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United States Department of Agriculture

Agricultural Research Service

Research Project: Integrated Aquatic Animal Health Strategies

Location: Aquatic Animal Health Research

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

Authors
item Lafrentz, Benjamin
item Welch, Timothy
item Shoemaker, Craig
item Drennan, John -
item Klesius, Phillip

Submitted to: Journal of Aquatic Animal Health
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 22, 2011
Publication Date: December 15, 2011
Repository URL: http://handle.nal.usda.gov/10113/55869
Citation: Lafrentz, B.R., Welch, T.J., Shoemaker, C.A., Drennan, J.D., Klesius, P.H. 2011. Modified live Edwardsiella ictaluri vaccine, AQUAVAC-ESC, lacks multidrug resistance plasmids. Journal of Aquatic Animal Health. 23(4):195-199.

Interpretive Summary: Plasmids are circular extrachromosomal DNA molecules in bacteria that encode genes with varied functions, including antibiotic resistance. Plasmid mediated antibiotic resistance has been discovered in bacterial fish pathogens, and these pose a potential threat to commercial aquaculture and may represent a human health risk due to the ability of plasmids to be transferred to other bacterial species. Such plasmids have been identified in Edwardsiella ictaluri, a bacterial fish pathogen that causes large economic losses in the commercial channel catfish industry. Due to the identification of multidrug resistance plasmids in aquaculture, we sought to determine whether the modified live E. ictaluri strain, contained in a commercially available vaccine for E. ictaluri (AQUAVAC-ESC®), harbors such plasmids to ensure that use of this vaccine does not contribute to the dissemination of plasmid-borne resistance. The vaccine strain was analyzed for the presence of a type of multidrug resistance plasmid (IncA/C) and other plasmid types using antibiotic sensitivity and molecular assays. The results demonstrated that the vaccine strain does not carry resistance to commonly used antimicrobials and does not harbor IncA/C or other plasmids associated with multidrug resistance. Therefore its use should not directly contribute to the dissemination of 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.

Technical Abstract: Plasmid mediated antibiotic resistance was first discovered in Edwardsiella ictaluri in the early 1990’s, and in 2007 an E. ictaluri isolate harboring an IncA/C plasmid was recovered from a moribund channel catfish infected with the bacterium. Due to the identification of multidrug resistance plasmids in aquaculture and the potential clinical importance of these, we sought to determine whether the modified live E. ictaluri vaccine strain in AQUAVAC-ESC® harbors such plasmids to ensure that use of this vaccine does 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 thirteen primer pairs specific for conserved regions of the IncA/C plasmid backbone, and no specific products were obtained following amplification. PCR-based replicon typing of the parent isolate and vaccine strain demonstrated an 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.

Last Modified: 4/19/2014
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