Electric current in gut attracts pathogens like Salmonella

By Pooja Toshniwal PahariaReviewed by Danielle Ellis, B.Sc.Sep 5 2024

Salmonella uses gut bioelectricity to find entry points for infection, study reveals. 

A recent study in Nature Microbiology investigated how Salmonella typhimurium targets the gut epithelium and localizes to the follicle-associated epithelium (FAE) of the small intestine.

Background

The human gut contains millions of beneficial microorganisms that aid in nutrient absorption, immune development, and defense against harmful bacteria. However, enteric bacteria like Salmonella inhabit the intestinal mucosa. Enterobacteria spread through contaminated food and water, causing intense enteritis and infections.

Salmonella uses a type III secretion pathway to penetrate the gut epithelium through microfold cells that line the FAE. However, the specific mechanism by which Salmonella localizes to the FAE remains unknown. Animal studies show that enteric pathogens prefer the FAE to enter and infect. The method by which a modest load of pathogens travels to the follicle-associated epithelium is unclear. Understanding bacterial bioelectric activity is critical for targeting pathogenic bacteria.

About the study

In the present study, researchers explored how intestinal pathogens like S. Typhimurium use galvanotaxis to find gut invasion sites.

The researchers studied bacterial tropism and competitive epithelial targeting in an ex vivo caecum model. They hypothesized that a regional electrical field alters S. Typhimurium target selection, and non-pathogenic E. coli exhibit distinct movement patterns in response to the field.

To investigate this, researchers modified E. coli and S. Typhimurium to produce different fluorescent proteins. They exposed the pathogens to an endogenous electrical field in vitro. Subsequently, they mapped the bioelectric activity in mouse caecal epithelia. Quantitative spatial fluorescence intensity patterns validated the bacterial tropism. Competitive tropism experiments determined whether changing the electrical fields affected S. Typhimurium localization in the FAE.

Researchers used time-lapse recording to perform bacterial galvanotaxis. They measured net outward currents in the follicle-associated epithelium and net inward currents in the surrounding villi. They used the voltage-sensitive dye DiBAC4 to determine whether the FAE and villi have different Vms. They assessed TEP by inserting glass microelectrodes into the FAE and villi of the ex vivo mouse caecum model.

Researchers studied chloride's role in gut bioelectricity. They blocked the chloride channel using a specific CFTR inhibitor. They used a flagellar mutant strain to study the role of flagella in S. Typhimurium's galvanotaxis. Immunostaining quantifies flagellar orientation. They stained Salmonella flagella with antibodies to its O- and H-antigens before and after applying an electrical field.

Researchers used Bacillus subtilis to investigate whether other commensal bacteria respond to an applied electrical field in vitro. The researchers also studied the role of chemotaxis in galvanotaxis-mediated migration. They used a chemotaxis-deficient mutant in the S. Typhimurium 14028S background.

Results

In the ex vivo mouse caecum model, Salmonella typhimurium localizes to the follicle-associated epithelium without chemotaxis. The FAE polarizes with a persistent outward current and a low transepithelial potential. The adjacent villus depolarizes with a high transepithelial potential and an inward electric current from electrolyte uptake. S. Typhimurium moves toward the FAE by different electrical potentials, whereas Escherichia coli binds to the villi via galvanotaxis.

Chloride flow causes ionic currents near the FAE. Suppressing CFTR reduces S. Typhimurium FAE localization and increases E. coli recruitment. The FAE restricts ionic flow to the chloride input, creating an outward electric current. CFTR-driven chloride ion efflux may modify or reverse this.

The distribution and differentiation of the electrogenic carriers create varied potentials across intestinal membranes and epithelia. These create a bioelectric channel between the villus epithelia and the FAE. It directs the S. Typhimurium pathogen to the follicle-associated epithelium using galvanotaxis.

The S. Typhimurium recovery rate was almost five times that of the E. coli from the FAE. The flagellar mutant had a 20-fold poorer recovery than the wild type. This shows the importance of flagella in congregating at the FAE.

Conclusions

Salmonella typhimurium targets gut epithelial areas by local, persistent bioelectric signals mediated by CFTR, not by chemotaxis receptors. This might influence other intestinal bacterial illnesses. Given the ubiquity of mucosal infections, more enteropathogenic bacteria may use the regional bioelectrical pattern to navigate and participate in galvanotaxis in the in vivo environment.

Salmonella infections produce electric fields in intestinal epithelia, resulting in systemic infection. A difference in bioelectric potential forms between the villus epithelium and FAE. Salmonella and commensal E. coli exhibit distinct responses to electrical fields due to differences in surface electric properties. S. Typhimurium enters the FAE, although commensal E. coli avoids it.

The bioelectrical arrangement of the colon epithelium favors commensal flora over S. Typhimurium. This is likely due to the lack of Peyer's patches or follicles. Faulty bioelectric fields capture E. coli and cause hyperimmunity or autoimmune illness in the gut microbiome. Future studies might investigate whether susceptible individuals have aberrant bioelectric activity or malfunctioning bioelectric networks in their gut lining.