Microbio 1.2

Bacteria (sometimes referred to as Eubacteria, meaning ‘true bacteria’) constitute a large cohort of prokaryotic microorganisms. They can be considered the everyday bacteria and are common in human daily life. They have a variety of shapes by which they can be further classified.

Bacteria display many distinct morphologies (shapes), the most common include: coccus (round/spherical), bacillus (rod), vibrio (curved rod), or spirillum (spiral/corkscrew).

Aside from the shape of the bacterial cell, consider the cellular organization — cells may appear as isolated (individual) cells, in chains, or in clusters. Certain bacteria may also demonstrate motility (swimming movements), while others do not. Examples of Eubacteria are: Streptococcus, which have the appearance of round chains; E. coli, which have the appearance of rod; and Mycobacterium tuberculosis, a unicellular bacterium.

Archaea, the other prokaryotic class of microorganisms, have similar characteristics and shapes as Bacteria (Eubacteria) and yet remain genetically and compositionally distinct. Most notably, they can survive in extremely harsh environmental conditions, such as high salt levels, acid conditions, high temperatures, and oxygen-poor conditions. As such, Archaeons may also be referred to as extremophiles, based on the extreme or atypical environments in which they can be found.

Eukarya is a diverse category and consists of a wide range of microorganism. Due to high diversity and complexity, eukaryotes can be subdivided into multiple categories, a process still debated today. However, the unifying characteristic of all eukaryotic cells is the presence of a membrane-bound nuclear region within the cell. For the purposes of this course, eukaryotic microorganisms will be classified as either: Animalia, Plantae, Fungi, and Protista.

Animalia consists of multicellular eukaryotic organisms, and, as the name suggests, includes animals, as well as humans (and their cells) in their classification. All microorganisms in this category are heterotrophic, meaning they are incapable of producing their own energy, must absorb (consume) nutrients from the environment, and have the general characteristic of motility (the ability to move).

Plantae are also multicellular eukaryotes, but, unlike Animalia, they can obtain most of their energy from sunlight via photosynthesis—a process that converts light energy (sunlight) into chemical energy (sugars) within the organism and fuels its activities.

Fungi can be either multicellular or unicellular microorganisms, and, like Animalia, they are heterotrophic. A defining characteristic of fungi is the presence of chitin, a derivative of glucose, in their cell walls. Common examples of multicellular fungi are molds and mushrooms, while perhaps the most common unicellular fungus is yeast.

Protista are unicellular microorganisms that may form as colonies. However, the colonies (seemingly multicellular) do not form tissue layers and thus retain the unicellular classification. Protista is often used as a classification for any microorganism that does not clearly fit the criteria for being considered animal, plant, or fungi. Common examples range from amoeba, known for its motility; algae, known for its plant-like characteristics; and mold, known for its fungi-like properties.

Viruses, although they are considered neither prokaryotic nor eukaryotic, do play a large role in microbiology. Viruses are not considered living and do not replicate on their own, meaning they must replicate within a host. Outside of a host cell viruses have no metabolism and are essentially dormant — the virus takes up neither energy nor nutrients. Although viruses contain similar structures to the microorganisms described above, they are not cellular, and, as such, they do not fall within one of the categories described above. For instance, viruses contain a capsid, a membrane-like structure that contains genetic material, similar to the nucleus of a eukaryotic cell.