To investigate the mechanisms leading to an increase in the prevalence of blaCMY-2 conferring resistance to ceftiofur in pigs receiving a feed medicated with chlortetracycline and penicillin, and to examine the effect of supplementation with a clay mineral on this phenomenon.Methods and Results
In 138 blaCMY-2-positive Escherichia coli isolates from faeces of pigs receiving feed supplemented or not with 2% clinoptilolite, from day 2 to day 28 after weaning, isolates from the two groups differed significantly with respect to their phylogenetic group: phylotype A predominated in the supplemented group, whereas phylotypes B1 and D predominated in the control group, as determined by PCR. In 36 representative isolates, pulsed-field gel electrophoresis and antimicrobial susceptibility testing revealed that the blaCMY-2-positive E. coli isolates were polyclonal with diverse antimicrobial resistance patterns and blaCMY-2-carrying plasmids of incompatibility (Inc) groups, A/C, I1 and ColE were observed in transformants as detected by PCR. Enterobacter cloacae possessing blaCMY-2-carrying IncA/C plasmids were found in the pens before introduction of this batch of pigs. The blaCMY-2-positive E. coli isolates were more clonally diverse in the control group than the supplemented group.Conclusions
The blaCMY-2 gene appears to have spread both horizontally and clonally in this batch of pigs and may have spread from previous batches of pigs via plasmids carried by Ent. cloacae and expanded in animals of the present batch in the presence of the selection pressure due to administration of chlortetracycline and penicillin in the feed. Feed supplementation may have an effect on clonal diversity of blaCMY-2-positive isolates.Significance and Impact of the Study
Implementation of improved hygiene measures, decreased administration of certain antimicrobials on farm and feed supplementation with certain ingredients may limit antimicrobial resistance spread between and within batches of animals.