Functionality and safety assessment of psychobiotic prospective gamma-aminobutyric acid-producing Streptococcus thermophilus, Streptococcus lutetiensis, and Enterococcus faecium strains isolated from Egyptian dairy products

Psychobiotics are probiotics that produce neuroactive compounds, such as gamma-aminobutyric acid (GABA), which acts as a primary inhibitory neurotransmitter, regulates blood pressure, diuresis, and stress reduction. Lactic acid bacteria (LAB) naturally produce GABA and could be used to create GABA-enriched dairy products with health benefits. This study evaluated the GABA production capabilities of LAB isolates from traditional Egyptian dairy products, alongside their safety, probiotic potential, and technological properties to identify psychobiotic candidates. From 1005 LAB isolates screened via the glutamic acid decarboxylase (GAD) colorimetric assay, 72 showed strong glutamate-to-GABA conversion. GC-MS analysis revealed high variability, 20 isolates produced over 7 mg/L GABA, with Streptococcus lutetiensis FFNL3926 producing the highest amount at 5364 mg/L. Selected GABA-producing strains showed tolerance to bile salts well (≥70% survival) and variable acid resistance. Str. lutetiensis FFNL3926 demonstrated superior auto-aggregation and hydrophobicity, suggesting strong gut adhesion, while Enterococcus faecium FFNL3336 and Streptococcus thermophilus FFNL3339 showed moderate but significant adhesion traits. All strains assimilated prebiotics, including lactulose and inulin, and reduced cholesterol by 19–27%. The strains tolerated up to 6% salt and varied in milk acidification capacity, with pH reductions to 4.19 and 4.69 after 24h by Str. thermophilus FFNL3339 and E. faecium FFNL3336, respectively. Diacetyl production was highest in Str. thermophilus FFNL3339. In conclusion, Str. lutetiensis FFNL3926, Str. thermophilus FFNL3339, and E. faecium FFNL3336 exhibit promising probiotic, technological, and GABA-producing properties, supporting their use as psychobiotic cultures in functional fermented dairy products.