Microbial chatter: how bacteria talk to each other

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  • Published: May 1, 2016
  • Author: Ryan De Vooght-Johnson
  • Channels: HPLC
thumbnail image: Microbial chatter: how bacteria talk to each other

The language of bacteria

Just as communities of animals communicate with each other for protection and survival, bacteria work together to coordinate their gene expression. This process of cell-to-cell communication is called quorum sensing, and is used by bacteria to adapt to changing conditions, secrete harmful substances, escape the human immune response and defend themselves against antibiotics.

Just as communities of animals communicate with each other for protection and survival, bacteria work together to coordinate their gene expression. This process of cell-to-cell communication is called quorum sensing, and is used by bacteria to adapt to changing conditions, secrete harmful substances, escape the human immune response and defend themselves against antibiotics.

Quorum sensing relies on small signalling molecules called auto-inducers, which are produced inside the cell and released into the environment. Once the concentration of these signalling molecules reaches a threshold (in other words, when enough bacteria are present), changes in gene expression are initiated, allowing the coordination of group behaviour. This means bacteria only perform energy demanding processes when their impact will be felt. For example, the bioluminescence of Aliivibrio fischeri, a rod-shaped bacterium famous for its green glow, is only activated when the bacterial population reaches sufficient size. If only one cell emitted light it would go undetected, and therefore be a waste of energy.

Aliivibrio fischeri uses a group of chemicals called acylated homoserine lactones, or AHLs, as auto-inducers. These chemicals are also used by gram-negative bacteria, such as E. coli, Salmonella and cyanobacteria (algae).

The hidden life in marine sponges

In a recent study, researchers from the University of Haifa in Israel investigated what happens when bacteria try to communicate underwater. There has been some uncertainty regarding marine quorum sensing because AHLs are unstable at the high pH typical of seawater and because bacteria are few and far between in the ocean. AHL-mediated communication between bacteria is therefore usually confined to small habitats, typically inside invertebrates like sponges and corals.

This study focused on marine sponges, large and unusual organisms full of holes, which allow water – and bacteria – to move through them. Inside marine sponges, dense communities of microbes are found, which live in harmony with the sponge.

In early 2014, researchers collected a sample of a marine sponge from the depths of the Mediterranean Sea, within Israel’s Achziv marine reserve. From this sponge, the researchers isolated a type of bacteria which, based on its genetic sequence, they identified as a member of the Paracoccus genus, which is known to use AHL-based quorum sensing.

To detect the signalling molecules produced by the bacterium, the scientists used a biosensor that expresses a particular gene (called a reporter gene) in the presence of AHLs. To confirm the identity of the chemicals, the researchers used high-performance liquid chromatography alongside high-resolution tandem mass spectrometry.

The bacterium was found to produce a wide range of long-chain AHLs, including 12 saturated long-chain lactones and four unsaturated long-chain lactones. For many of the AHLs, this is the first time they have been detected in sponge-associated bacteria.

The discovery of such a wide range of AHLs suggests that the bacteria use a complex communication circuit, with several different genes used to synthesise several different chemicals. As different AHLs have different sensitivities to pH, the production of multiple types allows the bacteria to develop a fine-tuned regulatory network that can sense and respond to environmental cues.

An underestimated life form

Finally, to assess whether this bacterial strain is found in other environments, the researchers compared its gene sequence to microorganisms from over 100 sponge species and in 182 environmental samples, including seawater, freshwater and sediments from the bottom of the ocean.

The bacterium was found in several species of sponge and several samples of seawater and sediment, leading the authors to suggest that it can either live free or associated with a host. However, they say quorum sensing is likely only active when it is inside enclosed spaces, such as a marine sponge, where AHLs aren’t disabled by pH and can accumulate.

It seems then that the gene expression of this bacteria depends on its location. This is an exciting hypothesis that adds complexity to this seemingly simple life form, but more research is needed to prove it. The researchers hope to sequence the genome of the bacteria to identify the genes involved in producing and responding to AHLs, and to explore how these genes are regulated in different ways when free in the ocean or attached to a host.

Related Links

Isr. J. Chem. 2016., Saurav et al., Isolation of Marine Paracoccus sp. Ss63 from the Sponge Sarcotragus sp. and Characterization of its Quorum-Sensing Chemical-Signaling Molecules by LC-MS/MS Analysis.

Wikipedia: Quorum sensing

Bacterial Quorum Sensing: Its Role in Virulence and Possibilities for Its Control

Quorum Sensing: Bacteria Talk Sense

What is quorum sensing?

Article by Ryan De Vooght-Johnson

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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