Discovery of genes promoting the growth of Aeromonas salmonicida salmonicida in Atlantinc salmon (Salmon salar) mucus

Aeromonas salmonicida is the pathogen responsible for one of the key infectious fish diseases, namely furunculosis. This bacteria can infect both wild and farmed marine salmonids, and also non salmonids. The disease can occur at any stage of the fish life cycle, reducing farm productivity, and consequently a huge loss to the economy. Therefore, deciphering the mechanisms through which the pathogens cause disease is of great interest. Furunculosis infection in fish can occur in many diverse ways, one of which is direct contact and invasion of A. salmonicida through the fish skin. Thus, to commence infection A. salmonicida can grow in skin mucus; the first line of fish innate immune defence. Therefore, the objective of this present study was to identify virulence factors and the genes responsible for the survival of A. salmonicida in fish skin mucus. The present study included the construction of a mariner based transposon mutant library using the A. salmonicida Hooke isolate and then screening the growth of random transformants in the presence of fish skin mucus. In vitro screening of the A. salmonicida Hooke parent strain demonstrated enhanced growth in the presence of fish skin mucus. Among the 147 transposon mutant strains which were screened for attenuated growth, only 8 transformants had reduced growth in the presence of mucus. However, 3 transformants were sequenced further to identify the gene with transposon insertion. In two transformants, the transposon had inserted into genes encoding general proteins; namely the hydrogenase-2 expression/formation protein and 23s ribosomal RNA, which are involved in general functionality of the bacteria. The transposon insertion in the third transformant was identified in the gene encoding ABC-type transporter protein abcA, which is involved in many diverse functions in bacteria.In particular, The ABC-type transporter protein abcA of A. salmonicida affects the functionality of the virulence factor A-layer protein. This present study has furthered our understanding of the infection biology of the important fish pathogen A. salmonicida, and these findings may assist with new ways to reduce the burden of this bacterium in aquaculture.