Microbial Interactions: Parasitism

Microbial parasitism is an interaction in which parasitic microbes benefit by exploiting a host, causing harm to the host.

Parasitic microbes are of three types — external, or ectoparasites, which stay on the surface; internal, or endoparasites, which inhabit the tissues or internal structures; and intracellular parasites, which reside within the host’s cells.

Parasites can coexist with hosts to reproduce, acquire nutrients, or evade immunity.

Parasitic relationships involve continuous exploitation of a host. Outcomes depend on host defense mechanisms, parasite load, and environmental conditions.

When this balance shifts, effective host defenses or treatment can eliminate the parasite, whereas unchecked parasite growth may cause disease or death.

Sometimes, parasitism can lead to genomic reduction in the parasite, with genes becoming redundant due to reliance on the host.

As an example, bacterial species Buchnera aphidicola in aphids and Mycobacterium leprae in humans have shed many metabolic genes as a result of long-term host dependence for essential nutrients.