The Web of Life (33-6) Microbe Associations


Despite tribal antagonisms, microbes are generally gregarious, and broad-minded about it. Their cooperation commonly extends to mutualist associations with many other species.

 Teamwork in the Rhizosphere

Both bacteria and microscopic fungi need to be mobile to survive and thrive, including ones that live in the rhizosphere: the narrow area of soil that is directly influenced by plant root secretions. The 2 team up to get around. Fungal spores attach themselves to bacteria to hitch a ride, or bacteria entrap spores and wrap them in their flagella, then carry them along. (Soil outside the rhizosphere is called bulk soil.)

Paenibacillus vortex is exemplary. These bacteria will even recover fungal spores from a life-threatening locale, moving them to a new home where they can germinate and start new colonies.

The payback comes when a bacterium hits an air pocket canyon too large to cross. The bacterium releases its spores, allowing the fungi to germinate into a colony that spans the gap. The bacterium then strolls over the bridge made by the fungi’s mycelia (the branch-like weave that grows from the spores).


Plants interact with a multitude of beneficial bacteria who call their rooted host home. When pathogens threaten, homebody microbes pitch in to protect the plant, producing phytohormones that help keep plant tissue healthy.

A phytohormone is a plant hormone. Hormones in plants and animals are signaling molecules that regulate physiology and behavior.

Symbiosis among microbial species evolved to optimize the special skills that each species can contribute. Housing in larger hosts means a decent living, the company of fellows, and an upscale long-term residence.

Microbial associations may be beneficial, commensal, or pathogenic. Relationships vary between a microbe and its host by species, or even by population. Normally genteel microbes can turn nasty if their environment deteriorates.

Sometimes residence is a means to an end. Some microbes use plants as a way station on their way to infecting insects and other animals that visit or prey upon the plants. The easiest way to consumer is through producers.

Biofilms associated with plant roots promote a mutual exchange of nutrients. Biofilms form the microbiome of all animals, assisting in almost all facets of living.