Amoxicillin therapy of poultry flocks: effect upon the selection of amoxicillin-resistant commensal Campylobacter spp

Background: The aim of this study was to investigate the effect of amoxicillin therapy of poultry flocks upon the persistence of commensal Campylobacter spp. and the incidence of antibiotic resistance. Methods: Four poultry flocks naturally colonized with Campylobacter were treated with amoxicillin and monitored before, during and up to 4 weeks post-treatment. The numbers of Campylobacter were determined and the isolates speciated and typed by flaA short variable region (SVR) sequence analysis and PFGE. The susceptibility of the isolates to antibiotics, presence of the Cj0299 gene encoding a beta-lactamase and beta-lactamase production (nitrocefin hydrolysis) were also determined. Results: Amoxicillin-resistant Campylobacter were isolated from Flock 1 before and during treatment, but Campylobacter were not detected afterwards. Flock 2 was colonized by amoxicillin-susceptible strains throughout sampling. No amoxicillin-resistant isolates arose during or after treatment. Flock 3 contained amoxicillin-susceptible and -resistant types pre-treatment. Resistant isolates were detected during treatment, while anti biotic-susceptible isolates re-emerged at 3 weeks post-treatment. All Campylobacter isolates from Flock 4 were amoxicillin resistant, irrespective of sampling time. All but one of the 82 amoxicillin-resistant (MICs 16 to >128 mg/L) Campylobacter jejuni and Campylobacter coli tested for the presence of Cj0299 carried the gene and all of these produced beta-lactamase. Co-amoxiclav remained active against amoxicillin-resistant isolates. Conclusions: Amoxicillin therapy had little effect on the numbers of amoxicillin-resistant commensal Campylobacter except for one flock where amoxicillin-resistant Campylobacter temporarily dominated. Amoxicillin therapy did not select amoxicillin-resistant isolates from a previous susceptible strain. Co-amoxiclav remained active against amoxicillin-resistant isolates.