INTRACYTOPLASMIC GROWTH AND VIRULENCE OF LISTERIA-MONOCYTOGENES AUXOTROPHIC MUTANTS

The intracellular growth of several auxotrophic mutants of Listeria mo nocytogenes was examined in cell culture, and virulence was evaluated in mice by intravenops injection of log-phase bacteria. L. monocytogen es transposon insertion mutants requiring either uracil, phenylalanine , glycine, proline, or nicotinic acid for growth were fully virulent a nd grew similarly to the parental strain as shown by their growth rate s in cell culture. Those requiring all three aromatic amino acids (phe nylalanine, tryptophan, and tyrosine) or adenine were 1.5 log10 less v irulent than the wild type. A threonine auxotroph, which showed enhanc ed growth in the presence of threonine-containing peptides as compared with that in the presence of free threonine, was approximately 1 log1 0 less virulent than the wild type. When host cells were deprived of s pecific amino acids required by both the host cell and L. monocytogene s, the bacteria continued to grow intracellularly. These studies sugge st that the cytoplasm of eucaryotic cells behaves like rich medium, fa cilitating the growth of an intracellular bacterial pathogen with comp lex growth requirements. In addition, results related to amino acid de privation during intracellular growth and specific extracellular growt h requirements of a threonine auxotroph suggest that L. monocytogenes may utilize intracellular peptides as a source of amino acids.