Chemical communication helps bacteria boost their immune systems

Just as humans are susceptible to diseases like influenza and measles, bacteria also need to defend themselves against viruses. Now, research led by the University of Otago has found that bacteria can boost their immune systems by ‘talking’ to each other.

Associate Professor Peter Fineran, leader of the Otago research team, noted that bacteria often reside in close quarters in order to share resources. “However,” he said, “there are also potential drawbacks to community life as high-density bacterial populations are more vulnerable to the spread of viruses — just like people in a crowded bus or a daycare centre.”

While humans have evolved “sophisticated immune systems that enable our bodies to fight the viral infections that render us ill”, said Associate Professor Fineran, bacteria “often possess similar adaptive immunity called CRISPR-Cas systems”. These immune systems create genetic memories of specific past viral infections by taking little snippets of the viruses’ DNA and storing them in memory banks to aid in recognising and destroying future infections.

One of the least understood aspects of the CRISPR-Cas field is how bacteria control the activity of these systems — too much activity can result in an autoimmune-like disease, killing the host cell, but too little activity might allow viruses to wipe out entire bacterial communities. Writing in the journal Molecular Cell, the Otago team revealed that the ability of bacteria to gauge the number of cells in their communities enables them to boost the power of their CRISPR-Cas systems in order to generate the correct outcome.

As explained by Associate Professor Fineran, the bacteria sense the population density by ‘talking’ to each other using a form of chemical communication known as quorum sensing. “The higher the population density, the stronger the communication between cells becomes, which results in greater coordination of immune defences,” he said.

The outcome, said first author Adrian Patterson, is that the bacterial cells pre-emptively elevate their immunity when they are most at risk of a virus spreading through the population. “They both increase their ability to generate new immune memories and strengthen existing immunity by up to 500-fold,” he said.

Dr Simon Jackson, second author of the study, said the researchers have recently made significant advances in understanding how bacterial immune systems function.

“The really exciting part of our most recent discovery is that we predict the communication-based coordination of CRISPR-Cas immunity to be widespread throughout bacterial species,” he said.