Characteristics of Microorganisms

Most microorganisms, like these, can be seen only with the aid of a microscope.

Microorganisms are microscopic organisms usually made of a few cells or even one cell (unicellular organisms). They are the most abundant organisms on earth, as well as the most diverse -- living in virtually every environment on the planet. Microorganisms can be divided into several categories, each of which have distinct characteristics.

Bacteria

Bacteria are among the most common microorganisms on earth, and they are invisible to the naked eye with the exception of Thiomargarita namibiensis, which occasionally grows to about three-quarters of a millimeter. Bacteria vary in shape but are usually globular and live in colonies ranging in number from hundreds to millions. They are known for causing many sicknesses in larger organisms, but others are beneficial to life because they digest harmful chemicals. Bacteria reproduce asexually via budding and are typically able to reproduce every 10 minutes under ideal conditions. They are prokaryotes, meaning they lack a cell nucleus.

Protists

Protists are eukaryotic, meaning they have a cell nucleus, and the majority are unicellular. Most protists are photosynthetic, meaning they use light to produce energy and are the primary producers in many environments, particularly in oceans. Some types of algae are protists. Protists eat by filter feeding, meaning they trap passing particles. Others eat bacteria by engulfing them, a process known as phagocytosis. Protists reproduce both sexually and asexually. When two protists mate, is is known as conjugation; when a protist splits into two, it is known as binary fission.

Animals

Many animals are microscopic. Microscopic animals are multicellular and eukaryotic. Dust mites, spider mites, copepods and nematodes are all examples of microscopic animals. They feed on microscopic organic particles, often detritus (dead cells). Dust mites, for example, often feed on the flakes of dead skin using specialized mouth parts. Microscopic animals usually reproduce sexually, though some are born pregnant. Nearly all of them lay eggs, which are often capable of surviving environments that would kill the adult.

Fungi

Many fungi have unicellular or microscopic forms. Fungi have adaptive characteristics that allow them to thrive in various environments. For example, the Candida albicans can switch from a unicellular organism to an invasive multicellular organism when it infects a host. Microscopic fungi reproduce one of two ways. They either bud, like many other microorganisms, or produce spores known as basidiospores, which produce gametes when they come into contact with the badiospores of other fungi and eventually grow into mature adults.

References

• Biology Pages: Bacteria
• Proceedings of the National Academy of Sciences; "Metabolic Specialization Associated with Phenotypic Switching in Candida Albicans"; Chung-Yu Lan, et al; 2002
• Biology Corner: Protist
• Missouri S&T; Thiomargarita Namibiensis; Brandie Amsden
• Natural Resources Conservation Service: Soil Micro Animals
• Environment, Health and Safety Online; "Dust Mites: Everything You Might Not Want to Know!"; 2011