Context. Phototrophic purple non-sulfur bacteria (PNSB) have attracted considerable attention to upgrade wastewater resources to microbial protein for animal feed. Next to their outstanding nutritional properties, they possess the unique ability to grow selectively under anaerobic conditions in the light (i.e. high abundance of one dominant species and an uneven microbial community).1
Gap. The microbial selectivity of PNSB has mainly been attributed to kinetic dominance. We have recently shown that PNSB are also able to inhibit the growth of competing microbes2. This might also contribute to selective growth of PNSB on wastewater, yet the mechanisms are still unknown.
Aim. We aimed to elucidate the mechanisms of microbial inhibition induced by PNSB and identify whether it can support their selective growth.
Findings. Mining 166 reference genomes of PNSB for antimicrobials using the online tool BAGEL, showed that 59% of the strains contained antimicrobial coding regions. In 38% of genomes, gene clusters coding for sactipeptides with antimicrobial properties (i.e., bacteriocins) were present. Agar spot assays demonstrated the antimicrobial potential of Rhodopseudomonas palustris on Escherichia coli with an inhibition zone of 1.5 mm (Figure 1). PNSB were also found to inhibit each other. Rhodobacter blasticus displayed antimicrobial potential on Rhodopseudomonas palustris with an inhibition zone of 5.1 mm. Two-species flask-based competition experiment confirmed these findings, showing a higher relative abundance of Rhodobacter blasticus (19%; Illumina) compared to what would be expected with a kinetic model (7%). Peptidomics (HPLC-MS-MS) showed indications of a large number of peptides in the supernatant, including fragments of a non-ribosomal peptide synthetase involved in producing antimicrobials.
Utilization. We unraveled the mechanisms underlying selective growth of PNSB by combining genome mining tools with laboratory experiments. This knowledge will help scientists and engineers to better direct purple phototrophic bioprocesses towards efficient wastewater treatment and resource recovery.
