Selenium (Se) is an essential trace element for humans and animals, but high-dose supplementation with Se compounds, most notably selenite, may exert cytotoxic and other adverse effects. On the other hand, bacteria, including Escherichia coli (E. coli), are capable of reducing selenite to red elemental Se that may serve as a safer Se source. Here, we examined how a diet of Se-enriched E. coli bacteria affected vital parameters and age-associated neurodegeneration in the model organism Caenorhabditis elegans (C. elegans). The growth of E. coli OP50 for 48 h in medium supplemented with 1 mM sodium selenite resulted in reddening of the bacterial culture, accompanied by Se accumulation in the bacteria. Compared to nematodes supplied with the standard E. coli OP50 diet, the worms fed on Se-enriched bacteria were smaller and slimmer, even though their food intake was not diminished. Nevertheless, given the choice, the nematodes preferred the standard diet. The fecundity of the worms was not affected by the Se-enriched bacteria, even though the production of progeny was somewhat delayed. The levels of the Se-binding protein SEMO-1, which serves as a Se buffer in C. elegans, were elevated in the group fed on Se-enriched bacteria. The occurrence of knots and ruptures within the axons of cholinergic neurons was lowered in aged nematodes provided with Se-enriched bacteria. In conclusion, C. elegans fed on Se-enriched E. coli showed less age-associated neurodegeneration, as compared to nematodes supplied with the standard diet.

Aging is accompanied by progressive functional decline and morphological defects of cholinergic neurons. Some probiotic bacteria with proficiency for biofilm formation in the gut are being discussed as potentially neuroprotective. Here, we screened Bacillus subtilis (B. subtilis) strains for biofilm production, using an in vitro assay. Thereafter, a transgenic Caenorhabditis elegans (C. elegans) strain with its cholinergic neurons expressing green fluorescent protein (GFP) was fed for up to 15 days on either the identified biofilm-producing B. subtilis strain B4U 50 or the control B. subtilis strain 168, with or without addition of a standardized Ginkgo biloba preparation. The morphology of the cholinergic neurons was monitored at several consecutive days by fluorescence microscopy. The occurrence of morphological defects such as knots and ruptures within the axons of cholinergic neurons, as observed with increasing age, was significantly lower in nematodes provided with the B4U 50 strain. Even worms whose diet was switched to B4U 50 later in life showed less damage of their neurons, as compared to worms fed on the control strain. Supplementation with the Ginkgo biloba preparation was neuroprotective in worms fed on either B. subtilis strain, while a stronger beneficial effect was observed upon combined administration of B. subtilis B4U 50 and the Ginkgo biloba preparation. In conclusion, feeding a B. subtilis strain proficient for biofilm formation, B4U 50, was neuroprotective when compared to a diet consisting of control B. subtilis, delaying the onset of aging-associated morphological defects in cholinergic neurons of C. elegans. Graphical Abstract