Characterization and virulence factors distribution of blaCTX-M and mcr-1carrying Escherichia coli isolates from bovine mastitis

Aim To investigate the occurrence of ESBL and colistin-resistant E. coli and its genotypic characterization and identification of virulence determinants in cases of bovine mastitis in three provinces of China. Materials and methods Five hundred and thirty-five milk samples presenting mastitis symptoms were screened for the presence of ESBL, colistin-resistant and different virulence genes. Susceptibility testing was identified by the micro-dilution method. Escherichia coli isolates were used to detect ESBL genes (bla(CTX-M), bla(SHV) and bla(TEM)) and colistin-resistant genes mcr-(1-9). Multiplex PCR approach was used for the detection of major bla(CTX-M) groups, different phylogroups and virulence genes. The clonal relationship was then evaluated with MLST, MLSA and PFGE. Results Multi-drug resistance (MDR) was found in more than 85% of ESBL and colistin-resistant E. coli isolates. Genotypic characterization showed a dominance of the bla(CTX-M-1) group, and the most prevalent alleles observed were bla(CTX-M-28) (38.37%), bla(CTX-M-14) (17.44%), bla(CTX-M-66) (13.95%) and bla(CTX-M-55) (10.46%). The targeted virulence genes were detected in 97.89% of isolates. Sequence types ST58 and ST410 were the most predominant (2/20 = 20%). The majority of the E. coli isolates carrying ESBL and mcr-1 were clonally unrelated. Conclusion High level of association was observed between ESBL-producing and COL-resistance in E. coli of bovine mastitis. Significance and Impact of the Study To the best of our knowledge, this is the first report which shows the genetic diversity of ESBL and mcr-1, and various virulent features of E. coli strains isolated from bovine clinical mastitis in three different provinces of China. The major carriers of the bla(CTX-M-1) and bla(CTX-M-9) were bla(CTX-M-28) and bla(CTX-M-14) alleles respectively. The association of ESBL-producing E. coli with mcr-1 is of particular concern.