Modified Live Edwardsiella ictaluri Vaccine, AQUAVAC-ESC, Lacks Multidrug Resistance Plasmids

Abstract

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.

Received May 6, 2011; accepted July 22, 2011

ACKNOWLEDGMENTS

The authors thank Drew Eason, Jen Harper, and Ning Qin of the U.S. Department of Agriculture–Agricultural Research Service (USDA–ARS) for their technical assistance. The authors also thank Alessandra Carattoli of the Department of Infectious, Parasitic, and Immune-Mediated Diseases at the Istituto Superiore di Sanità (Rome) for providing positive control strains for PCR-based replicon typing. This research was funded by USDA-ARS Current Research Information System project 6420-32000-024-00D (Integrated Aquatic Animal Health Strategies). Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture.