Cephalexin is a
first generation cephalosporin commonly used in dogs for treatment of
pyoderma. The objective of this study was to evaluate the in vivo effects of
cephalexin on selection of Escherichia coli resistant to extended-spectrum
cephalosporins. A cohort study was conducted on 13 dogs presenting clinical signs of
pyoderma and treated with
cephalexin and 22 healthy dogs that had not been treated with
antibiotics during the previous six months. Selective plating of faeces on MacConkey
agar plates containing
cefotaxime (CTX) yielded growth of CTX-resistant E. coli for eight of the 13 treated dogs (62%), whereas no growth was observed for any of the control dogs (Fisher exact test, P<0.001). PCR and sequence analysis identified
bla(CMY-2) in all eight dogs. PCR-based replicon typing and restriction fragment length polymorphism (RFLP) of E. coli transformants revealed location of
bla(CMY-2) on indistinguishable IncI1 plasmids in five of the eight dogs. One representative of these five epidemiologically related IncI1 plasmids was further characterized as sequence type (ST2) by plasmid multilocus sequence typing (pMLST). E. coli from the remaining three dogs harboured
bla(CMY-2) on distinct plasmids with non-typeable replicons. A single isolate was classified as an extraintestinal pathogenic E. coli (ExPEC) due to the presence of iutA, papC and sfa/foc. The results provide a strong indication that
cephalexin selects for E. coli producing plasmid-borne CMY-2 β-lactamase. The isolation of a specific IncI1 plasmid carrying
bla(CMY-2) from five epidemiologically unrelated dogs suggests that
cephalexin use may contribute to the spread of this plasmid lineage among Danish dogs.