Classification of Microorganisms

Classified by shape, arrangement, Gram reaction, oxygen requirement, spore formation, motility, and biochemical characteristics.

Basic morphological groups:

Further classifying criteria:

• Gram reaction: positive (retains crystal violet, thick peptidoglycan) versus negative (counterstains pink, thin peptidoglycan plus outer membrane). Full detail in a dedicated later subsection.
• Oxygen requirement: obligate aerobe, facultative anaerobe, obligate anaerobe, microaerophile, aerotolerant. Full detail under Microbial Growth and Replication.
• Spore formation: spore-forming (e.g. Bacillus, Clostridium) versus non-spore-forming.
• Motility: flagellated/motile versus non-motile.
• Biochemical characteristics: carbohydrate fermentation patterns, enzyme production (catalase, coagulase, oxidase, urease), used extensively in laboratory identification.

Classified almost entirely by genome and structural characteristics, since viruses lack the morphological/biochemical features used for bacteria.

• Genome type: DNA versus RNA.
• Strandedness: single-stranded versus double-stranded.
• RNA sense (RNA viruses only): positive-sense (genome directly translatable, functions as mRNA) versus negative-sense (genome must first be transcribed by a virion-carried RNA-dependent RNA polymerase before translation).
• Envelope: enveloped (lipid membrane, host-derived) versus nonenveloped (naked capsid only).
• Capsid symmetry: icosahedral, helical, or complex.
• Replication strategy: the basis of the Baltimore classification system (covered in full in Virology), grouping viruses by the specific pathway their genome takes to produce mRNA.

• Yeasts: unicellular, reproduce by budding.
• Moulds: multicellular, form branching hyphae and mycelium.
• Dimorphic fungi: switch between yeast and mould forms depending on temperature (environmental temperature = mould; body temperature, 37°C = yeast). This temperature-dependent switch is a recurring exam point, covered further in Fungal Structure and Basic Biology.

• Protozoa: unicellular eukaryotes.
• Helminths: multicellular worms, subdivided into:
• Nematodes: roundworms.
• Cestodes: tapeworms.
• Trematodes: flukes.
• Ectoparasites: organisms living on the host's external surface (lice, mites, fleas).

• Full protozoal subgroup detail (amoebae, flagellates, ciliates, apicomplexans) and parasite life-cycle terminology are covered in the dedicated Parasite Classification and Basic Terminology subsection.