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.