Purpose: The purpose of this study was to identify the earliest time when animals are exposed to ESBL-producing E. coli.
Methods: Meconium samples were collected from the rectal anal junction of 322 newborn calves. ESBL-producing E. coli were identified from the samples by plating on MacConkey agar supplemented with Cefotaxime (4 µg/mL). Isolates were further characterized with ChromAgar E. coli and CTX-M gene amplification using PCR. Illumina MiSeq was employed for Whole Genome Sequencing (WGS) of 37 strains from 24 calves. After assembly using SPAdes2.0, nineteen representative strains were selected, based on their Sequencing Types (STs) and whole genome architecture, for further bioinformatics analyses and antimicrobial susceptibility test.
Results: ESBL-producing E. coli was detected in 7.5% (24/322) of meconium samples of newborn calves. Following WGS, phylogenetic analysis revealed that these strains clustered into 8 groups that coincided with their STs. All the isolates carried a variety of virulence genes and were resistant to multiple antibiotics, suggesting that these strains may threaten public health if they contaminate food products. In particular, we identified hyper-virulent strains of ST117 that harbored Shiga toxin-encoding genes (stxAB), which may cause severe human diseases.
Significance: This was the first study that accessed the prevalence and characterization of ESBL-producing E. coli in meconium of newborn calves, indicating animals are even start to be exposed to AMR bacteria in the uterus.