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The Variety and Roles of Microbes

Author(s): Barbara Tharp, MS, and Nancy Moreno, PhD

Comparing Groups of Microbes

Next, have your students discuss the physical and biological similarities and differences between the four groups that contain at least some microscopic members (viruses, bacteria, protists and fungi). Use the table on this slide as a guide. 

Viruses are not considered to be cells, because they are made only of genetic material (RNA or DNA) in a protein coat. In fact, they are not even considered living organisms, because these microscopic pathogens cannot grow or reproduce on their own. Instead, they invade a host cell and use its cellular machinery to produce new copies of themselves, sometimes at great harm to the host organism. 

Bacteria are single-celled and microscopic, with cell membranes and usually cell walls. Bacteria do not have membrane-bound organelles or well-defined nuclei. Bacteria are a diverse group. Some use CO2 as a carbon source to obtain energy (autotrophic) through processes like photosynthesis. Others obtain energy by breaking down organic material as a carbon source (heterotrophic). Many bacterial microbes carry out important roles in the environment and ecosystem, but some can cause disease.

Protozoa are animal-like, single-celled, heterotrophic. They are an extremely diverse, informal group of organisms classified under different kingdoms of the Eukarya domain. Some are microscopic; others macroscopic. Some are single-celled, while others are multi-celled. All protists have cell membranes, well-defined nuclei and organelles. Some also have cell walls. There are three informally recognized groups of protists.

Algae are plant-like, single- or multi-celled, autotrophic. Brown algae (seaweed) are multicellular and can grow 180 feet in a single year. 

Water and Slime Molds are fungus-like, single- or multi-celled, heterotrophic.

Fungi are another diverse group, which contain microbes, whose sizes vary from micro- to macroscopic. Some fungi are single-celled, while others are multicellular. All fungi have a cell membrane, well-defined nucleus and organelles. Most have cell walls. These organisms obtain their nutrients from dead or decaying materials in the environment, making them natural recyclers. It is interesting note that fungi secrete digestive enzymes to breakdown organic matter externally and then reabsorb the digested organic matter, unlike the internal digestion systems of many animals.

Funded by the following grant(s)

Science Education Partnership Award, NIH

Science Education Partnership Award, NIH

Grant Number: 5R25RR018605