Abstract
Extended spectrum β-lactamases (ESBLs) are enzymes produced by bacteria that confer resistance to cephalosporins, antibiotics that are widely used to treat infections in animals and humans. The emergence of plasmid-borne ESBL resistance in humans and animals, especially poultry, is of particular concern. Avian pathogenic E. coli (APEC) cause colibacillosis, an economically important disease of poultry, but have also been linked to human disease. Of growing concern is the potential transfer of ESBL-producing plasmids between E. coli isolates and their impact on the host bacterium. Thus, the aim of this study was to characterise ESBL plasmids in APEC isolated from UK poultry and examine their impact on host bacterial fitness and virulence. Plasmid profiling and genome sequencing revealed that all three APEC isolates harboured at least two plasmids, belonging to the IncF and IncI replicon types. The ESBL plasmid was identical in each isolate, being of 105,610 Kb in size, belonging to the IncI1-Iy family and carrying the blaCTX-M-1 allele. Upstream of blaCTX-M-1 gene was an ISEcp1 insertion element, which likely contributes to the transmissibility of the element. The plasmid also carried genes coding for tetracycline resistance that were co-transferred with the β-lactamase genes. The APEC isolates also harboured a large virulence plasmid of similar size, but belonging to the IncF replicon type. Competitive growth studies in rich or minimal media found no significant differences in the growth of CTX-M-1 plasmid-harbouring and plasmid-cured derivatives. However, the presence of the CTX-M-1 plasmid in APEC O78 was beneficial to their growth in iron-limited media, but caused a greater fitness burden at low pH. When present as the only plasmid in APEC, the ESBL-containing derivatives produced more biofilm at human and chicken body temperatures and were better able to invade and survive in human (THP-1) and chicken (HD11) macrophage cell lines than the cured isolates. Finally, the substrate utilisation profiles of the plasmid-containing derivatives differed to that of the parent strains, indicating that the plasmid may influence the metabolic capability of the organism. The studies presented here indicate that ESBL plasmids of the IncI1-1Y type spread readily to unrelated APEC isolates and can influence the fitness of APEC under certain conditions.