Listeria monocytogenes uses de novo purine synthesis to enhance fitness in Lyoner-type sausage

Listeria monocytogenes is an important food-borne pathogen with high hospitalization and case fatality rates. To cause disease, L. monocytogenes must gain access to a specific food matrix and in many cases be able to grow before the food is consumed. The fitness of this pathogen differs between individual foods and depends on its ability to adapt to various environmental stressors that are highly specific in each food matrix. Deli meats are an important cause of infections with L. monocytogenes. Here, we screened a transposon mutagenesis library in L. monocytogenes LL195 with 2640 individual mutants on a Lyoner-type deli meat to understand the fitness effect of individual genes. After the determination of 10 candidate mutants with confirmed phenotypes on Lyoner, inframe deletion mutants of these genes were created by allelic replacement. The fitness effects of these in-frame mutants were then confirmed by growth experiments under cold stress and on Lyoner separately, and in a combined condition of both. Mutants with deletions in the cytosine-specific DNA modification methyltransferase sau3AIM and the penicillin-binding protein pbp-B were impaired in their growth at cold temperatures in rich medium as well as on Lyoner, suggesting a temperature-dependent phenotype. In contrast, the purB deletion mutant exhibited reduced fitness that was specific to growth on Lyoner. Our results indicate an important role for the sau3AIM and pbp-B genes in cold stress adaptation, while purB, which is a central component of purine biosynthesis, may play a more specialized role on the fitness of L. monocytogenes during growth on Lyoner.