Seven-Year Evolution of β-Lactam Resistance Phenotypes in Escherichia coli Isolated from Young Diarrheic and Septicaemic Calves in Belgium

Antimicrobial resistance is a major worldwide hazard. Therefore, the World Health
Organization has proposed a classification of antimicrobials with respect to their importance for
human medicine and advised some restriction of their use in veterinary medicine. In Belgium, this
regulation has been implemented by a Royal Decree (RD) in 2016, which prohibits carbapenem use
and enforces strict restrictions on the use of third- and fourth-generation cephalosporins (3 GC and
4 GC) for food-producing animals. Acquired resistance to β-lactam antibiotics is most frequently
mediated by the production of β-lactamases in Gram-negative bacteria. This study follows the
resistance to β-lactam antibiotics in Escherichia coli isolated from young diarrheic or septicaemic
calves in Belgium over seven calving seasons in order to measure the impact of the RD. Phenotypic
resistance to eight β-lactams was assessed by disk diffusion assay and isolates were assigned to four
resistance profiles: narrow-spectrum β-lactamases (NSBL); extended-spectrum β-lactamases (ESBL);
cephalosporinases (AmpC); and cephalosporinase-like, NSBL with cefoxitin resistance (AmpC-like).
No carbapenemase-mediated resistance was detected. Different resistance rates were observed for
each profile over the calving seasons. Following the RD, the number of susceptibility tests has
increased, the resistance rate to 3 GC/4 GC has markedly decreased, while the observed resistance
profiles have changed, with an increase in NSBL profiles in particular.