Bacterial QS (Abisado et al. 2018,Schluter et al 2016)

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Bacterial Quorum Sensing and Microbial Community Interactions

DOI: 10.1128/mBio.02331-17

• Introduction:

In this review, they focus on models that have been developed to study how quorum sensing (QS) systems increase the success of individuals in multiple-strain and multiple-species communities.

As it has been extensively discussed in previous issues, the AHL (acyl-homoserine lactone) signals are the most studied QS signals. And here's how the process works:

Bioluminescence happens through the QS proteins LuxR and LuxI at high cell densities. LuxI is a signal synthase that produces N-3-oxo-hexanoyl-homoserine lactone (3OC6-HSL).

3OC6-HSL specifically binds to LuxR, a transcription factor that activates expression of the luxCDABEG operon, which contains the genes that enable bioluminescence.

1. LuxR proteins contain a conserved N-terminal AHL-binding domain and a C-terminal helix-turn-helix DNA-binding domain.

2. The N terminus of LuxR interacts with AHLs, and this interaction induces homodimerization of the LuxR homologue and subsequent binding to a specific DNA sequence called the luxbox in the target promoter.

3. The AHL-bound LuxR often induces transcription of its cognate luxI signal synthase gene, resulting in a positive-feedback loop that further increases the concentration of AHLs.

Interesting point brought by the article: QS models can cooperate based on protease production.

• Because the rise of cheaters can threaten cooperation in a population, a recurring question in evolutionary biology is how do cooperative systems persist, despite the ongoing threat of cheating?

For example, Pseudomonas aeruginosa cells with a defective lasR gene preventing them from responding to quorum sensing signals, can outcompete cooperating wild-type cells. This is because the mutants have a higher growth rate since they do not produce the costly cooperative secretions, but can benefit from the secretions of wild-type cells.

• Many bacterial species use QS to control production of secreted or cell-targeted toxins: for example, bacteriocins in Streptococcus species.

  Thus, QS activation would be predicted to influence species dynamics in polymicrobial communities.

[nathaliaraquelx]

The Evolution of Quorum Sensing as a Mechanism to Infer Kinship

DOI: 10.1371/journal.pcbi.1004848

Introduction:

The canonical explanation for the function of quorum sensing is that autoinducer concentrations can be used as a proxy for cell density. Estimating the density of cells in a given environment allows microbes to cooperate in a coordinated manner.

Results:

• Quorum sensing increases performance in clonal groups;
• Constitutive secretors lose when surrounded by competing genotypes;

Conclusion:

• Quorum sensing as a mechanism to infer kinship:

A strain is predicted to be more successful then, if it can match its cooperative secretions not to the cell density or total biomass, but to the appropriate level of local relative cell density or genetic relatedness.