Listeria monocytogenes relAandhptMutants Are Impaired in Surface-Attached Growth and Virulence

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Abstract
We describe here the identification and characterization of twoListeria monocytogenes(Tn917-LTV3)relAandhpttransposon insertion mutants that were impaired in growth after attachment to a model surface. Both mutants were unable to accumulate (p)ppGpp in response to amino acid starvation, whereas the wild-type strain accumulated (p)ppGpp within 30 min of stress induction. The induction of transcription of therelAgene after adhesion was demonstrated, suggesting that the ability to mount a stringent response and undergo physiological adaptation to nutrient deprivation is essential for the subsequent growth of the adhered bacteria. The absence of (p)ppGpp in thehptmutant, which is blocked in the purine salvage pathway, is curious and suggests that a functional purine salvage pathway is required for the biosynthesis of (p)ppGpp. Both mutants were avirulent in a murine model of listeriosis, indicating an essential role for the stringent response in the survival and growth ofL. monocytogenesin the host. Taken as a whole, this study provides new information on the role of the stringent response and the physiological adaptation ofL. monocytogenesfor biofilm growth and pathogenesis.