The ANS regulates involuntary physiological processes throughout the body, except for skeletal muscle, providing neural control (152). In the gastrointestinal system, both the SNS and PNS transmit afferent signals arising from the lumen to the CNS (via enteric, spinal and vagal pathways) and efferent signals from the CNS to the intestinal structures (153). The PNS, which includes the vagus nerve, provides both excitatory and inhibitory control over gastric, intestinal, and pancreatic functions (154). On the other hand, the SNS, predominantly inhibits gastrointestinal muscle and mucosal secretion and regulates blood flow through neural-dependent vasoconstriction. The ENS is the third and largest component of the ANS. The individual components of the MGB axis communicate bidirectionally within the ANS. Additionally, in combination with the HPA axis and neuroendocrine signaling, the ANS can induce CNS-modulated changes in the gut (155).

Neurotransmitters provide additional communication mechanisms between the GM and nervous system ( Figure 2 ). Microbes synthesize and metabolize several neurotransmitters, including dopamine, noradrenaline, serotonin, acetylcholine, histamine, and GABA (28). However, these neurotransmitters do not seem to cross the BBB and likely act indirectly to modulate brain function via the vagus nerve or ENS (78). Some neurotransmitter precursors synthesized in the gut may reach the CNS via the circulation and are able to cross the BBB via active transporters (194).

Animal studies provide evidence that microbial modulation of these neurotransmitters may impact host physiology, and preliminary human studies demonstrate that microbiota-based interventions can alter neurotransmitter concentrations (28). Germ-free mice studies have shown significant alterations in multiple neurotransmitter systems and their receptors in several brain regions (124). Similarly, antibiotic administration to deplete the GM can change the levels of neurotransmitters in the gut and blood (195, 196). Furthermore, microbial abundance has been shown to alter the expression of neurotransmitter receptors in the brain (51, 189, 191). Therefore, there is a growing body of evidence suggesting that the GM can ultimately influence the levels of neurotransmitters in the brain and alter brain function and cognition.

The endocannabinoid system (ECS) is a complex signaling system found throughout the body. The ECS is composed of endocannabinoids (eCBs), cannabinoid receptors, and enzymes involved in the synthesis and degradation of endocannabinoids. The two primary endocannabinoids are anandamide (AEA) and 2-arachidonoylglycerol (2-AG) (238). These bioactive lipid mediators are produced from the common phospholipid precursor arachidonic acid and released by various cell types in the body, including neurons, immune cells, and adipocytes (239). They bind high-affinity GPCRs, including cannabinoid receptors type 1 (CB1) and type 2 (CB2) (238). As neuromodulators, eCBs often act in retrograde, released from postsynaptic cells and traveling backward across synapses, where they transiently inhibit the release of either inhibitory GABA or excitatory glutamate from presynaptic terminals (240) ( Figure 4 ).

The ECS modulates a multitude of physiological processes, including the HPA axis (241), cognition, learning and memory (242), intestinal-barrier function (243), inflammation (244), energy metabolism (245), among others (reviewed recently (239)). In response to stress, eCB signaling modulates glucocorticoid and CRH signaling in the brain and is crucial in recovering homeostasis (241, 246, 247). The ECS is also widely expressed in neural tissue of the gut and is critically involved in the maintenance of intestinal homeostasis. It regulates barrier function and permeability through the immune system, epithelial tight junction proteins, and mucous secretion (248). Furthermore, it modulates myenteric neuron activity, SNS and vagal nerve function, and the release of neuropeptides such as ghrelin, leptin, and orexin (249).

The ECS and GM interact to regulate intestinal homeostasis resulting in relevant functional effects in the gut and CNS (238, 246, 248, 250). Dysbiosis affects eCB signaling, and vice versa (243). Germ-free animals demonstrate significant changes in the expression of CB1 and CB2, and synthetic and degradative enzymes throughout the gut (251). Vijay et al. studied the relationships between the ECS, inflammatory cytokines, and the GM using a six-week exercise intervention in humans (252). Changes in eCBs correlated with increased butyrate levels, and decreased TNF, IL-6 and IL-10. Hence, the anti-inflammatory effects of SCFAs may be partly mediated by the ECS. Healthy mice colonized with Candida albicans showed marked anxiety-like behavior and increased corticosterone concentrations that were inversely correlated with forebrain AEA, demonstrating disruption of the HPA axis through dysregulation of the ECS (253). Both animal and human studies have shown that the ECS and GM play a role in cognitive decline (254). Although microbes secrete eCBs, their role in host physiology remains unclear (255).

Short chain fatty acids (SCFAs) are small organic monocarboxylic acids produced by bacterial fermentation of non-digestible polysaccharides in the large intestine. The main SCFAs are butyrate (C4), propionate (C3), and acetate (C2) (132). SCFAs are absorbed by colonocytes via monocarboxylate transporters (MCTs) or via non-ionic diffusion across the epithelium (131, 273).

SCFAs are a source of energy and trophic factors for cells of the colon and liver (274). Additionally, they can bind GPCRs, specifically the free-fatty-acid receptors FFA2, FFA3, Olfr78, and GPR109a, located throughout the body, including enteroendocrine, immune, and neural cells (275–278). This suggests that SCFAs play a key role in neuro-immuno-endocrine regulation (279–282) ( Figure 4 ). Indeed, extensive evidence supports pleiotropic roles of SCFAs, which affect several host organs and systems, including the gut and CNS (127, 131, 132). SCFAs have several local effects that improve intestinal health, including the maintenance of intestinal barrier integrity, mucus production, and protection against inflammation (250). These processes are crucial to the gut’s first line of defense. SCFAs promote immunity and suppress inflammatory responses in the intestine and other organs by regulating immune cells such as lymphoid cells, T cells, and B cells (283–285).

By inhibiting histone deacetylase (HDAC) activity, SCFAs also regulate systemic functions, promoting histone acetylation and gene expression in host cells (250). This epigenetic mechanism has been described in gastrointestinal, immune and neurological cells [reviewed (131)].

SCFAs appear to play a significant role in MGB communication (286). Research indicates that SCFAs can indirectly modulate the PNS through expression of FFA3 in the enteric neural plexus, portal nerve, and autonomic and sensory ganglia (131). Activation of FFA3 receptors on vagus nerve cells can result in the activation of various neurons in the CNS, including dynamic regulation of hypothalamic neuronal circuitry (287).

SCFA-induced activation of receptors on enteroendocrine cells can promote gut-brain signaling by inducing hormones such as glucagon-like peptide 1 (GLP1) and peptide YY (PYY), as well as neurotransmitters like GABA and serotonin (276). SCFA-signaling can also induce other hormones, including leptin from adipocytes, and insulin from pancreatic β-cells (288). Additionally, SCFAs can modulate the levels of neurotransmitters and neurotrophic factors and regulate the expression of tryptophan 5-hydroxylase, the enzyme involved in the synthesis of serotonin, and tyrosine hydroxylase, the enzyme involved in the rate-limiting step in dopamine, noradrenaline, and adrenaline synthesis (18, 132, 164, 222).

Moreover, the abundant expression of MCTs on endothelial cells suggests that SCFAs can cross the BBB, which is supported by the presence of SCFAs in human cerebrospinal fluid (CSF) and in brain uptake studies (286). Accumulating evidence supports the idea that SCFAs are necessary for the maintenance of CNS homeostasis, learning and cognition, and reward-associated behaviors (25). SCFAs also influence the integrity of the BBB by upregulating the expression of tight junction proteins (263).

SCFAs may also improve neuronal homeostasis and function by influencing neurotrophic factors such as nerve growth factor, glial cell line-derived neurotrophic factor, and BDNF (119–122). These factors regulate the growth, survival, and differentiation of neurons and synapses in the CNS, and are important for learning and memory. SCFAs can modify neuroinflammation by affecting the morphology and function of glial cells (120, 286, 289, 290). By administering SCFAs to germ-free mice, Erny et al. were able to rescue deficits in microglial immaturity and morphology (120).

There is also evidence to suggest that SCFAs can modulate the HPA axis. In stressed mice, SCFA administration reduced HPA axis hyperactivity and intestinal permeability (291). In humans, a recent triple-blind, randomized, placebo-controlled intervention trial examined the effects of colonic SCFA-mixture delivery in men on responses to psychosocial stress and fear tasks (292). SCFA supplementation was shown to downregulate the HPA axis by significantly attenuating the cortisol response.

Altered SCFA production has also been demonstrated in a variety of neuropathologies (42, 116, 127, 130, 132, 292–295). These findings suggest that SCFAs regulate CNS processes through both direct and indirect mechanisms and may ultimately affect host cognition and response to stress.

Bile acids (BAs) are products of cholesterol metabolism primarily produced in the liver as primary BAs and modified by the GM into secondary BAs through processes such as deconjugation, dihydroxylation, dehydrogenation, and isomerization ( Figure 5 ) (30, 296, 297). While their role in enterohepatic circulation as detergents for lipid digestion is well established, recent studies have also revealed their function as hormones via receptors such as farnesoid X receptor (FXR) and Takeda G protein-coupled receptor 5 (TGR5), with significant regulatory and signaling activities (298). BAs can also activate pregnane X receptors, vitamin D receptors, and glucocorticoid receptors (299). Their functions encompass regulation of motor, sensory, and secretory functions of the gut, intestinal barrier permeability, inflammatory response, and several metabolic processes, including lipid and glucose metabolism, and hepatic gluconeogenesis (300).

The effects of BAs extend beyond the gut, impacting various tissues throughout the host. BA receptors are present in the brain, and BAs can either be synthesized locally or actively transported across the BBB by BA transporters from circulation (297, 301, 302). Consequently, circulating BA levels significantly influence the CNS’s BA profile (303). FXR knockout mice exhibit abnormal BA and neurotransmitter concentrations, resulting in impaired cognition and motor coordination (304). TGR5, expressed in brain and peripheral neurons, as well as glial and microglial cells, can be activated by several neurosteroids (305). Specific BAs demonstrate neuroprotective effects in cellular and animal models, with human clinical trials underway (306–308).

BAs play a role in regulating the HPA axis. BAs modulate HPA axis activity by inhibiting CRH release through FXR activation, expressed in the hypothalamus (309). Additionally, BAs can interact with TGR5, expressed in the hypothalamic PVN, stimulating the HPA axis by increasing CRH (310, 311). Cholestasis, associated with suppression of the HPA axis, likely due to BA interactions with glucocorticoid receptors in the brain (299, 312). The discovery of FXR and TGR5 receptors in the adrenal gland further connects BAs with glucocorticoid metabolism (311, 313–315). BAs might act via TGR5 in a cAMP/protein kinase A (PKA)-dependent fashion phosphorylating and thus activating steroidogenic acute regulatory protein (StAR) and hormone sensitive lipase (HSL) (316). FXR activation is known to regulate lipoprotein receptors and transporters, as well as enzymes in the steroidogenic pathway, and has been shown to increase corticosterone levels in mice (311).

The GM influences BA metabolism, and BAs affect the GM’s composition (296). Specific microbes directly contribute to BA transformation, impacting the BA pool’s composition and size (317). Some BAs serve as substrates for gut microbes, while others exhibit antimicrobial properties, actively shaping the GM at the highest taxonomic levels (318). The BA-microbiota axis modulates the immunoregulatory environment along the gut (303). Given this close bidirectional relationship between BAs and the GM, these metabolites have emerged as important modulators of the MGB axis, functioning directly via BA receptors in the ENS and brain or indirectly via GLP-1 or the FXR-FGF15/19 axis (297, 319). Changes in the GM’s composition correspond to changes in blood and brain BA profiles, which are essential because specific BA ligands’ distinct physicochemical properties determine the potency of BA receptor activation (320–323).

Altered BA profiles have been observed in several neuropathologies associated with cognitive decline, such as Alzheimer’s disease (302, 324). BA disorders are also associated with neural symptoms (300). BAs may substantially affect cognitive function by their affinity to muscarinic receptors, as well as GABA and NMDA receptors (325). Germ-free mice excrete less fecal BAs, have a larger BA pool, and have different gene expression profiles involved in BA metabolism, than wild-type mice (323). Postnatal maturation of the GM in newborn mice was shown to be dependent on BAs and neonatal cholestasis is associated with dysbiosis in infants (326, 327). Bile duct ligation alters the GM composition, and increases the permeability of the BBB (328, 329). In a study of patients with depression and anxiety, BA profiles associated with altered GM composition were significantly different in those with more severe symptoms, and specific BA parameters were able to distinguish treatment failures from remitters (330).

Branched-chain amino acids (BCAAs) are essential amino acids including leucine, isoleucine, and valine, which participate in various biochemical functions, including energy production, protein synthesis, insulin secretion, brain amino acid uptake, and immunity (331). The GM produces higher proportions of specific BCAAs (valerate, isobutyrate, and isovalerate) relative to other amino acids, which have been shown to influence epithelial and mucosal homeostasis (332). Additionally, BCAAs can be utilized by microbes, potentially regulating intestinal microbial species, diversity, and metabolism (333, 334).

BCAAs regulate key signaling pathways, most notably the activation of mechanistic target of rapamycin (mTOR), which serves as the master regulator of cell growth and proliferation (331). Mice supplemented with a BCAA-enriched cocktail exhibited improved physical endurance and an extended lifespan (335).

In the CNS, BCAAs play roles in protein synthesis, food intake regulation, and serve as nitrogen donors involved in intercellular shuttling and the synthesis of the neurotransmitters glutamate and GABA (both modulate the HPA axis) (336). Mice deprived of leucine had increased HPA axis activation via CRH expression (337). Excessive BCAA concentrations are considered toxic and can cause tissue damage, particularly in the CNS (338). Although exploratory studies remain in their infancy, evidence suggests that BCAA modulation may be useful in cognition disorders (339–341). Further research is required to determine the relationship between the GM, BCAAs, HPA axis, and cognition.

The microbiota and HPA axis develop rapidly and profoundly in the first years of life, and environmental stressors can affect both (146–148). Stress experienced during different periods of life can have varying physiological consequences. Early life stressors, and in utero stressors, can impact the development and function of the HPA axis (149). Stress during pregnancy disrupts the vertical transmission of microbes from mother to offspring, leading to alterations in the maternal microbiota, which are then transferred to the offspring (342). Jašarević et al. demonstrated changes in the microbiome of these offspring, as well as alterations in the metabolome of the gut and brain (343). In a subsequent study, it was shown that FMT from stressed dams into stress-naïve germ-free mice was sufficient to instill the phenotype observed in stress-exposed offspring (344).

Early life stress, such as maternal separation, activates the HPA axis with associated changes to the developing microbiota, ultimately leading to an imbalance in the GM and an inappropriate stress response (18). Several studies have demonstrated that neonatal stress can lead to short- and long-term alterations in the diversity and composition of the GM (345–347). Interestingly, these consequences appear to be age-dependent. In response to early life stress, younger rats had increased neurogenesis, decreased BDNF IV promoter histone methylation, with a complementary increase in hippocampal BDNF concentration, and associated improvements in spatial and non-spatial learning (348). In contrast, middle-aged rats demonstrated opposing changes, concomitant with impairments in hippocampal-dependent cognitive tasks. These discordant results illustrate the biphasic consequences of early life stress and indicate a role for epigenetic modification of BDNF expression. Furthermore, chronic antidepressant treatment post-exposure was able to rescue the neurological decline observed in the middle-aged rats (348).

A landmark paper by Sudo et al. provided evidence for the essential role of the GM in programming the stress response by illustrating the differences in HPA axis hormones and receptors in germ-free mice when compared to specific pathogen free (SPF) mice (51). Germ-free mice had increased acetylcholine, ACTH, and corticosterone responses following acute stress, indicative of enhanced HPA axis activity. Moreover, these animals showed decreased expression of NMDA receptor subunits (NR-1) in the cortex, and of NR-2a in the cortex and hippocampus, while BDNF levels were lower in the cortex and hippocampus. Following chronic restraint stress, germ-free mice showed significantly greater HPA axis activity, whereas the SPF mice exhibited more anxiety-like behaviors under the same stress (349). These findings have subsequently been reproduced, with both male and female mice demonstrating enhanced stress reactivity to a novel environmental stressors (189, 350, 351). Indeed, germ-free animals show widespread neurodevelopmental changes, associated with alterations in monoaminergic neurotransmission in the CNS (189).

Animal models of chronic stress demonstrate altered intestinal physiology and GM composition, with an increased secretory state and permeability (352–355). Alterations in gut barrier integrity enable bacteria (and microbial-products) to translocate across the mucosa and epithelium, interfacing with immune and neuronal cells (356, 357). Mounting evidence suggests that chronic interactions can lead to systemic, low-grade inflammation contributing to the development of autoimmune, metabolic, and cognitive disorders (358). Moreover, exposure to chronic stress and the subsequent disruption of GM stability has been shown to increase host susceptibility to infection. Mice exposed to prolonged restraint stress demonstrated changes in GM composition, such as bacterial overgrowth and reductions in diversity and richness (359). When challenged orally with the enteric murine pathogen C. rodentium, chronically stressed mice had an increased pathogen load and increased colonic TNF-α expression. Probiotics rescued these changes in the GM and the associated host-microbe interactions. A study by Allen et al. demonstrated that the GM is necessary for stress-induced immunomodulation, with enhancement of splenic macrophage reactivity occurring in colonized controls but not in germ-free mice, in response to social disruption stress (360).

The impact of the GM on the HPA axis can further be interrogated following deliberate interventions. Chronic antibiotic treatment led to a decrease in CRH receptor mRNA levels in the brains of rats (361). The introduction of pathogenic bacteria reduced cognitive abilities and heightened anxiety-like behaviors (362, 363). Exposure of neonatal animals to low-dose endotoxins resulted in the activation of TLRs (364, 365). In addition, they showed long-term HPA axis alterations in activity, as evidenced by increased mean glucocorticoid concentrations resulting from an increase in glucocorticoid pulse frequency and amplitude. In an animal model of diet-induced obesity, anxious and depressive-like behaviors were associated with decreased hippocampal levels of glucocorticoid receptors and an exaggerated HPA axis-mediated stress response to acute physical and social stress (366). The probiotic B. pseudocatenulatum (CECT 7765) reversed the glucocorticoid receptor and stress response abnormalities, along with the neuro-behavioral phenotype. Probiotic treatment with Lactobacillus sp. concurrent with early life maternal separation stress could normalize HPA activity (346). Similarly, pre-treatment with L. farciminis reduced HPA hyper-reactivity, intestinal permeability, and neuroinflammation resulting from restraint stress (367). When L. rhamnosus was administered to mice, region-dependent alterations in GABA receptor expression in the brain were found to parallel the reduction in stress-induced glucocorticoid levels (191). Notably, the neurochemical effects were not found in vagotomised mice, implicating the vagal pathway. Recently, the GM was shown to influence the expression of genes encoding proteins that participate in the HPA axis and the peripheral metabolism of glucocorticoids (368). A study by Mudd et al. reported a predictive relationship between levels of fecal Ruminococcus, serum cortisol, and brain N-acetylaspartate in young pigs (126).

Although there remains a dearth of evidence from human studies, a recent pilot study of 34 healthy infants found that GM composition at one month (measured as alpha diversity) was positively associated with HPA axis reactivity following a painful stressor (369). In a larger cohort of 193 babies (aged 2.5 months), the cortisol stress response was weakly associated with alpha diversity (370). In healthy adults, the experimental administration of LPS in a randomized control trial (RCT) caused a transient physiological stress response, with dose-related increases in cortisol, noradrenaline, body temperature, pulse rate, and cytokines (371). This stress response was associated with increased anxiety and depressed mood. Alterations in cognition occurred both in the short- and long-term, confirming mechanisms for both the promotion and inhibition of cognitive performance during acute inflammatory stress.

In another RCT, a combination probiotic (L. helveticus (R0052) and B. longum (R0175)) administered to healthy volunteers was associated with beneficial psychological effects in participants and a decrease in 24-hour urinary cortisol, suggesting attenuation of the HPA axis in response to stressors (372). A recent meta-analysis of RCTs focusing on the efficacy of probiotics on stress in healthy individuals showed that probiotic use generally reduced subjective stress levels and appeared to alleviate stress-related sub-threshold anxiety and depression (373). However, cortisol levels were not significantly altered. In a small but detailed four-week study investigating the role of dietary fibre and fermented foods on the GM profile and function, including stress and overall health, subtle GM composition changes were associated with significant changes in several faecal lipids and urinary tryptophan metabolites (374). Participants reported reductions in perceived stress, but these were not significantly different to controls, and markers of stress were unaffected. However, the reduction in perceived stress was dose-dependent, with higher dietary adherence resulting in larger reductions in stress.

Cognition is defined as the complex mental process of acquiring, understanding, and storing information through thought, experience, and the senses. Essentially, it is the ability to perceive and react, process and understand, store and retrieve, and make decisions and produce appropriate responses. Cognition is not a singular concept, and various ‘domains’ (functions) with several components have been identified. Cognitive dysfunction typically manifests as impairment in one or more aspects of memory, language, visuospatial, execution, computation, understanding, or judgment, amongst others. An increasing body of preclinical and human evidence demonstrates that the MGB axis plays important roles in the development and maintenance of various components of cognition (191, 362, 375).

Studies in germ-free mice have shown that, in the absence of microbes, the brain is markedly affected, exhibiting deficits in learning, memory formation and recognition, and social and emotional behaviors (58, 123, 124, 362). Gareau et al. demonstrated impaired short-term recognition and working memory in 5 to 6-week-old germ-free mice when compared to conventionally reared counterparts (362). Behavioral differences in germ-free mice include their anxiolytic-like manner when compared to SPF controls, although some studies report increased anxiety-like behavior in different species (124, 189, 351, 376). Microbial colonization has been shown to rescue these elements of cognition, but only if administered during early stages of life (124, 350). Germ-free mice present with social cognitive deficits, which may be associated with biochemical changes, such as decreased hippocampal BDNF and c-FOS expression, important in memory (124, 362, 376, 377). Further studies have demonstrated that reduced cognitive function is inversely associated with BDNF mRNA levels (124, 350).

Several metabolomic studies show alterations in the dopamine and serotonin systems (124, 189, 350, 378–380). These changes include increased hippocampal dopamine D1 receptor mRNA levels and decreased dopamine D1 receptor in the striatum and nucleus accumbens, as well as increased serotonin in the blood and hippocampus, with decreased serotonin receptor expression, respectively.

The role of sexual dimorphism must be noted, as several studies have documented sex-specific differences in the serotonergic system of germ-free animals (350). Female offspring displayed increased cognitive deficits and anxiety-like behavior following prenatal stress, which were associated with GM alterations in the pregnant females and increased IL-1β and decreased BDNF levels in utero (381). In contrast, male offspring presented with deficits in social cognition only, without disturbances in the other behavior or cognitive parameters measured (146).

The use of broad-spectrum antibiotics provides another modality to study the cognitive deficits induced by dysbiosis. Administration has been shown to alter tryptophan metabolism and BDNF, NMDA receptor subunit 2B, serotonin transporter, neuropeptide Y, oxytocin, noradrenaline, and vasopressin expression (382, 383). Möhle et al. administered long-term antibiotics to adult mice, resulting in a decrease in hippocampal neurogenesis and memory retention (384). Confirming a role for the GM, these deficits were reversed by a combination of gut flora reconstitution with probiotics and voluntary exercise. The function of the GM was further demonstrated in a recent study comparing antibiotic-treated and germ-free mice with regards to fear extinction learning (385). Without a complex microbiota, both types of mice exhibited altered fear-associated behavior, changes in gene expression in brain cells, and alterations in the firing patterns and rewiring ability of neurons. Selective colonization revealed a critical developmental period, indicating that microbiota-derived signals were required during early post-natal and adult life for normal learning. Given the well-established relationship between glucocorticoids and memory formation, studies like this suggest a complex relationship between the GM, HPA axis and cognitive processes (386).

Animal models can be further used to explore the role of prebiotics and probiotics on cognition and behavior, including changes in depression, anxiety, and stress associated with changes in immune markers, hippocampal synaptic efficacy, and tryptophan metabolism (376, 387). Administering Mycobacterium vaccae, a transient commensal microbe, as a probiotic or vaccine to young adult mice improved behavior, learning, and memory during cognitive testing, confirming the role of the immune and serotonergic systems in MGB pathways (388, 389). The probiotic B. longum (NCC3001) demonstrated vagal pathway-mediated anxiolytic effects in mice and B. longum (1714) showed improved learning and memory after 11 weeks of supplementation (192, 390). Supplementation with various Lactobacillus species resulted in improvements in cognitive abilities and social deficits and these changes were shown to occur alongside altered GABA expression in the brain, as well as significant changes in the oxytocin and vagus nerve pathways (191, 391). Six weeks of administration with the probiotic Clostridium butyricum restored cognitive function in a mouse model for vascular dementia, which was associated with increased butyrate levels in fecal and brain samples, and activation of the hippocampal BDNF-PI3K/Akt pathway (392).

Infection studies are another useful modality. Administration of C. rodentium, combined with acute stress, led to memory dysfunction in young adult germ-free mice (362). This deficit was prevented by daily administration of a Lactobacillus sp. probiotic combination before infection. Humann et al. showed that microbial PGs can traverse the mouse placenta, such that maternal administration of PGs could be detected in the fetal brain (64). The offspring exhibited decreased cognitive function related to TLR-2-mediated neuroproliferation via FoxG1 induction in the cortex. Neonatal LPS exposure leads to similar cognitive deficits (393). Bilbo et al. revealed that neonatal E. coli infection could impair memory in adult rats (394). Intriguingly, the deficit was only observed if an LPS challenge was administered at the time of learning and could be prevented by daily handling of the neonatal rats, which significantly altered their basal HPA axis activity (395).

Animal and human studies indicate an association between prenatal bacterial or viral exposure and the subsequent development of various disorders. Mid-gestation injection of maternal mice with low-dose immunostimulatory polyinosinic:polycytidylic acid (poly I:C) to mimic viral infection significantly impaired non-spatial memory, learning, and motor activity in the offspring at 3 and 9 weeks of age (396). Infection and/or the immune response to mimetics of infectious agents potentially have long-lasting effects on the cognitive abilities of offspring. In humans, the consequences of maternal infection for the microbiota of the offspring, and related cognitive abnormalities, remain unclear and warrant further investigation. These data highlight the interrelatedness of the microbes, the HPA axis, and cognition.