In the multicellular soil bacterium Streptomyces coelicolor, some cells start producing lots of antibiotics after mutations delete big chunks of their genomes. Now a computer model has helped to confirm that this is no accident but an evolved mechanism for dividing labour among cells in a colony.
鈥淲颈迟丑 Streptomyces, permanent differentiation happens by breaking the genome,鈥� says at the University of Cambridge.
While definitions vary on what makes an organism multicellular, for many biologists it isn鈥檛 simply about having lots of cells, but about having those different cells take on different jobs. For instance, our bodies contain specialised muscle cells, brain cells and so on.
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Contrary to popular perception, lots of bacteria are multicellular according to these criteria. For instance, in which some cells take on specialised roles such as fixing nitrogen for other cells to use.
In bacteria, as in other multicellular organisms, the cells doing different jobs are usually genetically identical. The division of labour comes about by key genes being switched on or off.
This conventional form of cell division happens in Streptomyces bacteria, too. Colonies consist of a branching network of filaments in the soil. When they run out of food, some filaments grow upwards and produce spores, very much like a fungus.
But recent studies by at Leiden University in the Netherlands have shown that some cells in Streptomyces colonies also mutate in ways that makes them pump out more antibiotics to help the colony compete against rival organisms. These mutations, however, impair their ability to reproduce. In other words, the mutated cells , just like almost all cells in multicellular organisms.
That suggests this phenomenon is no accident but an evolved mechanism of specialisation via mutation. To test this idea, Colizzi, Rozen and their colleagues created a computer model of Streptomyces and let the virtual bacteria evolve over 500 generations.
The model showed that genes related to reproduction become separated from those involved in antibiotic production, with reproductive genes moving to the end of the chromosome where they are more likely to be deleted by mutations. In the model, the rate of mutations also increases, with mutations occurring more often in places that lead to the loss of reproductive genes, says Colizzi. This matches what is seen in real life 鈥� Streptomyces has long been known to have an 鈥渦nstable鈥� genome with frequent major mutations.
鈥淲hat we are talking about here is division of labour within a colony,鈥� says at the John Innes Centre in Norfolk, UK. 鈥淟ong term, the cells with such mutations won鈥檛 go on to continue the line but they will benefit the wider community, their kin. That鈥檚 different, that鈥檚 interesting.鈥�
This kind of work isn鈥檛 just of academic interest. Streptomyces and related bacteria are capable of producing a huge range of antibiotics but some aren鈥檛 produced in lab conditions, says Bush. Understanding these microbes better could help us discover much-needed new antibiotics.
In addition to antibiotics, Streptomyces also produces the chemicals responsible for the smell of soil, called geosmin and 2-MIB. In 2020, Bush and his colleagues reported that Streptomyces produce these chemicals to attract springtails, which eat the bacteria but also disperse their spores.
Reference: bioRxiv,聽
