A Bacillus cereus NY5 strain from tilapia intestine antagonizes pathogenic Streptococcus agalactiae growth and adhesion in vitro and in vivo

Abstract

Bacillus cereus NY5 is a potential probiotic isolated from healthy tilapia (Oreochromis niloticus) gut. In this study, the antagonism of this bacterium against pathogenic Streptococcus agalactiae WC1535 from tilapia was evaluated in vitro and in vivo. Through antagonistic culture, artificial infection, histological micro- and ultramicroscopic detection, and gene expression monitoring, the ability of NY5 to inhibit the growth of WC1535, attenuate its ability to invade tilapia tissue, promote immune cell secretion, and affect host immune gene expression were studied. When NY5 and WC1535 were co-incubated in BHI (brain-heart infusion) or LB (Luria-Bertani) medium, NY5 effectively inhibited the growth of WC1535. Under immersion infection, NY5 attenuated the ability of WC1535 to adhere to the body surface of fish. In oral infection, NY5 effectively inhibited WC1535 from invading tilapia cells, increased the number of goblet cells in the midgut epidermis, and improved the fish survival rate. Moreover, NY5 increased the mitochondria in the midgut of S. agalactiae-infected tilapia. Under oral infection for 20 h, the relative expression levels of the immune-related genes GATA3, LgM, 1L-10, TCR-β, TNF, and TP3 were higher in tilapia treated with WC1535 than in tilapia treated with both NY5 and WC1535 (NY5-WC1535) and with NY5. The MHCII and TGF-β expression levels were higher in the NY5-WC1535 and NY5 treatments (p < 0.05), and the CD4 and MHCI expression levels were higher in WC1535 and NY5-WC1535 treatments (p < 0.05). These results indicate that B. cereus NY5 can effectively reduce tilapia mortality under S. agalactiae infection. The main antagonistic mechanism appears to be inhibiting the growth of S. agalactiae through nutrient deprivation or by secreting extracellular products and reducing its ability to invade and infect. Antagonism was also achieved by modulating specific and non-specific immunity in tilapia infected with S. agalactiae. The findings suggest that B. cereus NY5 may be a potential bacterial antagonist for controlling S. agalactiae infections in tilapia aquaculture.