Final answer:
Commensalism defines the symbiotic relationship where one organism benefits and the other is not affected. It is distinct from mutualism, where both benefit, and parasitism, where one benefits at the expense of the other.
Step-by-step explanation:
When microbes are in a close nutritional relationship where one organism benefits and the other is not harmed, the interaction is called commensalism. Commensalism is a type of symbiotic relationship that is characterized by a situation where one species benefits while the other species is not affected. An example of this would be certain mites that attach themselves to larger insects for transportation without causing harm to their host.