c-di-AMP Accumulation Impairs Muropeptide Synthesis in Listeria monocytogenes

Abstract

Cyclic di-AMP (c-di-AMP) is an essential and ubiquitous second messenger among bacteria. c-di-AMP regulates many cellular pathways through direct binding to several molecular targets in bacterial cells. c-di-AMP depletion is well known to destabilize the bacterial cell wall, resulting in increased bacteriolysis and enhanced susceptibility to cell wall targeting antibiotics. Using the human pathogen <i>Listeria monocytogenes</i> as a model, we found that c-di-AMP accumulation also impaired cell envelope integrity. An <i>L. monocytogenes</i> mutant deleted for c-di-AMP phosphodiesterases (<i>pdeA pgpH</i> mutant) exhibited a 4-fold increase in c-di-AMP levels and several cell wall defects. For instance, the <i>pdeA pgpH</i> mutant was defective for the synthesis of peptidoglycan muropeptides and was susceptible to cell wall-targeting antimicrobials. Among different muropeptide precursors, we found that the <i>pdeA pgpH</i> strain was particularly impaired in the synthesis of d-Ala-d-Ala, which is required to complete the pentapeptide stem associated with UDP-<i>N</i>-acetylmuramic acid (MurNAc). This was consistent with an increased sensitivity to d-cycloserine, which inhibits the d-alanine branch of peptidoglycan synthesis. Finally, upon examining d-Ala:d-Ala ligase (Ddl), which catalyzes the conversion of d-Ala to d-Ala-d-Ala, we found that its activity was activated by K⁺ Based on previous reports that c-di-AMP inhibits K⁺ uptake, we propose that c-di-AMP accumulation impairs peptidoglycan synthesis, partially through the deprivation of cytoplasmic K⁺ levels, which are required for cell wall-synthetic enzymes.<b>IMPORTANCE</b> The bacterial second messenger c-di-AMP is produced by a large number of bacteria and conditionally essential to many species. Conversely, c-di-AMP accumulation is also toxic to bacterial physiology and pathogenesis, but its mechanisms are largely undefined. We found that in <i>Listeria monocytogenes</i>, elevated c-di-AMP levels diminished muropeptide synthesis and increased susceptibility to cell wall-targeting antimicrobials. Cell wall defects might be an important mechanism for attenuated virulence in bacteria with high c-di-AMP levels.