Bacterial Cell Differentiation

Abstract

Four bacterial developmental systems are described. In the dimorphic cell cycle of Caulobacter crescentus, differences in the proteins assembled at cell poles cause cell division to generate a stalked cell rich in regulator DivK-P, and competent for continued proliferation, and a swarmer cell rich in regulator CtrA-P, and unable to proliferate until it discards its flagellum and grows a stalk. The other three systems all lead to the formation of spores, but by completely different routes. In Bacillus subtilis, an endospore forms inside a mother cell; in the mycelial Streptomyces coelicolor, long hyphae grow into the air and then turn into chains of spores; whereas in Myxococcus xanthus, which hunts in motile swarms to prey on other bacteria, the swarm aggregates into a mound to form a fruiting body, inside which cells change into spores. The regulatory cascades leading to differentiation evolved completely independently in the four systems, but show some common strategies.

Key Concepts

• Bacterial cells can be both organisationally and developmentally complex.

• Bacterial development is usually driven forward by positively acting regulatory cascades, often reinforced by positive feedback loops.

• Cascades activating bacterial differentiation often respond to environmental or physiological information through the action of repressors or other negatively acting mechanisms.

• Diverse extracellular signals are often employed to allow communication between cells, leading to coordination of bacterial development.

• Sporulation has evolved completely independently in different groups of bacteria.

• Some kinds of protein recur frequently in bacterial developmental systems, including sigma factors, phosphoproteins and proteases.