Most bacteria are capable of producing hydrogen sulfide and use a trio of enzymes to do so. Classically, this gas was considered a by-product of sulfur metabolism, but knowing that nitric oxide protects Gram-positive bacteria against oxidative stress, Shatalin et al. (see the Perspective by Belenky and Collins) discovered that H2S probably does likewise. When the H2S-producing enzymes were inactivated, bacteria became more susceptible to antibiotics unless supplied with a source of H2S.
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P. Belenky, J. J. Collins, Antioxidant strategies to tolerate antibiotics. Science 334, 915–916 (2011). [Abstract] [Full Text]