL/pAmpC genes identification (sequencing), and plasmid characterization (pMLST). Regarding beta-lactam resistance, amoxicillin- (AMX) and cefotaxime- (CTX) resistant Enterobacteriaceae were detected in 70 and 18% of the dogs, respectively, whereas for quinolone/fluoroquinolone-resistance, Nalidixic acid- (NAL) and ciprofloxacin- (CIP) resistant Enterobacteriaceae were detected in 36 and 18% of the dogs, respectively. Medical rather than preventive consultation was a risk marker for the presence of NAL and CIP resistance. CTX resistance was mainly due to a combination of specific ESBL/pAmpC genes and particular conjugative plasmids already identified in human patients: bla_CTX−M−1/IncI1/ST3 (n = 4), bla_CMY−2/IncI1/ST12 (n = 2), and bla_CTX−M−15/IncI1/ST31 (n = 1). bla_SHV−12 (n = 3) was detected in various plasmid lineages (InI1/ST3, IncI1/ST26, and IncFII). ESBL/pAmpC plasmids were located in different genetic lineages of E. coli, with the exception of two strains in France (ST6998) and two in Spain (ST602). Our study highlights dogs as a potential source of Q/FQ-resistant and ESBL/pAmpC-producing bacteria that might further disseminate to humans, and notably a serious risk of future acquisition of CTX-M-1 and CMY-2 plasmids by the owners of dogs.

We demonstrate using an in vitro pharmacodynamic model that the likelihood of selection of Escherichia coli mutants resistant to a fluoroquinolone was increased when the initial size of the bacterial population, exposed to fluoroquinolone concentrations within the mutant selection window, was increased.