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The intracellular growth of several auxotrophic mutants of Listeria monocytogenes was examined in cell culture, and virulence was evaluated in mice by intravenous injection of log-phase bacteria. L. monocytogenes transposon insertion mutants requiring either uracil, phenylalanine, glycine, proline, or nicotinic acid for growth were fully virulent and grew similarly to the parental strain as shown by their growth rates in cell culture. Those requiring all three aromatic amino acids (phenylalanine, tryptophan, and tyrosine) or adenine were 1.5 log10 less virulent than the wild type. A threonine auxotroph, which showed enhanced growth in the presence of threonine-containing peptides as compared with that in the presence of free threonine, was approximately 1 log10 less virulent than the wild type. When host cells were deprived of specific amino acids required by both the host cell and L. monocytogenes, the bacteria continued to grow intracellularly. These studies suggest that the cytoplasm of eucaryotic cells behaves like rich medium, facilitating the growth of an intracellular bacterial pathogen with complex growth requirements. In addition, results related to amino acid deprivation during intracellular growth and specific extracellular growth requirements of a threonine auxotroph suggest that L. monocytogenes may utilize intracellular peptides as a source of amino acids.
Marquis et al. (Wed,) studied this question.