The Wolf Pack in the Soil
Beneath your feet in the soils of Colorado and around the world, a microbial drama unfolds. The bacterium Myxococcus xanthus lives, hunts, and survives through remarkable social cooperation. These rod-shaped, gram-negative bacteria are predators, but they don't hunt alone. When they detect prey, such as E. coli or other microbes, they swarm together in coordinated groups that function like a wolf pack. This collective behavior allows them to secrete a high concentration of enzymes and antibiotics that kill and dissolve their prey.
M. xanthus cells lack flagella, the whip-like tails many bacteria use to swim. Instead, they glide across surfaces using two distinct motility systems. "Adventurous" or A-motility allows individual cells to explore, moving at a rate of 2-4 microns per minute on firm surfaces. "Social" or S-motility, powered by the extension and retraction of Type IV pili, enables cells to move together in groups or rafts. This social movement is essential for forming the predatory swarm. When preying on colonies of other bacteria, the swarm organizes into macroscopic traveling waves called ripples, an efficient strategy for consuming the nutrients released from their lysed victims.
The genome of M. xanthus is unusually large for a bacterium, at over 9 million base pairs (Mb). This genetic complexity supports its sophisticated social behaviors. Its genome contains at least 18 distinct gene clusters for producing secondary metabolites, including potent antibiotics like myxovirescin (also known as TA), which are used in predation.
An Ancient Form of Altruism
When food becomes scarce, the predatory swarm shifts its behavior toward survival. The bacteria begin a developmental process that is an example of primitive multicellularity. Approximately 100,000 individual cells will aggregate, communicating through direct contact and chemical signals. Over a period of about 24 hours, they construct a three-dimensional structure called a fruiting body.
These haystack-shaped mounds can reach heights of 77 micrometers (μm) and diameters of about 138 μm. Within this structure, a division of labor occurs that involves cellular sacrifice. A majority of the cells die, likely providing nutrients for the developmental process. About 30 percent of the cells remain as metabolically active "peripheral rods" outside the main structure. Only about 10 percent of the original population differentiates into spherical, dormant myxospores inside the fruiting body. These spores are highly resistant to heat, desiccation, and UV, allowing the colony to survive until favorable conditions return. When nutrients become available again, the spores germinate, and the predatory life cycle begins anew.