Indigenous Lactic Acid Bacteria in Fish and Crustaceans

The gut microbiota of fish plays an important role in mediating and stimulating host gastrointestinal (GI) development, aiding digestive function, maintaining mucosal tolerance, stimulating the host immune response and providing a level of protection against gastric infections. Lactic acid bacteria (LAB), which are generally considered as favourable bacteria due to their abilities to antagonize bacterial pathogens, are often identified as components of the gut microbiota of fish. Members of the Lactobacillus, Lactococcus, Leuconostoc, Enterococcus and Streptococcus genera have been isolated by culture-dependent methods as minor components of the indigenous GI populations in salmonids. Carnobacterium species, such as C. (piscicola) maltaromaticum, C. mobile, C. divergens, C. alterfunitum-like, C. inhibens and Carnobacterium spp., have been thought to be the dominant/core LAB genus in salmonids and can account for up to 15% of the viable culturable populations. Recent culture-independent molecular analyses have led us to question whether Carnobacterium truly are the most prevalent LAB genus in salmonids. Techniques such as denaturing gradient gel electrophoresis (DGGE), temporal temperature gradient electrophoresis (TTGE), and 16S rRNA clone libraries confirm the presence of Carnobacterium spp. but have also identified Lactobacillus spp., Streptococcus spp., Lactococcus lactis, Weissella cibaria and Pediococcus sp. as indigenous species. In warm water fish species (e.g. sturgeon, zebrafish, carp, tilapia, grouper and cobia etc.), members of the genera Lactobacillus, Lactococcus, Enterococcus, Pediococcus, Leuconostoc and Weissella have commonly been identified as the indigenous GI populations, while Carnobacterium species have rarely been identified. The dominant LAB in shellfish belong to the Pediococcus and Lactobacillus genera, including the species Lb. plantarum, Lb. acidophilus, Lb. bulgaricus, Lb. sporogenes, Lb. casei, Lb. brevis, Lb. helveticus and P. acidilactici. Although Bifidobacterium are commonly reported in the GI tract of mammals and terrestrial organisms, they are seldom reported in fish. Future experiments should attempt to quantify these bacterial groups using qPCR, FISH, metagenetic and metagenomic approaches. The importance of these bacteria in mediating host benefits, how they respond to dietary and environmental changes, and their potential to antagonize enteric pathogens, should be topics of future investigations.