In their natural environment, most bacteria prefer to live as complex multicellular surface colonies called biofilms. Biofilms start with a few cells that adhere to a surface, where they multiply to form a mature colony. When conditions worsen, cells can leave the biofilm. This spreading is considered a necessary process that modifies the overall biofilm architecture and promotes the colonization of new environments.

A new study shows how some bacteria living in a biofilm sacrifice themselves to ensure the survival of the community.

Yves Brun, a professor in the Department of Microbiology, Infectious Diseases and Immunology at the Université de Montréal, said: “For the bacterium Caulobacter crescentus, the biofilm becomes a kind of prison forever. Once cells are attached to a surface by a strong glue at one end of the cell, they can’t leave the biofilm.”

“However, when these attached cells divide, their non-attached ‘daughter’ cells can either join the biofilm or swim away.”

Mature bacterial biofilm of Caulobacter cells
Mature bacterial biofilm of Caulobacter cells adhered to a glass surface. CREDIT: CECILE BERNE

How do cells decide to stay or leave the biofilm?

Scientists showed in 2010 that when Caulobacter cells die in the biofilm, they release their DNA, preventing daughter cells from attaching to the biofilm. It therefore promotes relocation from environments where the mortality rate is increasing.

Therefore, scientists want to determine whether cell death was random as environmental quality decreased or whether it was a controlled process responding to a particular signal.

Team member Cécile Berne, the study’s lead author, said: “We showed that Caulobacter uses a programmed cell death mechanism that causes some cells to self-sacrifice when conditions in the biofilm deteriorate.”

“This mechanism is known as a toxin-antitoxin system and uses a toxin that targets a vital function and its corresponding antidote, the antitoxin. The toxin is more stable than the antitoxin, and when programmed cell death is initiated, the amount of antitoxin is reduced, resulting in cell death.”

Attached Caulobacter cells
Attached Caulobacter cells form a biofilm around a dust particle. CREDIT: CECILE BERNE

“Using a combination of genetics and microscopy, we have shown that the toxin-antitoxin system is activated when oxygen becomes scarce as the biofilm expands and cells compete for available oxygen.”

Some of the cells that die as a result release DNA, which promotes the spread of their living siblings to a more suitable environment. This prevents overcrowding, which would further affect the environmental quality of the biofilm.

Biofilms affect our daily lives in both positive and harmful ways. Biofilm-forming bacteria are commonly used in the food, wastewater treatment and pollution control industries.

Brun said, “The downside is that the biofilm lifestyle is also a strategy that disease-causing bacteria use to become more resistant to antibiotics.”

“Understanding the mechanisms that determine the balance between cells that join the biofilm and cells that swim away will help us develop solutions to the challenge of antibiotic resistance, to promote the formation of biofilms when we want them, and to eliminate them from to row when we don’t want to.”

Magazine reference:

  1. Cecile Berne, Sebastien Zappa, Yves V Brun et al. EDNA-stimulated cell spreading of Caulobacter crescentus biofilms under oxygen limitation depends on a toxin-antitoxin system. eLife. DOI: 10.7554/eLife.80808