Probiotics and gut microbiota-brain axis: exploring bioelectrical communication through enterococci

Abstract

Emerging evidence underscores the pivotal role of the microbiota-gut-brain (MGB) axis in the etiology and progression of certain neuropsychiatric and neurological conditions, attributed to dysbiosis and alterations in bacteria-neuron signalling. Notably, inter-bacterial communication via bioelectrical signals within biofilms suggests potential implications for long-distance signalling within the MGB axis. Despite the recognized effects of enterococci, inherent members of the human microbiota, as probiotic adjuncts in addressing intestinal dysbiosis, their bioelectrical properties and interactions with neurons remain largely unexplored. Here, we investigate the bioelectrical profile (electroma) of the ubiquitous Enterococcus faecalis (E. faecalis) and its responsiveness to neural stimuli, aiming to start elucidating the interplay between nervous signals, microbiota, and probiotics. Using a voltagesensitive fluorescent dye, bis-(1,3-dibutylbarbituric acid) trimethine oxonol (DiBAC4(3)), as a membrane potential (Vmem) indicator, we revealed and quantified i) the dynamic evolution of the bioelectrical profile during E. faecalis growth, ii) its response to two types of neural signals – the inhibitory neurotransmitter γ-aminobutyric acid (GABA) and the excitatory neurotransmitter Glutamate (Glu), and iii) the consequent impact of neurotransmitter-induced bioelectrical changes on bacterial growth, viability, and culturability, evaluated through absorbance readings, live/dead fluorescent probes, and viable counts. We observed a substantial impact of growth dynamics and neurotransmitters on the bioelectrical profile of bacteria. Notably, there was a marked increase in Vmem levels (depolarization) throughout bacterial growth. However, both types of neural signals significantly decreased membrane depolarization, while leaving growth, viability, and culturability unaffected. These findings deepen our comprehension of E. faecalis' involvement in gut-brain communication and provide insights into the effects of enterococci-based probiotics on the gut microbiota-brain axis.