Table 1.

Potential microbial effectors of synaptic plasticity

	Target nervous system	Hypothesized plasticity‐related mechanism affected
Lactobacillus reuteri (LR)	ENS	Calcium‐dependent channels in a subpopulation of enteric sensory neurons, implication in intestinal‐disorder derived pain (Kunze et al., 2009). LR probiotic‐treated mice exhibited reduced capsaicin‐induced excitatory response, accompanied by decreased expression of the nociceptive signal‐transmitting TRPV1 (Perez‐Burgos et al., 2015). Probiotic administration rescued social deficits in ASD mice in a vagus‐dependent manner.
Bifidobacterium infantis	CNS	Probiotic administration led to reduction of the serotonin intermediate 5‐HIAA in the frontal cortex, together with plasma increases of tryptophan, kynurenic acid, dihydroxyphenylacetic acid and noradrenaline (El Aidy et al., 2012).
Lachnospiraceae, Syntrophococcus, Shuttleworthia, Gemella, Allobaculum, and Hydrogenoanaerobacterium	CNS	Changes in dopamine receptor expression, known to contribute to induction of stimulus‐dependent plasticity in areas of the brain responsible for emotional regulation (Jadhav et al., 2018).
Bifidobacterium	CNS	Deficient ventral striatal responses in a reward anticipation fMRI test (Aarts et al., 2017).
Enterobacteriaceae	CNS	Correlated with severity of Parkinson's symptoms (Parashar & Udayabanu, 2017).
Lactobacillus rhamnosus JB‐1 (JB‐1)	CNS	Immune‐mediated modulation of anxiolytic and depressive behaviours (Liu et al., 2020). Affects anxiolytic and depressive behaviours in a vagus‐dependent manner (Perez‐Burgos et al., 2014), and has also been shown to reduce GABA receptor expression in specific brain areas known to undergo extensive learning and emotional‐regulation related plasticity (Bravo et al., 2011). L. rhamnosus ameliorated LPS‐induced impairment of hippocampal CaMKII‐α gene expression.
Bifidobacterium pseudocatenulatum	CNS	Prevention of stress‐induced HPA axis activation and consequent increases of corticosterone and hypothalamic catecholamines (Moya‐Pérez et al., 2017).
Campylobacter jejuni		Increase of anxiolytic behaviours, as well as Fos activity in vagal afferents and the solitary tract nucleus (Fülling et al., 2019).
Lactobacillus acidophilus, Bifidobacterium lactis and/or Lactobacillus fermentum	CNS	Probiotic cocktail led to improvement in learning task performance, as well as rescue of hippocampal EPSP response and markedly improved spatial memory, facilitated by an improved ability of the brain to undergo LTP (Davari et al., 2013).
L. rhamnosus, L. reuteri and/or L. plantarum	CNS	Probiotic administration impeded neuroinflammation‐induced memory deficits in mice (Zolfaghari et al., 2020).
Lactobacillus johnsonii CJLJ103		Probiotic administration of L. johnsonii CJLJ103 (LJ) to mice. LJ corrected two detrimental effects caused by LPS: NF‐κB‐induced hippocampal neuroinflammation and reduced BDNF expression (Lee et al., 2018)

5‐HIAA, 5‐hydroxyindoleacetic acid; ASD, autism spectrum disorder; BDNF, brain‐derived neurotrophic factor; CaMKII‐α, calcium/calmodulin‐dependent protein kinase type II subunit alpha; CNS, central nervous system; ENS, enteric nervous system; EPSP, excitatory postsynaptic potential; fMRI, functional magnetic resonance imaging; Fos, transcription factor belonging to the immediate early gene family; GABA, gamma aminobutyric acid; HPA, hypothalamus–pituitary–adrenal axis; LPS, lipopolysaccharide; LR, Lactobacillus reuteri; LTP, long‐term potentiation; NF‐κB, nuclear factor kappa‐light‐chain‐enhancer of activated B cells; TRPV1, transient receptor potential vanilloid 1.