Study on selection and transfer of antimicrobial resistant Escherichia coli from broiler breeders to their progeny

Escherichia coli is a normal inhabitant of the intestinal tract of chickens and is harmless as long as its growth and colonization is inhibited by other commensal intestinal microbial populations. When an imbalance in bacterial flora of the intestinal tract occurs, E. coli may grow and cause extraintestinal infections. E. coli disease outbreaks, manifested as septicaemia in both parent flocks and their progeny, are often due to clonal strains which can acquire multiple antibiotic resistance characters along the antimicrobial therapy and eventually transfer it to other intestinal bacteria circulating between birds. The aim of this study was to evaluate the transmission of E. coli clones and their antimicrobial resistant characters from broiler breeders to their progeny in absence or presence of an ongoing E. coli infection. Three groups of chicks (i.e., A1, B1, C1) characterised by omphalitis symptoms and originated from the same group of broiler breeders (R1) were chosen during the first week of life along with three groups (i.e., A2, B2, C2) of healthy chicks originated from the same group of broiler breeders (R2). Among each group 5-10 animals were picked out. Consistently from each group of birds 5-10 animals at 21 days and at the end of the productive cycle were selected. In particular, for sick birds (A1, B1, C1) 5-10 animals were collected before and 5-10 animals after the antimicrobial therapy at 21 days. In parallel, three pools of environmental dust were sampled. Two weeks after the collection of healthy and sick chicks, 5-10 animals of each of the corresponding broiler breeder group (R1 or R2) were also selected. All selected animals were humanely euthanized and from each of them different organs (i.e., liver, spleen, intestine, heart, lungs and tracheal mucus) were collected and tested for the presence of E. coli. In brief, 10 g of dust or sampled organ were diluted 1:10 in Ringer’s Solution (Oxoid, Hampshire, UK). An aliquot was plated on Eosin Methylene Blue agar (EMB) and incubated at 37 °C for 24h in duplicate. From each plate, two colonies showing characteristic E. coli morphology were selected for confirmation through biochemical tests. E. coli isolates were molecular characterised by Pulsed Field Gel Electrophoresis (PFGE), using the standard PulseNet protocol with XbaI as restriction enzyme, and automated ribotyping according to the manufacturer instructions with EcoRI as restriction enzyme (Oxoid, Hampshire, UK). The antimicrobial sensibility of E. coli isolates toward amoxicillin, tetracycline, spectinomycin, enrofloxacin and ciprofloxacin was investigated by the microdilution method following standard procedures reported in the document M31-A2 of the Clinical Laboratory Standard Institute (CLSI). Results on the selection and transfer of antimicrobial resistant E. coli isolates from broiler breeders to their progeny will be discussed at the conference.