No abstract

Microbes are present as communities in the environment. However, the importance of minor populations has not been well studied experimentally. In this study, we evaluated the role of Bacillus subtilis with a 1% population and its effect on co-incubated Enterococcus faecalis with a 99% population. Here we used an azo dye-decolorizing Enterococcus faecalis strain T6a1 and non-dye-decolorizing Bacillus subtilis strain S4ga. The dye decolorization assay enabled the investigation of the effects of Bacillus subtilis S4ga on the activity of Enterococcus faecalis T6a1, even when Bacillus subtilis S4ga was present at only 1% relative abundance or lower. We found that non-decolorizing Bacillus subtilis S4ga enhanced the dye decolorization activity of Enterococcus faecalis T6a1, shortened the lag time of Enterococcus faecalis T6a1 to start decreasing the dye concentration, and increased the time for Enterococcus faecalis T6a1 to continue dye decolorization. These effects were correlated with redox potential values. We compared the extracellular amino acids between each incubation culture of Enterococcus faecalis T6a1 and Bacillus subtilis S4ga, which revealed their mutual relationship by cross-feeding of specific amino acids. We also compared the intracellular primary metabolites between co-incubation and sole incubation of E. faecalis T6a1. The arginine deiminase (ADI) pathway in the co-incubated E. faecalis T6a1 was activated compared to that of E. faecalis T6a1 incubated solely. These findings explained that co-incubation with Bacillus subtilis S4ga promoted ATP production in Enterococcus faecalis T6a1 cells to a greater extent and enhanced dye-decolorization activity.

The public sewage (PS) and night soil mixed with johkasou sludge (JO) have similar chemical compositions; however, the concentrations of organic matter and nitrogen compounds were different. We investigated the microbial community of the rotating biological contactor (RBC) units treating PS and JO, in which the RBC was submerged in the mixed liquor of activated sludge. Here, we observed that the microbial community compositions at the phylum and class levels were similar between the PS-RBC and JO-RBC, whereas the relative abundances of several phyla (Euryarchaeota, Acidobacteria, Chloroflexi, Firmicutes, Patescibacteria, and Betaproteobacteria) significantly differed between them. The microbial community composition of RBC (an attached growth process) was similar to that of the activated sludge (a suspended growth process). The microbial community of activated sludge likely affected that of RBC. The relative abundance of total denitrifying bacteria in the PS-RBC was twice as much as that in JO-RBC, while nitrifying bacterial phylotypes had a similar relative abundance. The predominant denitrifying genera were different between the PS-RBC and JO-RBC, as well as in the cross-sectional layers of the PS-RBC, suggesting the functional diversity of denitrifying bacterial genera inhabiting the RBC.

Microbes live in communities in biological wastewater treatment plants and in the intestines. However, limited information is currently available on the mechanisms by which minority bacterial populations assist other bacteria besides syntrophic relationships as well as on the microbial food web. Therefore, the present study investigated the effects of non-dye-decolorizing Bacillus subtilis strain S4ga at population levels ranging between 0.04 and 4% on the activity of dye-decolorizing Enterococcus faecalis strain T6a1 using a dye decolorization assay. The results obtained revealed that the minority population of B. subtilis S4ga enhanced the dye-decolorizing activity of E. faecalis T6a1, resulting in a shorter lag time and longer active time of dye decolorization. These effects were related to redox potential values rather than O 2 concentrations. Comparisons of the extracellular metabolites in individual incubations of E. faecalis T6a1 and B. subtilis S4ga and a co-incubation suggested a mutual relationship through the cross-feeding of specific amino acids (tyrosine, methionine, tryptophan, phenylalanine, valine, and leucine from B. subtilis S4ga to E. faecalis T6a1; glutamine, histidine, aspartic acid, and proline from E. faecalis T6a1 to B. subtilis S4ga). An ana­lysis of intracellular primary metabolites indicated that the arginine deiminase (ADI) pathway, an ATP-producing energy-generating process, was more strongly activated in co-incubated E. faecalis T6a1 than in E. faecalis T6a1 incubated alone. These results suggest that a co-incubation with B. subtilis S4ga promoted ATP production by E. faecalis T6a1 cells and enhanced its dye-decolorizing activity.