Abstract
Edwardsiella ictaluri is a Gram-negative intracellular pathogen causing enteric septicemia of channel catfish (ESC). Type six secretion system (T6SS) is a sophisticated nanomachine that delivers effector proteins into eukaryotic host cells as well as other bacteria. In the current work, we in-frame deleted the E. ictaluri evpB gene located in the T6SS operon by allelic exchange. The safety and efficacy of EiΔevpB as well as Aquavac-ESC, a commercial vaccine manufactured by Intervet/Merck Animal Health, were evaluated in channel catfish (Ictalurus punctatus) fingerlings and fry by immersion exposure. Our results showed that the EiΔevpB strain was avirulent and fully protective in catfish fingerlings. The EiΔevpB strain was also safe in catfish fry, and immersion vaccination with EiΔevpB at doses 106 and 107 CFU/ml in water resulted in 34.24 and 80.34% survival after wild-type immersion challenge compared to sham-vaccinated fry (1.79% survival). Catfish fry vaccinated with EiΔevpB at doses 106, 107, and 108 CFU/ml in water exhibited dose-dependent protection. When compared with Aquavac-ESC, EiΔevpB provided significantly higher protection in catfish fingerlings and fry (p < 0.05). Results indicate that the EiΔevpB strain is safe and can be used to protect catfish fingerlings and fry against E. ictaluri.
Highlights
Channel catfish (Ictalurus punctatus) is the most significant aquaculture commodity in the United States, and Edwardsiella ictaluri causes enteric septicemia of channel catfish (ESC) (Hawke, 1979)
We successfully introduced an in-frame deletion to the evpB gene in the E. ictaluri chromosome
The resulting Ei evpB strain contained a deletion of 1,167 bp out of 1,488 bp open reading frame (78.42%), resulting in loss of 389 amino acids from the E. ictaluri evpB gene
Summary
Channel catfish (Ictalurus punctatus) is the most significant aquaculture commodity in the United States, and Edwardsiella ictaluri causes enteric septicemia of channel catfish (ESC) (Hawke, 1979). Use of antibiotic-added feed (Smith et al, 1994; Plumb et al, 1995) and feed restriction (Wise et al, 2004) are traditional means used to control ESC. These practices could reduce mortalities, feed restriction results in reduced production through lost feeding days. Aquavac-ESC is safe in catfish fry (Klesius and Shoemaker, 1999), but it has not been accepted widely due to marginal economic returns (Bebak and Wagner, 2012). Another live attenuated E. ictaluri vaccine was
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