Helmholtz-Centre for Infection Research, Division of Cell Biology, Braunschweig, Germany.
BMC Microbiol. 2010 Feb 23;10:58. doi: 10.1186/1471-2180-10-58.
One of the key virulence determinants of Streptococcus mutans, the primary etiological agent of human dental caries, is its strong acid tolerance. The acid tolerance response (ATR) of S. mutans comprises several mechanisms that are induced at low pH and allow the cells to quickly adapt to a lethal pH environment. Malolactic fermentation (MLF) converts L-malate to L-lactate and carbon dioxide and furthermore regenerates ATP, which is used to translocate protons across the membrane. Thus, MLF may contribute to the aciduricity of S. mutans but has not been associated with the ATR so far.
Here we show that the malolactic fermentation (mle) genes are under the control of acid inducible promoters which are induced within the first 30 minutes upon acid shock in the absence of malate. Thus, MLF is part of the early acid tolerance response of S. mutans. However, acidic conditions, the presence of the regulator MleR and L-malate were required to achieve maximal expression of all genes, including mleR itself. Deletion of mleR resulted in a decreased capacity to carry out MLF and impaired survival at lethal pH in the presence of L-malate. Gel retardation assays indicated the presence of multiple binding sites for MleR. Differences in the retardation patterns occurred in the presence of L-malate, thus demonstrating its role as co-inducer for transcriptional regulation.
This study shows that the MLF gene cluster is part of the early acid tolerance response in S. mutans and is induced by both low pH and L-malate.
变形链球菌是人类龋齿的主要病原体，其强耐酸性是其主要毒力决定因素之一。变形链球菌的耐酸应答（ATR）包括几种机制，这些机制在低 pH 值下被诱导，使细胞能够快速适应致死 pH 值环境。苹果酸-乳酸发酵（MLF）将 L-苹果酸转化为 L-乳酸和二氧化碳，并进一步再生 ATP，ATP 用于将质子跨膜转运。因此，MLF 可能有助于变形链球菌的耐酸性，但迄今为止与 ATR 无关。
我们发现，苹果酸-乳酸发酵（mle）基因受酸诱导启动子的控制，在没有苹果酸的情况下，酸冲击后 30 分钟内，这些启动子被诱导。因此，MLF 是变形链球菌早期耐酸应答的一部分。然而，酸性条件、调节剂 MleR 和 L-苹果酸的存在是实现所有基因（包括 mleR 本身）最大表达所必需的。mleR 的缺失导致 MLF 能力下降，并且在存在 L-苹果酸时在致死 pH 值下的生存能力受损。凝胶阻滞试验表明 MleR 存在多个结合位点。在存在 L-苹果酸时，阻滞模式存在差异，因此证明了它作为转录调控的共诱导剂的作用。
本研究表明，MLF 基因簇是变形链球菌早期耐酸应答的一部分，可被低 pH 值和 L-苹果酸诱导。
