The internal surface of the animal gastrointestinal tract is covered by microbial biofilms. They play an important role in the development and functioning of the host organism and protect it against pathogens. Microbial communities of gastrointestinal biofilms are less elucidated than luminal microbiota. Therefore, the studies of biofilm formation by gastrointestinal microorganisms are a topical issue. For the first time, we report the formation of a biofilm in vitro by the strains of bioluminescent bacteria isolated from the intestines of marine fish. These bacteria exhibit co-aggregation and tend to attach to solid surfaces. The attachment of cells is accompanied by appearance of the pili. Then, we observed the formation of microcolonies and the production of extracellular polymer substances (EPSs) connecting bacterial cells into an integrated system. The presence of acidic polysaccharides is shown in the EPS when using the ruthenium red staining. Acidic polysaccharides in this matrix is a biochemical evidence of microbial biofilms. On the fibers of the polymer matrix, these bacteria form the "mushroom body"-type structures. Matured biofilms exhibit a specific three-dimensional architecture with pores and channels formed by cells and EPS. We also demonstrated the formation of a biofilm by binary culture of the luminous enterobacterium Kosakonia cowanii and a Gram-positive Macrococcus sp. The data obtained help to understand the role of these bacteria in the intestines of fish. They lead to a new study in the field of investigation of the intestinal microbiome of fish.
Keywords: Aliivibrio; Biofilm; Bioluminescent bacteria; Fish gut microbiota; Photobacterium; Schewanella.