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Microbiology is the study of “small life” – in other words the study of all living organisms that are too small to be visible with the naked eye and are usually visible only through a microscope. Such organisms are referred to as microorganisms or microbes. Microorganisms were first observed over three hundred years ago and it is estimated that only one percent of the species of microorganisms that exist have been studied so far. Microbiology is a broad term that includes bacteriology, virology, mycology, phycology, parasitology, and other branches of biology.

Microorganisms include bacteria, archaea, viruses, protozoa, protists, microscopic fungi and moulds, yeasts, and microscopic algae. Note that viruses, though not always strictly classed as living organisms, are included. Bacteria are a large group of unicellular prokaryotic microorganisms. A few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals. Archaea are single-celled microorganisms. Like bacteria, Archaea are prokaryotes and have no cell nucleus or any other organelles within their cells. Generally, archaea and bacteria are quite similar in size and shape however archaea possess genes and several metabolic pathways that are more closely related to those of eukaryotes. Viruses are sub-microscopic infectious agents that are unable to grow or reproduce outside a host cell. Microbiologists debate whether or not viruses are living organisms. They do not meet all the criteria used in the common definitions of life, however, viruses have genes and evolve by natural selection.

Fungi are eukaryotic, heterotrophic organisms. The majority grow as multicellular filaments called hyphae forming a mycelium but some fungal species grow as single cells. Fungi that are largely invisible to the naked eye are classed as microorganisms. Yeasts, moulds and mushrooms are examples of fungi. Algae include many single-celled organisms that are also considered protozoa, such as Euglena. Blue-green algae are in fact bacteria that obtain their energy through photosynthesis. They are generally, and more correctly, referred to as cyanobacteria.

Protozoa are mostly single-celled, motile protists that feed by phagocytosis, though there are numerous exceptions. Examples of protozoa include Euglena, Amoeba, Paramecium and Toxoplasma. Protists are a diverse group of eukaryotic microorganisms. Protists were traditionally subdivided into the one-celled animal-like protozoa, the plant-like protophyta (mostly one-celled algae), and the fungus-like slime molds and water molds. These groups have been replaced by phylogenetic-based classifications but are still used as informal names for describing the various protists.

Microbiology encompasses both prokaryotic and eukaryotic microorganisms but the majority of microbes are prokaryotes. Prokaryotes are a group of organisms that do not have a membrane-bound nucleus; they do not possess a nuclear membrane or nuclear envelope. They have DNA but the DNA is not enclosed in a membrane or envelope. In addition, they do not possess other membrane-bound organelles. Most are unicellular, but a few prokaryotes such as Myxobacteria have multicellular stages in their life cycles. The prokaryotes are divided into two domains: the bacteria and the archaea. Eukaryotes are defined as possessing a nuclear membrane enclosing their nucleus. Many eukaryotic cells also contain other membrane-bound organelles such as mitochondria, chloroplasts and Golgi bodies. Animals, plants, fungi and protists are eukaryotes.

More information and resources on microbiology at Microbiology and recent scientific developments in microbiology at the Microbiology Blog.

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