Multiple nutritional requirements of lactobacilli: genetic lesions affecting amino acid biosynthetic pathways

Genetic lesions responsible for amino acid requirements in several species of multiple auxotrophic lactobacilli were investigated. Systematic attempts were made to isolate mutants that could grow in the absence of each of the amino acids required by the parental strains of Lactobacillus plantarum, L. casei, L. helveticus, and L. acidophilus. After treatment with appropriate mutagens, such mutants could be obtained with respect to many but not all required amino acids. Successful isolation of mutants for a given amino acid means that a minor genetic lesion reparable by single-step mutations affects its biosynthesis; a failure to isolate mutants suggests the involvement of more extensive lesions. Analysis of these results as well as the specific requirements exhibited by the parental strains revealed certain regularities; some of the biosynthetic pathways for individual amino acids were virtually unaffected by more extensive lesions in at least species tested, whereas others were affected by more extensive lesions in at least some species. Both the number and the kind of pathways affected by extensive lesions differed appreciably among different species. Furthermore, the growth response of the parental strains to some putative amino acid precursors revealed a clear correlation between the extent of genetic lesions and the occurrence and location of a genetic block(s) for a given pathway. These findings are discussed in relation to the phylogeny, ecology, and evolution of lactic acid bacteria.