Probiotic Bacillus subtilis, but Not a Lactobacillus spp., Ameliorates Cognitive Impairment in a Mouse Model of LPS and Zidovudine-Induced Neuroinflammation

<b>Background/Objectives:</b> The gut-brain axis is increasingly recognized as a critical modulator of cognitive function. This study investigated the neurotoxic effects of combined exposure to bacterial lipopolysaccharide (LPS) and the antiretroviral drug zidovudine (ZDV) in a mouse model, and evaluated the protective potential of two probiotic interventions: Bacillus subtilis and a mixture of lactobacilli. <b>Methods:</b> Cognitive function was assessed using the Morris water maze (MWM). Gut microbiota composition was analyzed by 16S rRNA sequencing, and intestinal morphology was examined histologically. Gene expression of neuroinflammatory markers and mitophagy-related genes in brain tissue was quantified by RT-PCR. Plasma levels of cell-free mitochondrial DNA (cf-mtDNA) were measured as a marker of mitochondrial damage. <b>Results:</b> Combined LPS + ZDV exposure induced systemic inflammation, impaired spatial memory, damaged the intestinal mucosa, and caused dysbiosis characterized by an increase in pro-inflammatory <i>Muribaculaceae</i>. In the brain, LPS + ZDV significantly upregulated <i>Tnfa</i> expression, confirming neuroinflammation. <i>Bacillus subtilis</i> administration prevented cognitive deficits, maintained <i>Tnfa</i> at control levels, and significantly reduced <i>Il1b</i> and <i>Il6</i> expression compared to the LPS + ZDV group. This was accompanied by activation of the PINK1/PTEN-dependent mitophagy pathway, prevention of cf-mtDNA release, and restoration of gut microbial diversity. In contrast, the <i>Lactobacilli</i> mixture not only failed to improve outcomes but was associated with exacerbated intestinal damage, more pronounced cognitive dysfunction, and no reduction in neuroinflammatory markers. <b>Conclusions:</b> Combined exposure to LPS and ZDV induces gut-brain axis dysfunction characterized by neuroinflammation, cognitive impairment, intestinal damage, and dysbiosis. <i>Bacillus subtilis</i> effectively preserves cognitive function through activation of PINK1/PTEN-dependent mitophagy and suppression of neuroinflammation, highlighting its potential as a therapeutic candidate for cognitive impairments associated with gut-brain axis dysfunction. The contrasting effects of the lactobacilli mixture underscore the critical importance of strain-specificity in probiotic interventions.