Modified live Edwardsiella ictaluri vaccine, AQUAVAC-ESC, lacks multidrug resistance plasmids

Plasmid-mediated antibiotic resistance was first discovered in Edwardsiella ictaluri in the early 1990s, and in 2007 an E. ictaluri isolate harboring an IncA/C plasmid was recovered from a moribund channel catfish Ictalurus punctatus infected with the bacterium. Due to the identification of multidrug resistance plasmids in aquaculture and their potential clinical importance, we sought to determine whether the modified live E. ictaluri vaccine strain in AQUAVAC-ESC harbors such plasmids, so that the use of this vaccine will not directly contribute to the pool of bacteria carrying plasmid-borne resistance. Antimicrobial sensitivity testing of the E. ictaluri parent isolate and vaccine strain demonstrated that both were sensitive to 15 of the 16 antimicrobials tested. Total DNA from each isolate was analyzed by polymerase chain reaction (PCR) using a set of 13 primer pairs specific for conserved regions of the IncA/C plasmid backbone, and no specific products were obtained. PCR-based replicon typing of the parent isolate and vaccine strain demonstrated the absence of the 18 commonly occurring plasmid incompatibility groups. These results demonstrate that the vaccine strain does not carry resistance to commonly used antimicrobials and provide strong support for the absence of IncA/C and other commonly occurring plasmid incompatibility groups. Therefore, its use should not directly contribute to the pool of bacteria carrying plasmid-borne resistance. This work highlights the importance of thoroughly investigating potential vaccine strains for the presence of plasmids or other transmissible elements that may encode resistance to antibiotics.