Effect of ampicillin, cephalexin, ceftiofur and tetracycline treatment on selection of resistant coliforms in a swine faecal microcosmos.

AIMS: To analyse and compare the effect of selection power for antimicrobial resistance (AMR) in coliforms of two kinds of ß-lactams-aminopenicillins; ampicillin (Amp) and cephalosporins; cephalexin (Cpn) and ceftiofur (Cef)-and tetracycline (Tet) using an approach based on a swine faecal microcosmos. METHODS AND RESULTS: Sixteen faecal samples from 32 pigs (mixed two by two) were treated with Amp, Cpn, Cef and Tet for 6 h (T6h) at concentrations expected to reach the animals gut when using in vivo standard doses. Controls (no drug added) were also tested. Next, samples were 1 : 100 diluted and left under the same conditions (no antimicrobial added) for further 20 h (T20h). The proportion of resistant coliform bacteria (R coliforms) to each antimicrobial was analysed just before starting the treatment (T0), at T6h and at T20h. Coselection was also studied by replica plating. Treatment for 6 h yielded significant increase in proportion of R coliforms, regardless of the drug and lack of selection pressure showed different effects at T20h depending on the antimicrobial used. Selective pressure was associated with the type of the ß-lactam with Amp selecting for significantly higher numbers of R coliforms than cephalosporins. CONCLUSIONS: AMR development was observed following short treatment, and for Amp and Tet treatment, resistance persisted 20 h beyond the interruption of treatment. An association between kind of ß-lactam and power of selection was found. SIGNIFICANCE AND IMPACT OF THE STUDY: AMR represents a threat to human health globally and antimicrobial treatment of livestock has a direct impact on this problem. Through our approach based on a swine faecal microcosmos, we demonstrated the effect on AMR development of several drugs commonly used in livestock. Cephalosporins, representing last-line antimicrobials in human medicine, exerted lower selective pressure than Amp under the conditions used and yielded higher proportion of multidrug-R strains.

Genetic relatedness of commensal Escherichia coli from nursery pigs in intensive pig production in Denmark and molecular characterization of genetically different strains.

AIMS: To determine the genetic relatedness and the presence of virulence and antibiotic resistance genes in commensal Escherichia coli from nursery pigs in Danish intensive production. METHODS AND RESULTS: The genetic diversity of 1000 E. coli strains randomly picked (N = 50 isolates) from cultured faecal samples (N = 4 pigs) from five intensive Danish pigs farms was analysed by repetitive extragenic palindromic-PCR (REP-PCR) and 42 unique REP-profiles were detected (similarity <92%). One profile was dominant (67.2% of strains) but farms differed significantly in the diversity of commensal E. coli: between eight and 21 different profiles per farm were detected. One to three strains representing each REP-profile were characterized by multilocus typing scheme-typing, as well as for presence of antimicrobial and virulence genes and serogrouping through microarray analysis. The 42 REP-profiles were classified into 22 different sequence types (ST) with ST10 being the most common, encompassing 10 REP-profiles. Resistance and virulence genes were detected in most of the isolates. Genes encoding AmpC-ß-lactamases and quinolone resistance were found in one and three isolates, respectively. Toxin-producing genes were observed in 20 isolates. CONCLUSIONS: A low genetic diversity was found in commensal gut E. coli from nursery pigs in Denmark. No correlation was observed between REP-profiles, ST-types and resistance/virulence patterns. SIGNIFICANCE AND IMPACT OF THE STUDY: This is the first study analysing in depth the genetic variability of commensal E. coli from pigs in Danish intensive pig production. A tendency for higher diversity was observed with in nursery pigs that were treated with zinc oxide only, in absence of other antimicrobials. Strains with potential to disseminate virulence and antibiotic resistance genes to pathogenic subgroups of E. coli were found to be wide-spread.