Deletion of D-Lactate Dehydrogenase A in Neisseria meningitidis Promotes Biofilm Formation Through Increased Autolysis and Extracellular DNA Release

Abstract

<i>Neisseria meningitidis</i> is a Gram-negative bacterium that asymptomatically colonizes the human nasopharyngeal mucosa. Pilus-mediated initial adherence of <i>N. meningitidis</i> to the epithelial mucosa is followed by the formation of three-dimensional aggregates, called microcolonies. Dispersal from microcolonies contributes to the transmission of <i>N. meningitidis</i> across the epithelial mucosa. We have recently discovered that environmental concentrations of host cell-derived lactate influences <i>N. meningitidis</i> microcolony dispersal. Here, we examined the ability of <i>N. meningitidis</i> mutants deficient in lactate metabolism to form biofilms. A lactate dehydrogenease A (<i>ldhA</i>) mutant had an increased level of biofilm formation. Deletion of <i>ldhA</i> increased the <i>N. meningitidis</i> cell surface hydrophobicity and aggregation. In this study, we used FAM20, which belongs to clonal complex ST-11 that forms biofilms independently of extracellular DNA (eDNA). However, treatment with DNase I abolished the increased biofilm formation and aggregation of the <i>ldhA</i>-deficient mutant, suggesting a critical role for eDNA. Compared to wild-type, the <i>ldhA</i>-deficient mutant exhibited an increased autolytic rate, with significant increases in the eDNA concentrations in the culture supernatants and in biofilms. Within the <i>ldhA</i> mutant biofilm, the transcription levels of the capsule, pilus, and bacterial lysis genes were downregulated, while <i>norB</i>, which is associated with anaerobic respiration, was upregulated. These findings suggest that the absence of <i>ldhA</i> in <i>N. meningitidis</i> promotes biofilm formation and aggregation through autolysis-mediated DNA release.