Amino acid and C1 carbon source metabolism in the Salmonella typhimurium phagosome: Role of amino acids and glycine conversion in intracellular growth

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We constructed a unique collection of mutants of Salmonella typhimurium defective in amino acid biosynthesis and uptake, peptide and oligopeptide uptake and amino acid catabolism and determined their intracellular growth characteristics in intestinal epithelial cells. Our studies showed that intracellular Salmonella not only have access to host-derived amino acid pools but are also capable of using peptides to satisfy amino acid requirements in the absence of biosynthetic and uptake systems. One exception is a strain with a mutation in glyA, encoding serine hydroxymethyltransferase (SHMT), responsible for the reversible conversion of serine to glycine, which shows a severe intracellular growth defect. In culture, glycine can correct the growth defect of a glyA mutant in glucose minimal medium; however, within host cells, a glyA mutant shows a severe growth defect. In addition, we found that the attenuation of aroA mutant strains of Salmonella is not due to defects in aromatic amino acid biosynthesis, but likely limitations in para-aminobenzoic acid, also derived from the same precursor, chorismic acid. Mutants defective in biosynthesis of all three aromatic amino acids show no intracellular growth defects, whereas mutants in pabA, pabB and/or pabC show severe growth defects within host cells. Finally, a previously uncharacterized gene, ltaA, was found to play a central role in maintaining intracellular growth by regulating the flow of threonine into either the glycine or branched chain amino acid pathways. The results indicate that intracellular Salmonella preferentially acquires amino acids from the host to maximize intracellular growth, and shows a high dependence on maintenance of amino acid pools involved in C1 carbon unit biosynthesis.