Role of metAB in Methionine Metabolism and Optima Chicken Colonization in Campylobacter jejuni

Campylobacter jejuni is a zoonotic pathogen and is one of the leading causes of human gastroenteritis worldwide. C. jejuni IA3902 (representative of the sheep abortion clone) is genetically similar to C. jejuni W7 (representative of strain type NCTC 11168); however, there are significant differences in the ability of luxS mutants of these strains to colonize chickens. LuxS is essential for the activated methyl cycle and generates homocysteine for conversion to L-methionine. Comparative genomics identified differential distribution of the genes metA and metB, which function to convert homoserine for downstream production of l-methionine, between IA3902 and W7, which could enable a secondary pathway for l-methionine biosynthesis in a W7 Delta luxS but not in an IA3902 Delta luxS strain. To test the hypothesis that the genes metA and metB contribute to l-methionine production and chicken colonization by Campylobacter, we constructed two mutants for phenotypic comparison, the W7 Delta metAB Delta luxS and IA3902 Delta luxS::metAB mutants. Quantitative reverse transcription-PCR and tandem mass spectrometry protein analysis were used to validate MetAB transcription and translation as present in the IA3902 Delta luxS::metAB mutant and absent in the W7 Delta metAB Delta luxS mutant. Time-resolved fluorescence resonance energy transfer fluorescence assays demonstrated that l-methionine and S-adenosyl methionine concentrations decreased in the W7 Delta metAB Delta luxS mutant and increased in the IA3902 Delta luxS::metAB mutant. Assessment of chicken colonization revealed that the IA3902 Delta luxS::metAB strain partially rescued the colonization defect of the IA3902 Delta luxS strain, while the W7 Delta metAB Delta luxS strain showed significantly decreased colonization compared to that of the wild-type and the W7 Delta luxS strain. These results indicate that the ability to maintain l-methionine production in vivo, conferred by metA and metB in the absence of luxS, is critical for normal chicken colonization by C. jejuni.