Some bacteria living in a biofilm sacrifice themselves to ensure the survival of the community

Life and death of an "altruistic" bacterium.

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In their natural environment, most bacteria preferentially live as complex surface-attached multicellular colonies called biofilms. Biofilms begin with a few cells adhering to a surface, where they multiply to form a mature colony. When conditions deteriorate, cells can leave the biofilm. This dispersion 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 community’s survival.

Yves Brun, a professor in the Department of Microbiology, Infectious Diseases and Immunology at Université de Montréal, said, “For the bacterium Caulobacter crescentus, the biofilm becomes a kind of prison in perpetuity: once cells are attached to a surface through a strong adhesive at one end of the cell, they cannot leave the biofilm.”

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

Mature bacterial biofilm of Caulobacter cells
Mature bacterial biofilm of Caulobacter cells attached to a glass surface. CREDIT: CÉCILE BERNE

How do cells decide to stay or leave the biofilm?

Scientists, in 2010, showed that when Caulobacter cells die in the biofilm, they release their DNA, inhibiting daughter cells from adhering to the biofilm. It, therefore, promotes relocation from environments where the death rate increases.

Therefore, scientists want to determine if cell death was random when environmental quality decreased or if 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 sacrifice themselves when the conditions inside the biofilm deteriorate.”

“Known as a toxin-antitoxin system, this mechanism uses a toxin that targets a vital function and its associated 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 forming a biofilm around a dust particle. CREDIT: CÉCILE BERNE

“‘Using a combination of genetics and microscopy, we showed that the toxin-antitoxin system is activated when oxygen becomes sparse as the biofilm becomes larger and cells compete for the available oxygen.”

A portion of the cells that die as a result release DNA, which encourages the dispersal of their live siblings to maybe more suitable surroundings. This prevents overcrowding, which would further degrade the biofilm’s environmental quality.

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

Brun said“The downside is that the biofilm lifestyle is also a strategy used by pathogenic bacteria to become more resistant to antibiotics.”

“Understanding the mechanisms driving the balance between cells joining the biofilm and cells swimming away will help us develop solutions to the challenge of antibiotic resistance, to promote the formation of biofilms when we want them, and eradicate them when we do not.”

Journal Reference:

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

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