ABSTRACTThe opportunistic pathogenStreptococcus agalactiaeis the major cause of meningitis and sepsis in a newborn’s first week, as well as a considerable cause of pneumonia, urinary tract infections, and sepsis in immunocompromised adults. This pathogen respires aerobically if heme and quinone are available in the environment, and a functional respiratory chain is required for full virulence. Remarkably, it is shown here that the entire respiratory chain ofS. agalactiaeconsists of only two enzymes, a type 2 NADH dehydrogenase (NDH-2) and a cytochromebdoxygen reductase. There are no respiratory dehydrogenases other than NDH-2 to feed electrons into the respiratory chain, and there is only one respiratory oxygen reductase to reduce oxygen to water. AlthoughS. agalactiaegrows well invitroby fermentative metabolism, it is shown here that the absence of NDH-2 results in attenuated virulence, as observed by reduced colonization in heart and kidney in a mouse model of systemic infection. The lack of NDH-2 in mammalian mitochondria and its important role for virulence suggest this enzyme may be a potential drug target. For this reason, in this study,S. agalactiaeNDH-2 was purified and biochemically characterized, and the isolated enzyme was used to screen for inhibitors from libraries of FDA-approved drugs. Zafirlukast was identified to successfully inhibit both NDH-2 activity and aerobic respiration in intact cells. This compound may be useful as a laboratory tool to inhibit respiration inS. agalactiaeand, since it has few side effects, it might be considered a lead compound for therapeutics development.IMPORTANCES. agalactiaeis part of the human intestinal microbiota and is present in the vagina of ~30% of healthy women. Although a commensal, it is also the leading cause of septicemia and meningitis in neonates and immunocompromised adults. This organism can aerobically respire, but only using external sources of heme and quinone, required to have a functional electron transport chain. Although bacteria usually have a branched respiratory chain with multiple dehydrogenases and terminal oxygen reductases, here we establish thatS. agalactiaeutilizes only one type 2 NADH dehydrogenase (NDH-2) and one cytochromebdoxygen reductase to perform respiration. NADH-dependent respiration plays a critical role in the pathogen in maintaining NADH/NAD+redox balance in the cell, optimizing ATP production, and tolerating oxygen. In summary, we demonstrate the essential role of NDH-2 in respiration and its contribution toS. agalactiaevirulence and propose it as a potential drug target.
Biochemical Characterization of
Toxoplasma gondii
Mitochondrial Type II NADH Dehydrogenase Isoform I as a potential Drug Target
