Microbes require energy for growth. "Green" pigmented organisms like plants and algae are capable of capturing the sun's energy and using inorganic carbon (such as carbon dioxide) to produce energy rich compounds like carbohydrates. The photosynthetic and/or chemotrophic organisms are commonly referred to as autotrophic (self-feeders). Other microbes including small animals and fungi, as well as many bacteria and protozoans, require previously manufactured carbon compounds like carbohydrates, proteins, and lipids as their source of energy. These organisms are called heterotrophic and may use a host (parasite) or nonliving material (saprobe) as the source of their food.

Using a microscope and proper stains, it is possible to sometimes identify a distinct nucleus in the cells of living organisms. A distinct nucleus is observable in all microbes except the monera/procaryotae; so fungi, protists, plants, and animals are called eucaryotic. The more primitive organisms like bacteria lack a distinct nucleus and are called procaryotic. The hereditary molecule (DNA) is found in all living cells and essential for reproduction.