Characterization of Pseudomonas aeruginosa growth on O-acylcarnitines and identification of a short-chain acylcarnitine hydrolase.

To survive in various environments, from host tissue to soil, opportunistic bacterial pathogens must be metabolically flexible and able to use a variety of nutrient sources. We are interested in Pseudomonas aeruginosa's catabolism of quaternary amine compounds that are prevalent in association with eukaryotes. Carnitine and acylcarnitines are abundant in animal tissues, particularly skeletal muscle, and are used to shuttle fatty acids in and out of the mitochondria, where they undergo β-oxidation. We previously identified the genes required for carnitine catabolism as the first four genes in the carnitine operon (caiX-cdhCAB; PA5388 to PA5385). However, the last gene in the operon, PA5384, was not required for carnitine catabolism. We were interested in determining the function of PA5384. Bioinformatic analyses along with the genomic location of PA5384 led us to hypothesize a role for PA5384 in acylcarnitine catabolism. Here, we have characterized PA5384 as an l-enantiomer-specific short-chain acylcarnitine hydrolase that is required for growth and hydrolysis of acetyl- and butyrylcarnitine to carnitine and the respective short-chain fatty acid. The liberated carnitine and its downstream catabolic product, glycine betaine, are subsequently available to function as osmoprotectants in hyperosmotic environments and induce transcription of the virulence factor phospholipase C, plcH. Furthermore, we confirmed that acylcarnitines with 2- to 16-carbon chain lengths, except for octanoylcarnitine (8 carbons), can be utilized by P. aeruginosa as sole carbon and nitrogen sources. These findings expand our knowledge of short-chain acylcarnitine catabolism and also point to remaining questions related to acylcarnitine transport and hydrolysis of medium- and long-chain acylcarnitines.