Adhesion and colonization properties of potentially probiotic Bacillus paralicheniformis strain FA6 isolated from grass carp intestine

As a prerequisite for probiotics to establish and settle in the host intestine, adhesion is an essential criterion for probiotic screening. However, adhesion of probiotics in the intestines of one of the most important cultured fish species globally, the grass carp Ctenopharyngodon idellus, remains understudied. Here, we assessed the adhesion properties of a novel potentially probiotic bacterial strain isolated from the grass carp intestine: Bacillus paralicheniformis FA6 (henceforth FA6). To indirectly determine the adhesion, we compared the hydrophobicity of FA6 with eight other bacterial strains, and found that FA6 had comparatively high hydrophobicity. To assess adhesion directly, we supplemented the feed with FA6 for 2 weeks, and then used immunofluorescence to study the dynamics of FA6 for 220 days after discontinuing the supplementation. The FA6 fluorescence signal plateaued between days 40 and 60, and remained visible in the grass carp guts until day 80. In addition, FA6 was re-isolated on day 220 from intestinal contents. Our results suggest that FA6 has high adhesion ability, and can colonize the grass carp intestine for long periods of time. This research provides a promising foundation for the application of FA6 in aquaculture.