Analysis of luminescent intestinal microflora of fish in cold seas: the White, Bering and Okhotsk Seas

The present study investigates the species composition of luminescent bacteria that live in the intestinal tract of fish in cold sea waters in summer and winter. Samples were collected during two expeditions — winter 2010 and summer 2014 — in order to study the possibility of seasonal changes in the composition of intestinal microflora or free-living pelagic flora. To conduct the search of fluorescent symbionts and parasites, including potential pathogens, samples were collected from fish of the White, Bering and Okhotsk seas. The result of the expedition was the allocation of 21 winter strains of luminescent bacteria and 25 ones in summer samples. Species replacement of bacteria inhabiting the intestines of fish from the Bering and Okhotsk seas was demonstrated. It was probably associated with seasonal fluctuations in temperature of the sea surface waters: in summer P. phosphoreum bacteria species displace other bacteria and A. logei were prevailing in the winter cold. It is remarkable that both species belong to psychrophilic. Presence of psychrophilic luminescent bacteria in intestinal microflora of fish found in the waters of icy seas is frequent, sometimes dominant. Sequence analysis of «house-keeping» genes of two strains identified as A. logei by their biochemical parameters, these strains showed proximity to the species A. salmonicida. This in consistency between data of biochemical and phylogenetic analysis for a number of bioluminescent strains demonstrates the need for reclassification of the species. The variability of non-coding region between conservative genes luxR1 and luxC, contained in the structure of the lux-operon of A. logei was shown. Apparently, the variability of that area connected with the relatively recent loss of ability to express luxI gene, which residues are present in the area of luxR1-luxClux-operon