Strain Diversity of CTX-M-Producing Enterobacteriaceae in Individual Pigs: Insights into the Dynamics of Shedding during the Production Cycle

Abstract

ABSTRACTThe aim of this study was to evaluate the population dynamics of CTX-M-producingEnterobacteriaceaein individual pigs on a farm positive for CTX-M-14-producingEscherichia coli. Fecal samples were collected once around the farrowing time from five sows and four times along the production cycle from two of their respective offspring. Multiple colonies per sample were isolated on cefotaxime-supplemented MacConkey agar with or without prior enrichment, resulting in 98 isolates identified by matrix-assisted laser desorption ionization–time of flight mass spectrometry and tested forblaCTX-M. CTX-M-positive isolates (n= 86) were typed by pulsed-field gel electrophoresis (PFGE). Plasmids harboringblaCTX-Mwere characterized in 22 representative isolates by replicon typing and restriction fragment length polymorphism. Based on the PFGE results, all individuals shed unrelated CTX-M-14-producingE. colistrains during the course of life. Concomitant shedding of CTX-M-2/97-producingProteus mirabilisorProvidencia rettgeriwas observed in two sows and two offspring. At least two genetically unrelated CTX-M-producingE. colistrains were isolated from approximately one-fourth of the samples, with remarkable differences between isolates obtained by enrichment and direct plating. A clear decrease in strain diversity was observed after weaning. Dissemination ofblaCTX-M-14within the farm was attributed to horizontal transfer of an IncK plasmid that did not carry additional resistance genes and persisted in the absence of antimicrobial selective pressure. Assessment of strain diversity was shown to be influenced by the production stage from which samples were collected, as well as by the isolation method, providing useful information for the design and interpretation of future epidemiological studies of CTX-M-producingEnterobacteriaceaein pig farms.