ScienceWeek

Science 26 October 2007:
Vol. 318. no. 5850, pp. 578 - 579
DOI: 10.1126/science.1150704

http://scienceweek.com/2007/071028b.htm

Microbiology: Bacteria and Multicellularity

Roberto Kolter

Constituent cells of an organism communicate with each other through chemical signals to coordinate growth and differentiation. Cells also perish along the way, their programmed death benefiting the organism's survival (1). Bacteria also "talk" to each other through chemical signals and, on occasion, kill themselves (2, 3). This suggests that they are capable of multicellular behaviors (4). On page 652 of this issue, Kolodkin-Gal et al. show that the bacterium Escherichia coli releases a signaling molecule that activates a programmed cell death pathway, supporting the concept that multicellularity is a general bacterial trait (5).

Programmed cell death mechanisms in bacteria often follow a common theme: A lethal toxin is constitutively produced and, at the same time, an antitoxin is made for protection (3). When synthesis of both is maintained, the bacterial cell survives even though the antitoxin is usually more labile than the toxin. But when their synthesis is arrested, the greater lability of the antitoxin eventually unmasks the toxin's activity, leading to the cell's demise. The lethal effect of these toxins can be due to a variety of activities, such as inhibiting DNA replication or translation.

Bacterial toxin-antitoxin systems were first found encoded in plasmids, autonomously replicating extrachromosomal elements whose inheritance is not always guaranteed upon cell division. However, any cell that fails to inherit the plasmid will soon find itself without any antitoxin and dead. The surprise came when similar toxin-antitoxin coding modules were found in bacterial chromosomes. Why would a bacterium want to kill itself? Such a response makes little sense for an individual cell. But when that cell is a member of a multicellular aggregate, its death could benefit the rest. For example, the development of fingers in a human embryo requires the cells between the fingers to undergo programmed cell death.

BOOK SOURCES:

bacteria
multicellularity
Escherichia coli
biology of colonial organisms