Characterization of plasmids encoding extended-spectrum β-lactamases (ESBLs) and QnrS1 in Avian Pathogenic Escherichia coli (APEC) isolated from commercial poultry flocks in Italy.

Avian pathogenic Escherichia coli (APEC) cause infections with high morbidity and mortality in poultry flocks. The aim of this study was to characterize the mobilizable pool mediating resistance to cephalosporins and fluoroquinolones in APEC collected in Italy between 2008 and 2012. Non-repetitive APEC from turkeys (n=109), broilers (n=98) and layers (n=22) were examined. Isolates resistant to third-generation cephalosporins were screened for presence of blaTEM, blaSHV, blaCTX-M and blaCMY-2 and for chromosomal ampC promoter mutations, while all isolates were tested by PCR for all known plasmid-mediated quinolone resistance (PMQR) genes. ESBL/AmpC or PMQR-harboring plasmids were typed by traditional typing methods. Twenty-eight (12%) isolates displayed resistance to third-generation cephalosporins either mediated by mutations leading to chromosomal ampC overproduction (n=10) or by the following plasmid/gene combinations: IncI1/ST26/blaSHV-12 (n=1), IncI1/ST3/blaCTX-M-1 (n=7), IncI1/ST26/blaCTX-M-1 (n=1), IncI1/ST36/blaCTX-M-1 (n=2), IncI1/STnew/blaCTX-M-1 (n=1), IncN/blaCTX-M-1 (n=1), IncI1/ST26/blaCTX-M-2 (n=1), IncFII/blaCTX-M-14 (n=1), IncK/blaCTX-M-14 (n=1), IncI1/ST26/blaCMY-2 (n=1) and IncK/blaCMY-2 (n=1). Plasmids measured approximately 40 to 200 kb and mainly exhibited different RFLP profiles. Sixty (26%) and 21 (9%) isolates displayed resistance to nalidixic acid and ciprofloxacin, respectively. qnrS1 was detected in two isolates on IncX2 and a non-typeable plasmid of ca. 30 and 40 kb, respectively. The APEC population in Italian poultry harbours diverse ESBL-encoding genes and plasmids, often in association with fluroquinolone resistance. Interestingly, IncI1/ST26 plasmids were associated with four β-lactamases (SHV-12, CTX-M-1, CTX-M-2 and CMY-2), suggesting that this plasmid lineage is well adapted in APEC isolated from Italian poultry production. These findings underline the need to develop new strategies for prevention and therapy of multidrug-resistant APEC infections.