Regulation and Function of Versatile Aerobic and Anaerobic Respiratory Metabolism in Pseudomonas aeruginosa

Abstract

Pseudomonas aeruginosa is a ubiquitously distributed opportunistic pathogen that inhabits soil and water as well as animal-, human-, and plant-host-associated environments. The ubiquity would be attributed to its very versatile energy metabolism. P. aeruginosa has a highly branched respiratory chain terminated by multiple terminal oxidases and denitrification enzymes. Five terminal oxidases for aerobic respiration have been identified in the P. aeruginosa cells. Three of them, the cbb(3)-1 oxidase, the cbb(3)-2 oxidase, and the aa(3) oxidase, are cytochrome c oxidases and the other two, the bo(3) oxidase and the cyanide-insensitive oxidase, are quinol oxidases. Each oxidase has a specific affinity for oxygen, efficiency of energy coupling, and tolerance to various stresses such as cyanide and reactive nitrogen species. These terminal oxidases are used differentially according to the environmental conditions. P. aeruginosa also has a complete set of the denitrification enzymes that reduce nitrate to molecular nitrogen via nitrite, nitric oxide (NO), and nitrous oxide. These nitrogen oxides function as alternative electron acceptors and enable P. aeruginosa to grow under anaerobic conditions. One of the denitrification enzymes, NO reductase, is also expected to function for detoxification of NO produced by the host immune defense system. The control of the expression of these aerobic and anaerobic respiratory enzymes would contribute to the adaptation of P. aeruginosa to a wide range of environmental conditions including in the infected hosts. Characteristics of these respiratory enzymes and the regulatory system that controls the expression of the respiratory genes in the P. aeruginosa cells are overviewed in this article.

Keywords: Pseudomonas aeruginosa; denitrification; nitric oxide; respiration; terminal oxidase.

Figure 1

Branched respiratory chain of P. aeruginosa. Under aerobic conditions, oxygen is utilized as a terminal electron acceptor and reduced to water by five terminal oxidases. Two quinol oxidases, the bo₃ oxidase and the cyanide-insensitive oxidase, receive electrons directly from quinol. Three cytochrome c oxidases, the aa₃ oxidase and the two cbb₃ oxidases, receive electrons via the cytochrome bc₁ complex and c-type cytochromes or a small blue-copper protein azurin. Under anaerobic conditions, electrons are transferred to nitrogen oxides via the denitrification enzymes.

Figure 2

Schematic model of the regulatory network controlling the multiple terminal oxidases in P. aeruginosa. The sensing signals for the regulators are shown in the left column. Affinity for oxygen and upregulation conditions of the terminal oxidases are shown in the right columns. Activation is indicated by arrows. Inhibition is indicated by bars with dotted lines.

Figure 3

Schematic model of the regulatory network controlling the denitrification genes in P. aeruginosa. ANR activates the expression of DNR under anaerobic or low oxygen conditions. DNR activates the expression of all denitrification genes in response to nitric oxide. A two-component nitrate sensing regulator, NarXL is required for expression of the nar genes encoding nitrate reductase. Both ANR and DNR can activate the nar gene expression. NirQ is predicted to be involved in the fine tuning of the activities of nitrite reductase and nitric oxide reductase. NAR, NIR, NOR, and N₂OR indicate nitrate reductase, nitrite reductase, nitric oxide reductase, and nitrous oxide reductase, respectively.

Figure 4

Hypothetical model of the role of pyocyanin in anaerobic survival in biofilm and construction of the multilayered structure of the multispecies biofilm. (A) Pyocyanin is predicted to act as an electron acceptor for the anaerobic cells and shuttle electrons between anaerobic and aerobic niches. Pyocyanin stimulates excretion of pyruvate. The secreted pyruvate is expected to be utilized for the anaerobic pyruvate fermentation. PYO_red and PYO_ox indicate the reduced- and oxidized-forms of pyocyanin, respectively. pO₂ indicates partial oxygen pressure. (B,C) FISH images of three-species biofilms in vertical sections. A pyocyanin-overproducing P. aeruginosa strains P1 (P1) and its pyocyanin-non-producing derivative (ΔphzM) appear blue with a Cy3-labeled probe, A pyocyanin-resistant Raoultella strain (R1) and a pyocyanin-sensitive Brevibacillus strain (S1) appear green and red with the FITC- and Cy5-labeled probes, respectively. Strain P1 forms a multilayered biofilm with strains R1 and S1 (B). An intermingled biofilm is formed when strain ΔphzM is used (C). Small arrows indicate the bottom of biofilms. White bars indicate 50 μm.