TABLE 2.

Bacterial shapes and possible selective forces

Shape	Possible selective forces
Symmetrical	Cell division apparatus
	Equipartition to daughter cells
Various widths	Nutrient availability
	Flotation requirements
	Efficient chemotaxis in gradients
Small size	Nutrient limitation
	Passive dispersal
	Sieving through geological strata
	Protistan predation
Larger rods	Reduced dispersal
	Fluid shear stress
	Efficient motility
	Motility near surfaces
	Swarm cell differentiation
	Protistan predation
Filamentation	Nutrient limitation
	Fluid shear stress
	Stability to washing out of soil
	Gliding (slime extrusion) motility
	Swarm cell differentiation
	Protistan predation
	Immune system
	Multiorganism symbiosis
Prosthecate cells	Nutrient limitation
	Attachment
	Protistan predation
Helical/spiral	Motility in viscous environments
cells	Motility near surfaces
	Protistan predation
Bifids (Y shapes)	Polar-localized protein complexes
	Symbiosis requirements