To assess the ability of human intestinally derived strains of Lactobacillus and Bifidobacterium to produce γ‐aminobutyric acid (GABA).

Strains of Lactobacillus and Bifidobacterium were grown in medium containing monosodium glutamate (MSG). Growth of the bacteria and conversion of MSG to GABA were measured. Of 91 intestinally derived bacteria assessed, one Lactobacillus strain and four strains of Bifidobacterium produced GABA. Lactobacillus brevis DPC6108 was the most efficient of the strains tested, converting up to 100% of MSG to GABA. The ability of the cultured intestinal strains to produce GABA was investigated using a simple pH‐controlled anaerobic faeces‐based fermentation, supplemented with 30 mg ml⁻¹ MSG. The addition of Lact. brevis DPC6108 to a faeces‐based fermentation significantly increased the GABA concentration (P < 0·001), supporting the notion that this biosynthesis could occur in vivo.

The production of GABA by bifidobacteria exhibited considerable interspecies variation. Lactobacillus brevis and Bifidobacterium dentium were the most efficient GABA producers among the range of strains tested. The addition of Lact. brevis DPC6108 to the culturable gut microbiota increased the GABA concentration in fermented faecal slurry at physiological pH.

Identification of optimal MSG conversion to GABA by particular cultured elements of the commensal intestinal microbiota and the demonstration that this can occur under simulated in vivo conditions offer new prospects for microbiota modulation to promote health.