Clonal spread of Escherichia coli O101:H9-ST10 and O101:H9-ST167 strains carrying fosA3 and blaCTX-M-14 among diarrheal calves in a Chinese farm,with Australian Chroicocephalus as the possible origin of E.coli O101:H9-ST10

During a 2018 antimicrobial resistance surveillance of Escherichia coli isolates from diarrheal calves in Xinjiang Province,China,an unexpectedly high prevalence(48.5%)of fosfomycin resistance was observed.This study aimed to reveal the determinants of fosfomycin resistance and the underlying transmission mechanism.Polymerase chain reaction(PCR)screening showed that all fosfomycin-resistant E.coli carried the fosA3 gene.Pulsed-field gel electrophoresis(PFGE)and southern blot hybridization revealed that the 16 fosA3-positive isolates belonged to four different PFGE patterns(i.e.,A,B,C,D).The fosA3 genes of 11 clonally related strains(pattern D)were located on the chromosome,while others were carried by plasmids.Whole-genome and long-read sequencing indicated that the pattern D strains were E.coli O101:H9-ST10,and the pattern C,B,and A strains were O101:H9-ST167,O8:H30-ST1431,and O101:H9 with unknown ST,respectively.Among the pattern C strains,the blaCTX-M-14 gene was co-localized with the fosA3 gene on the F18:A-:B1 plasmids.Interestingly,phylogenetic analysis based on core genome single nucleotide polymorphisms(cgSNPs)showed that the O101:H9-ST10 strains were closely related to a Australian-isolated Chroicocephalus-origin E.coli O101:H9-ST10 strain producing CTX-M-14 and FosA3,with a difference of only 11 SNPs.These results indicate possible international dissemination of the high-risk E.coli clone O101:H9-ST10 by migratory birds.