ARS | Publication request: Antibiotic sensitivity and molecular analyses demonstrate a lack of IncA/C plasmid in modified live Edwardsiella ictaluri vaccine strain

Submitted to: Annual Eastern Fish Health Workshop
Publication Type: Abstract Only
Publication Acceptance Date: April 25, 2010
Publication Date: May 25, 2010
Citation: Lafrentz, B.R., Welch, T.J., Shoemaker, C.A., Klesius, P.H. 2010. Antibiotic sensitivity and molecular analyses demonstrate a lack of IncA/C plasmid in modified live Edwardsiella ictaluri vaccine strain [abstract]. Eastern Fish Health Workshop. p.77.

Technical Abstract: Plasmid mediated antibiotic resistance was first discovered in Edwardsiella ictaluri in the early 1990’s and in 2007, an E. ictaluri strain harboring an IncA/C plasmid was isolated from a moribund channel catfish infected with the bacterium. Due to the recent identification of IncA/C plasmids in aquaculture and the clinical importance of these plasmids, we sought to determine whether the modified live E. ictaluri vaccine strain harbors such plasmids to ensure that use of this vaccine does not contribute to the dissemination of plasmid-borne resistance. Antimicrobial sensitivity testing of the parent and vaccine strains of E. ictaluri demonstrated that both are sensitive to all five of the antimicrobials tested. Genomic DNA from both strains was analyzed by PCR using a set of 13 primer pairs that are specific for regions of the IncA/C plasmid backbone. These results demonstrated a lack of the IncA/C plasmid in these strains; however, one primer set did produce a specific PCR product for both strains. Further analysis of additional E. ictaluri strains confirmed that this DNA sequence is a common feature of the E. ictaluri genome and its presence predates use of the live E. ictaluri vaccine. Our results demonstrate that the vaccine strain does not carry resistance to commonly used antimicrobials and does not show evidence of harboring an IncA/C plasmid and therefore its use should not directly contribute to the dissemination of plasmid-borne resistance. Furthermore, this work highlights the importance of thoroughly investigating potential vaccine strains for the presence of plasmids, or other transmissible elements, encoding resistance to antibiotics.