Heat-killed bacterial preparations ameliorate maternal separation-induced social deficits in rats through species-associated effects

Early-life stress (ELS) is a major risk factor for persistent social dysfunction; however, effective post-weaning intervention strategies remain limited. This study investigated the neurobehavioral effects of post-weaning supplementation with two heat-killed bacterial preparations, Lactiplantibacillus plantarum KABP-051 and Bifidobacterium longum KABP-042, using a maternal separation (MS)-induced ELS model. Sprague–Dawley rats were exposed to MS during postnatal days (PD) 2–20 and fed diets containing either preparation from PD21 onward. Both preparations ameliorated MS-induced deficits in social novelty preference in the three-chamber test (3-CST) in adulthood, but their broader effects differed between the two species-derived preparations. In addition to social novelty deficits, L. plantarum KABP-051 specifically reversed MS-associated increases in total distance traveled during the 3-CST and working memory impairments in the Y-maze test in adulthood. Furthermore, microbiota and transcriptomic analyses during the juvenile period revealed restoration of cecal butyrate-producing potential, as indicated by butyryl-CoA:acetate CoA-transferase gene abundance, and a broad reversal of MS-induced colonic alterations related to energy metabolism, inflammatory signaling, and epithelial remodeling. By contrast, B. longum KABP-042 exhibited more limited colonic effects than L. plantarum KABP-051 but uniquely ameliorated MS-induced alterations in the medial prefrontal cortex (mPFC), partially rescuing parvalbumin-positive interneuron density and social behavior-evoked neuronal activation, as assessed by c-Fos-positive cell density, in adulthood. Our findings suggest that post-weaning supplementation with heat-killed bacterial preparations can mitigate ELS-induced neurodevelopmental vulnerabilities, particularly social deficits, through species-associated mechanisms. These nonviable, food-compatible preparations may provide a practical dietary strategy for modulating the microbiota–gut–brain axis following early-life adversity.