Molecular Characterization of Extended-Spectrum β-Lactamase-Producing Escherichia coli Isolates from Milk Samples of Dairy Cows with Mastitis in Bavaria, Germany.

The aim of this study was to determine the rate of extended-spectrum β-lactamase (ESBL)-producing microorganisms among Escherichia coli isolates causing bovine mastitis, including molecular characterization of these isolates. Therefore, a total of 490 bovine E. coli isolates from milk samples of dairy cows with mastitis were investigated for ESBL production by antimicrobial susceptibility testing, PCR-based detection, and sequencing of ESBL encoding genes, which were identified in 22 isolates (4.5%). Moreover, resistance to the fluoroquinolones enrofloxacin and marbofloxacin occurred in 15 of 22 ESBL-producing isolates (68.2%). All ESBL-producing isolates carried a blaCTX-M-like gene, with blaCTX-M-14 (n = 10) as the most prevalent type. Seven isolates producing CTX-M-14 and belonging to phylogenetic group A were further investigated for genetic relatedness by multilocus sequence typing. Five of them could be assigned to four different sequence types (STs): ST10 (n = 2), ST167 (n = 1), ST410 (n = 1), and ST744 (n = 1), whereas the remaining two isolates could not be assigned. To conclude, the rate of ESBL-producing E. coli associated with cattle mastitis was 4.5%. Furthermore, a high proportion of fluoroquinolone coresistance could be detected. Therefore, careful and continuous surveillance of ESBL-producing E. coli in cattle and consequent implementation of prevention measures are needed to avoid a further spread of these multidrug-resistant bacteria.