6 Classification of Microorganisms - MicroCore Review Center

What is the science of biological classification?

• Taxonomy

• Classification

• Nomenclature

• Systematics

Answer explanation

Taxonomy is the science of classification according to a pre-determined system, with the resulting catalog used to provide a conceptual framework for discussion, analysis, or information retrieval. Image from: https://www.britannica.com/science/taxonomy

Which of the following is not true about systematics?

• Study of the diversity of life on the planet earth, both past and present, and the relationships among living things through time

• Used to understand the evolutionary history of life on earth

• Describing, identifying, classifying, and naming of organisms

• Systematics is different from taxonomy

Answer explanation

Systematics is often used synonymously with "taxonomy" and scientific classification." However, taxonomy is the describing, identifying, classifying, and naming of organisms.  Systematics uses taxonomy as a means to understand organisms, as nothing about an organism's relationships with other living things can be understood without it first being properly studied and described in sufficient detail to identify and classify it correctly.

Which of the following is not true about a species and a strain?

• A strain is a typical representative of the species to which it may be assigned

• Species share a recent common ancestor for their 16S rRNA genes

• Species are defined operationally as a group of strains

  sharing a high degree of similarity in many traits

• A strain is a low-level taxonomic rank used in three related ways, usually at the infraspecific level (within a species)

Answer explanation

ATCC 12814 strain of Escherichia coli is non-motile and produces orange-pigmented colonies and a capsule – a combination of characteristics not expected for typical strains of E. coli. Image from: https://www.jlindquist.com/generalmicro/102usage.html

Which of the following describes natural classification?

• Only a few characters are considered

• More stable classification

• Members of different groups are usually not similar in hereditary pattern

• Related phylogenetically

Answer explanation

An artificial system of classification is based upon one or a few easily observable characteristics and is devised and used for a limited purpose where convenience and speed are important. It conveys little information. On the other hand, the natural system of classification is based on a large number of characters.

Which of the following is true about the principles of nomenclature?

• The specific name can change if the organism is assigned to another genus because of new information

• A law of priority ensures the use of the newest available legitimate name

• The generic name is stable

• The oldest epithet for a particular organism takes precedence and must be used when you consider the specific name

Answer explanation

When the name of a genus is changed, the specific epithets of the species included under the original generic name must be retained for the same species if they are transferred to the new genus. (From the International Code of Nomenclature of Bacteria: https://www.ncbi.nlm.nih.gov/books/NBK8808/)

Which group is described as using inorganic chemicals as energy source and carbon dioxide as carbon source?

• Photoheterotroph

• Chemoautotroph

• Photoautotroph

• Chemoheterotroph

Answer explanation

Chemoautotrophs are capable of utilizing inorganic compounds such as hydrogen sulfide, sulfur, ammonium, and ferrous iron as reducing agents, and synthesize organic compounds from carbon dioxide. Image from: https://slideplayer.com/slide/13753593/

Which of the following is true of psychrotrophs?

• Grows between -5°C and 15°C, but can grow at temperatures between 45°C and 70°C

• Grows between 15°C and 30°C, but can grow at lower temperatures as well

• Grows between 25°C and 45°C

• Grows between 45°C and 70°C

Answer explanation

The term psychrotrophs (also denominated psychrotolerant) refers to microorganisms that have the ability to grow at low temperatures but have optimal and maximal growth temperatures above 15°C and 20°C, respectively. Image from: https://www.bartleby.com/questions-and-answers/hyperthermophile-mesophile-thermophile-psychrotroph-psychrophile-10-o-10-20-30-40-50-60-70-80-90-100/2b92fddb-4018-45de-b0bc-884108f68ea4

Which of the following illustrates the growth of an aerotolerant anaerobe?

• a

• b

• c

• d

• e

Answer explanation

Aerotolerant anaerobes grow throughout the tube. Growth is not better near the surface because these organisms can only ferment. Image from: https://swo.today/what-are-bacteria/aerobic-bacteria/

Which of the following is true about the cell wall of bacteria and archaea?

• The lipid A of Gram negative bacteria like other glyerol lipids contains glycerol to which long-chain hydrocarbons are attached to the hydroxyl groups via carboxylic acid ester linkages

• Cell walls of certain methanogens have pseudomurein which replaces NAG with NAT

• The glycosidic bonds between the sugar derivatives in archaea is β-1,4 instead of β-1,3

• Cell walls of some archaea may be composed of polymers of glucose, glucuronic acid, galactosamine uronic acid, and acetate

• Gram positive and Gram negative bacteria produce acidic molecules called teichoic acids embedded in their cell wall

Answer explanation

Image from: Legat, A., Denner, E. B., Dornmayr-Pfaffenhuemer, M., Pfeiffer, P., Knopf, B., Claus, H., Gruber, C., König, H., Wanner, G., & Stan-Lotter, H. (2013). Properties of Halococcus salifodinae, an Isolate from Permian Rock Salt Deposits, Compared with Halococci from Surface Waters. Life (Basel, Switzerland), 3(1), 244–259. https://doi.org/10.3390/life3010244

Which of the following statements are not true?

• A value of 70% or less genomic hybridization and a difference in SSU rRNA gene sequence of 3% or more between two organisms is taken as evidence that the two are distinct species

• The genomic similarity between the two organisms is calculated as a percentage of probe hybridized to target relative to a control (probe DNA hybridized to target DNA from the same organism)

• Pairs of strains that share greater than 70% 16S rRNA gene sequence similarity and 97% genomic hybridization values are likely members of the same species

• The degree of DNA–DNA hybridization between the genomes of two organisms provides a measure of their genomic similarity

Answer explanation

Pairs of strains that share greater than 97% 16S rRNA gene sequence similarity and 70% genomic hybridization values, and thus are likely members of the same species. Image from: Madigan, M. T., Martinko, J. M., Bender, K. S., Buckley, D. H., Sattley, W.M. & Stahl, D. A. (2018). Brock biology of microorganisms (Fifteenth edition.). Boston: Pearson.