Variations of theEscherichia colipopulation in the digestive tract of broilers

We explored the between-group and temporal variations in the intestinalEscherichia colipopulations of broilers under experimental conditions, taking both antimicrobial resistance and virulence into consideration. Four replicates of 45 commercial chicks were reared in four animal facilities. On their first day of life (Day 0), they were orally inoculated with two extended-spectrum-cephalosporin-resistant (ESCR)E. coli(2.72 log(10)CFU of abla(CMY-2-)and 2.55 log(10)CFU of abla(CTX-M-)carryingE. coli). Faecal samples were then collected weekly and caecal samples were obtained from birds sacrificed on Days 21 or 42. The total, ESC-, ciprofloxacin- and gentamicin-resistantE. colipopulations were enumerated on MacConkey (MC) and MC-supplemented media, and eight virulence-associated genes (VAGs) (iroN, iutA, iss, ompT, hlyF, vat,frz(orf4), andfyuA) were sought by PCR on isolates obtained on MC agar. The results showed significant between-group differences in the size of the resistant sub-populations and the presence of VAGs. Contrary tobla(CTX-M)-positive strains,bla(CMY)-positive strains persisted up to Day 42, but represented only a minor fraction of the totalE. colipopulation. The ESC-, gentamicin- and ciprofloxacin-resistant populations decreased over time. Isolates obtained during the first week contained a mean of 5.1 VAGs. The percentages of some VAG profiles differed between faecal isolates on Day 41 and caecal isolates on Day 42. The fluctuations or differences betweenE. coliisolates according to group, age, and faecal or caecal origin need to be considered when designing experimental protocols and seeking to improve colibacillosis control.