Identification of b-Lactamase-Encoding (bla) Genes in Phenotypically b-Lactam-Resistant Escherichia coli Isolated from Young Calves in Belgium

The bla genes identification present in 94 phenotypically resistant Escherichia coli isolated from feces or intestinal contents of young calves with diarrhea or enteritis in Belgium was performed by microarrays (MA) and
whole genome sequencing (WGS). According to their resistance phenotypes to 8 b-lactams at the disk diffusion
assay these 94 E. coli produced a narrow-spectrum-b-lactamase (NSBL), an extended-spectrum-b-lactamase
(ESBL) or a cephalosporinase (AmpC). All ESBL-encoding genes identified by MA and WGS belonged to the
blaCTX-M family, with a majority to the blaCTX-M-1 subfamily. Two different genes encoding an AmpC, blaCMY-2,
and blaDHA-1 were detected in isolates with an AmpC phenotype. The blaTEM-1 and the blaOXA-1 were detected
alone in isolates with a NSBL phenotype or in combination with ESBL-/AmpC-encoding bla genes. Furthermore, the WGS identified mutations in the ampC gene promoter at nucleotides -42 (C>T) and/or -18 (G>A) that
could not be identified by MA, in several isolates with an AmpC-like resistance phenotype. No carbapenemaseencoding gene was detected. To our knowledge this is the first survey on the identification of bla genes in E. coli
isolated from young diarrheic or septicemic calves in Belgium.