Biological diversity of animal-associated lactic acid bacteria as natural producers of gamma-aminobutyric acid (GABA)

Gamma-aminobutyric acid (GABA) is a crucial bioactive compound widely distributed in animal nervous systems, where it serves as the primary inhibitory neurotransmitter and plays a pivotal role in regulating physiological and behavioral processes. In recent years, increasing attention has been directed toward natural, biodiversity-based sources of GABA, particularly microorganisms associated with animals and animal-derived environments. Lactic acid bacteria (LAB), commonly occurring in animal gastrointestinal microbiota and traditional animal-based fermented foods, represent an important but underexplored component of animal-associated microbial biodiversity. This review synthesizes current knowledge on the diversity of animal-associated LAB capable of GABA biosynthesis and highlights their ecological, genetic, and functional significance. Special emphasis is placed on the glutamate decarboxylase (GAD) system (gadA, gadB, and gadC genes), which enables LAB to convert L-glutamate into GABA as part of an acid-resistance mechanism essential for survival in animal-related niches. Comparative analysis of published studies reveals substantial interspecific and intraspecific variation in GABA production among LAB species, with Lactobacillus brevis, L. paracasei, and L. buchneri emerging as prominent GABA producers. Optimal GABA synthesis is generally associated with acidic conditions, moderate temperatures, and sufficient substrate availability. The review further discusses the relevance of animal-derived fermented foods as reservoirs of microbial biodiversity and emphasizes the potential of indigenous LAB strains from animal-origin products as natural sources of GABA. Understanding the biodiversity and functional traits of animal-associated LAB contributes not only to microbial ecology and wildlife-related microbiomes but also to the development of naturally derived functional foods with potential benefits for animal and human health.