Cell-Cell communication in Streptococcus mutans – From a global view to the single cell perspective

Abstract

The LuxS protein is responsible for the synthesis of the autoinducer-2 (AI-2) signaling molecule. A luxS mutant was complemented genetically, physiologically by heterologous expression of the S-adenosylhomocysteine hydrolase (SahH) and chemically by the addition of pure AI-2. Differences due to the lack of luxS became only obvious using a comparative transcriptome analysis showing differences in the glutamate and sulfur metabolism, which could be complemented by expression of SahH. An influence of AI-2 on the transcriptome and various phenotypes could not be shown. In a previous study it was shown that the transcriptional regulator MleR was induced upon addition of AI-2. This could not be confirmed. It was shown that MleR is a positive regulator for the genes involved in malolactic fermentation. These genes belonged to the early acid tolerance response in S. mutans and were shown to protect against acid stress. To understand the role of the CSP (Competence Stimulating Peptide) peptide in more detail a comparative transcriptome analysis of competent and non-competent cells, which were sorted using flow cytometry, was carried out. A strong enrichment of transcripts of all regulators, transformasome genes and the mutacin immunity protein CipI was achieved, whereas expression of mutacin related genes was similar in all cells. Using microscopy it was shown that a part of the competent subpopulation lysed. This study demonstrates that the expression of the spxB gene was induced upon addition of CSP and that the histidine kinase HK9 was necessary for that induction. Furthermore it was shown that SpxB is a repressor of the CSP mediated competence development since a reduction in the expression of ComX and DNA-uptake was caused by SpxB.