Homeostasis and Catabolism of Choline and Glycine Betaine: Lessons from Pseudomonas aeruginosa

Abstract

Most sequenced bacteria possess mechanisms to import choline and glycine betaine (GB) into the cytoplasm. The primary role of choline in bacteria appears to be as the precursor to GB, and GB is thought to primarily act as a potent osmoprotectant. Choline and GB may play accessory roles in shaping microbial communities, based on their limited availability and ability to enhance survival under stress conditions. Choline and GB enrichment near eukaryotes suggests a role in the chemical relationships between these two kingdoms, and some of these interactions have been experimentally demonstrated. While many bacteria can convert choline to GB for osmoprotection, a variety of soil-and water-dwelling bacteria have catabolic pathways for the multistep conversion of choline, via GB, to glycine and can thereby use choline and GB as sole sources of carbon and nitrogen. In these choline catabolizers, the GB intermediate represents a metabolic decision point to determine whether GB is catabolized or stored as an osmo-and stress protectant. This minireview focuses on this decision point in Pseudomonas aeruginosa, which aerobically catabolizes choline and can use GB as an osmoprotectant and a nutrient source. P. aeruginosa is an experimentally tractable and ecologically relevant model to study the regulatory pathways controlling choline and GB homeostasis in choline-catabolizing bacteria. The study of P. aeruginosa associations with eukaryotes and other bacteria also makes this a powerful model to study the impact of choline and GB, and their associated regulatory and catabolic pathways, on host-microbe and microbe-microbe relationships.

Characterization of the GbdR Regulon in Pseudomonas aeruginosa

“…cules (GB and DMG), GbdR also promotes its own inactivation. Analogous to BetI control of the choline oxidation genes, we predict that the dual competing activities of GbdR on the cytosolic pool of GB are regulated to maintain GB homeostasis (33,36).…”

A nonpyrrolysine member of the widely distributed trimethylamine methyltransferase family is a glycine betaine methyltransferase

“…GB can be produced as an intermediate during choline and carnitine degradation (9,10). The route of catabolic bacterial degradation of GB differs markedly depending on the presence or absence of oxygen.…”

Bacterial Sphingomyelinases and Phospholipases as Virulence Factors

“…In P. aeruginosa, PlcH and the phosphorylcholine phosphatase PchP might have a role in phosphate scavenging and osmoprotection (307). Under phosphate limitation, in niches where eukaryotic membrane debris are present, PlcH would be induced to generate phosphate monoesters that would be subsequently hydrolyzed by PchP to phosphate and choline (308)(309)(310)(311).…”

“…Under phosphate limitation, in niches where eukaryotic membrane debris are present, PlcH would be induced to generate phosphate monoesters that would be subsequently hydrolyzed by PchP to phosphate and choline (308)(309)(310)(311). Choline could be used as an energy source or as a precursor for the synthesis of glycine betaine, an important protectant for survival in environments where osmolarity is not constant (307). Homologous enzymes to P. aeruginosa PlcH and PchP are present in several species of Actinobacteria and Proteobacteria (311).…”