Infectious Diseases

A clonal population of hyper virulent Aeromonas hydrophila (vAh) has caused the loss of over 10 million kilograms of farmed catfish across the southeastern United States since 2009. Phylogenomic analyses suggest that U.S. catfish isolates emerged from the importation of Asian carp, with outbreaks of A. hydrophila in carp species documented in China since 1989. An ongoing worldwide survey of A. hydrophila isolates revealed that vAh strains have disseminated to multiple countries and can infect diverse fish hosts. We developed two strategies for vAh control, using either probiotic bacteria or an attenuated vaccine. Probiotic studies: We identified Bacillus spp. strains that have the ability to reduce fish mortality due to multiple bacterial pathogens, including vAh. The effect of probiotic-amended feed (106–107 CFU spores/g feed) over 10 or more weeks on catfish growth performance indicated that B. velezensis AP193 induced a mean 9-14% increase in growth compared to control fish and significantly reduced mortality (3%) when challenged with vAh compared to control fish (60% mortality).

Vaccine studies: Comparative genomic analysis of vAh strains from the US and China identified many genetic loci that are uniquely present in vAh strains, including a novel O-antigen biosynthesis gene cluster. Genetic knockouts in the gfc operon responsible for O antigen capsule assembly were found to attenuate vAh virulence, reduce biofilm formation and affect protein secretion. Furthermore, a gfcD mutant was observed to induce an adaptive immune response that protected catfish from challenge with wild-type vAh in aquaria and pond studies.