Characterisation and mobilisation of IncA/C plasmid-mediated antibiotic resistance in Edwardsiella ictaluri

Abstract

Edwardsiella ictaluri is an important pathogen in farmed raised catfish. Recently, we showed that resistance to tetracycline and florfenicol in the E. ictaluri MS-17-156 strain isolated from channel catfish was facilitated by acquisition of a 135 kb plasmid (named pEIMS-171561). We described the genetic structure of pEIMS-171561. Plasmid copy number and stability within E. ictaluri strain MS-17-156 was determined. We also investigated the in vitro and in vivo transferability of pEIMS-171561 using catfish as a model for in vivo transfer. pEIMS-171561 belonged to the IncA/C group and contained florfenicol efflux major facilitator superfamily (MFS) (floR), sulfonamides (sul2), and tetracycline efflux MFS (tetD) genes. The plasmid contained two conjugative transfer-associated regions and encoded six transposases and insertion sequences. In vitro conjugation experiments demonstrated that the IncA/C plasmid can transfer from E. ictaluri to Escherichia coli. The plasmid was stable in E. ictaluri without selection pressure for 33 days. We showed that pEIMS-171561 did not transfer from E. ictaluri MS-17-156 to endogenous microbiota in catfish. Moreover, we could not detect in vivo conjugal transfer of pEIMS-171561 from E. ictaluri to E. coli. Results from real-time PCR revealed upregulation of the floR gene in the intestines of catfish receiving florfenicol-medicated feed, compared with that in catfish receiving unmedicated feed. This study demonstrated that pEIMS-171561 did not disseminate from E. ictaluri to gut microbiota under selective pressure. This result suggests a limited role of the fish microbiota as a reservoir for this plasmid and for the spread of resistance.