Potent and Selective Inhibition of Sulfate-Reducing Bacteria by Neutral Red

Sulfate-reducing bacteria (SRB) are anaerobic microorganisms that use sulfate as a terminal electron acceptor for the oxidation of organic compounds or H₂. These organisms can cause a serious problem in, for example, the offshore oil industry, due to the production of sulfide. Thus, it is of fundamental and practical importance to identify potent and selective inhibitors of SRB. In this study, neutral red was identified as a previously unrecognized selective inhibitor of SRB, with several orders of magnitude higher potency than most commonly used industrial biocides and inorganic oxyanions. Neutral red remained a potent inhibitor of SRB growth under fermentative conditions and was tolerated by nitrate-reducing bacteria. After 30 days of exposure to 14.2 μM neutral red, the sulfidogenesis activity of SRB-enriched biomass was reduced by 98.3%, and the abundance of SRB populations declined from 25.5% to 0.76%. Transcriptomic analysis revealed that the inhibition of the central sulfate reduction pathway was implicated in the mechanism of neutral red toxicity against SRB growth. Furthermore, downregulation of two electron transport complexes (QmoABC and DsrMKJOP), ATP synthase, as well as cytoplasmic/periplasmic hydrogenase suggested the collapse of the proton gradient. These findings have implications for environmental control of SRB and may enhance economic benefits in industrial operations.