Streptococci survive by communicating

HZI researchers investigate how caries pathogen vanquishes its competitors


Streptococcus mutans, a caries bacterium, under the fluorescence microscope. All cells producing bacteriocin are stained blue. The pale green cells are also capable of taking up foreign DNA.

In order to survive in their environment and to remain competitive, bacteria have to keep acquiring new properties. They often do this by taking up foreign DNA and integrating them into their own genome. Scientists of the Helmholtz Centre for Infection Research (HZI) in Braunschweig recently were the first to show how this mechanism works in detail in Streptococcus mutans, which causes caries. They published their results in the professional magazine, "Plos Genetics".

The bacterium, Streptococcus mutans, is thought to be the main cause of caries and dwells in the human plaque. It is in constant competition with other germs in this location fighting for survival. Only if it keeps developing and acquiring new properties it can survive this fight. For this purpose, the bacterium incorporates DNA from its environment into its own genome. The ability to do so is called genetic competence. "It was already known that the competence is controlled by quorum sensing, which means by cell-cell communication. We recently were the first to show that the crucial signals for this process, XIP and CSP, interact with each other," says Prof Irene Wagner-Döbler, who is the director of the "Microbial Communication" research group at the HZI. "For this purpose, we linked the promoters of the participating genes along the entire signal chain to fluorescent proteins."

In addition, Wagner-Döbler and her team showed that another mechanism is very closely correlated to this competence: Bacteriocin synthesis. Bacteriocins are peptide antibiotics killing other Streptococci. "Our results show that the production of the bacteriocins is regulated by the same Gene that controls the competence ability," says Dr Michael Reck, who is a scientist in the research group of Wagner-Döbler and the lead author of the study. "Both processes are therefore linked much more closely than was previously presumed."

The close linkage of the two quorum sensing systems leads to the conclusion that Streptococcus mutans kills other bacteria not only to get rid of its competitors, but also makes use of the released genetic substance to expand its own Gene repertoire. "The ability to do so ensures survival despite the heavy competition in its natural habitat and explains why this bacterium is so prevalent," says Wagner-Döbler.

In the long-term, the new insights into the mechanisms may also yield new approaches for the fight against this pathogen. "Once we know exactly how a pathogen survives and acts in its natural habitat, we can develop strategies aimed at reducing its spread," says Reck.

Original publication:

PLoS Genet 11(7): e1005353. DOI:10.1371/journal.pgen.1005353.