The pathogen Neisseria meningitidis requires oxygen, but supplements growth by denitrification. Nitrite, nitric oxide and oxygen control respiratory flux at genetic and metabolic levels

Abstract

SummaryThe human pathogen Neisseria meningitidis is the major causative agent of bacterial meningitis. The organism is usually treated as a strict aerobe and is cultured under fully aerobic conditions in the laboratory. We demonstrate here that although N. meningitidis fails to grow under strictly anaerobic conditions, under oxygen limitation the bacterium expresses a denitrification pathway (reduction of nitrite to nitrous oxide via nitric oxide) and that this pathway supplements growth. The expression of the gene aniA , which encodes nitrite reductase, is regulated by oxygen depletion and nitrite availability via transcriptional regulator FNR and two-component sensor-regulator NarQ/NarP respectively. Completion of the two-step denitrification pathway requires nitric oxide (NO) reduction, which proceeds after NO has accumulated during batch growth under oxygen-limited conditions. During periods of NO accumulation both nitrite and NO reduction are observed aerobically, indicating N. meningitidis can act as an aerobic denitrifier. However, under steady-state conditions in which NO is maintained at a low concentration, oxygen respiration is favoured over denitrification. NO inhibits oxidase activity in N. meningitidis with an apparent K i NO = = = = 380 nM measured in intact cells. The high respiratory flux to nitrite after microaerobic growth and the finding that accumulation of the denitrification intermediate NO inhibits oxygen respiration support the view that denitrification is a pathway of major importance in N. meningitidis .

Expression of Factor H Binding Protein of Meningococcus Responds to Oxygen Limitation through a Dedicated FNR-Regulated Promoter

“…N. meningitidis is able to live in anaerobic niches as part of its normal transmissioncolonization and infectious cycles within the host. It has been shown that although N. meningitidis fails to grow under strictly anaerobic conditions, under oxygen limitation the bacterium expresses a denitrification pathway that supplements growth (1,39). The nasopharynx can generally be thought of as an environment of fluctuating oxygen concentrations, as both strict aerobes and strict anaerobes are routinely isolated from this niche (9).…”

Regulation of Denitrification Genes in Neisseria meningitidis by Nitric Oxide and the Repressor NsrR

“…NO is present at high concentrations in the nasopharynx, as judged by its concentration in exhaled nasal breath (12). The ability of N. meningitidis to use NO is important for its lifestyle in several respects: (i) denitrification supports growth (23); (ii) NO is a toxic free radical gas, the toxicity of which is controlled by NO reductase in both pure culture (1) and tissue and organ culture models of N. meningitidis colonization (28); and (iii) bacterial NO reduction impacts upon host processes such as apoptosis (30).…”

“…Denitrification in N. meningitidis, and in its close relative Neisseria gonorrhoeae, is regulated by oxygen availability and also by the availability of nitrite and nitric oxide (10,13,23). The expression of the nitrite reductase gene aniA in N. meningitidis is controlled by oxygen via the transcriptional regulator FNR (fumarate and nitrate reduction regulator) and by nitrite via the two-component sensor-regulator NarQ/P (23).…”

The Nitric Oxide (NO)-Sensing Repressor NsrR of Neisseria meningitidis Has a Compact Regulon of Genes Involved in NO Synthesis and Detoxification

“…For genes which are simply derepressed in an NsrR-dependent fashion (e.g., norB and dnrN), a concentration of around 20 M NONOate (which is equivalent to approximately 1 M NO) is sufficient to bring about considerable derepression. NO accumulates to around 1 M during the transition from aerobic growth to denitrification, and concentrations of around 1 M are sufficient to bring about a transient cessation of growth and inhibition of oxidase activity (30). The physiological concentrations of NO in human tissue are likely to be in the range of 0.1 to 1 M (21, 13, 17), indicating that the activation of gene expression in an NsrRdependent/NO-dependent manner is tuned to be appropriate for the response to physiologically relevant NO concentrations.…”

“…We have found that N. meningitidis synthesizes NO detoxification proteins (NO reductase, NorB, and cytochrome cЈ [CycP]) that protect the organism from being killed by macrophage-generated NO (36). The environment in the pharyngeal mucosa is subject to various levels of oxygen availability, and the meningococcus has adapted and proliferates under conditions of oxygen limitation by using the alternative respiratory pathway of denitrification (30). In this process, nitrite is reduced to nitrous oxide, producing NO as a freely diffusible intermediate.…”