Bacteria are the most numerous lifeforms on the planet. Most bacteria live as single cells that grow and multiply independently within larger communities of microbes. However, some bacterial cells assemble to form more complex structures where individual cells might perform distinct roles. Such bacteria are referred to as ‘multicellular bacteria’. For example, cells of bacteria known as Streptomyces collectively form filaments that help the bacteria collect nutrients from their food sources, and aerial structures bearing reproductive spores.
Bacteria in these filaments may come into contact with many other microbes in their surroundings including other bacteria within the same filament, other species of bacteria, and viruses. These contacts often lead to conflict, for example, if the microbes compete with each other for nutrients or if a virus tries to attack the bacteria.
Bacteria have evolved immune systems that detect other microbes and use antibiotics, toxins and other defense mechanisms against them. Compared to single-celled bacteria, multicellular bacteria may be more vulnerable to threats from viruses because once a virus has overcome the defenses of one cell in the multicellular assembly, it may be easier for it to kill, or spread to the other cells. However, it is not clear how these systems evolved to deal with the unique problems of multicellular bacteria. Now, Kaur, Burroughs et al. have used computational approaches to search for new immune systems in diverse multicellular bacteria. The new classes of systems they found are each made of different molecular components, but all require a large input of energy to be activated. This activation barrier prevents the bacterial cells from deploying weapons unless the signal from the enemy microbe crosses a high enough threshold.
Many tools used in molecular biology, and increasingly in medicine, have been derived from the immune systems of bacteria, such as the enzymes that cut or edit DNA. The findings of Kaur, Burroughs et al. may aid the development of new tools that specifically bind to viruses or other dangerous microbes, or inhibit their ability to interact with components in cells. The next step would be to perform experiments using some of the immune systems identified in this work.