3 Isolation & Cultivation of Mcgs - MicroCore Review Center

Which of the following does not describe a pure culture?

• Contains a single species of microorganism

• Axenic culture

• Contains a microorganism with cells having the same Gram-reaction

• All of them describe a pure culture

Answer explanation

Cells may be of the same Gram reaction but of different species (E.g. S. aureus and B. cereus). Image from: https://commons.wikimedia.org/wiki/File:Bacillus_cereus_Gram.jpg and https://www.researchgate.net/publication/266028804_Prevalence_of_Bacterial_Contamination_when_using_a_Diversion_Pouch_during_Blood_Collection_A_Single_Center_Study_in_Malaysia/figures?lo=1

Which of the following is described as the increase in microbial population by providing their nutritional and physical requirements?

• Cultivation

• Isolation

• Preservation

• Serial dilution

Answer explanation

Both sufficient nutrients and proper conditions (temperature, pH, etc) should be provided in order to cultivate microorganisms. Image from: https://www.tgw1916.net/Bacillus/stearothermophilus.html

Which culture media (according to its physical state) is the best to use for studying motility?

• Liquid

• Solid

• Semi-solid

• Solid, Liquid, and Semi-solid

Answer explanation

Motility by bacterium is mostly demonstrated in a semi solid agar medium. In semi-solid agar media, motile bacteria 'swarm' and give a diffuse spreading growth that is easily recognized by the naked eye. Image from: https://microbiologyinfo.com/motility-test/#:~:text=Motility%20by%20bacterium%20is%20mostly,recognized%20by%20the%20naked%20eye.

Which culture media has yeast extract?

• Complex

• Synthetic

• Chemically-defined media

• Both complex and synthetic

Answer explanation

A chemically-defined (synthetic) medium is one in which the exact chemical composition is known. A complex (undefined) medium is one in which the exact chemical constitution of the medium is not known.

Which of the following types of culture media allows the growth of a specific type/group of microorganisms only, while inhibiting the growth of others?

• General Purpose

• Selective

• Differential

• All of the aforementioned

Answer explanation

The non-selective media on the right allows of the growth of several different bactarial species and is overgrown with bacteria (whitish lines). While the plate on the right selectively only allows the bacteria Neisseria gonorrhoeae, to grow (white dots). Image from: https://bio.libretexts.org/Bookshelves/Microbiology/Book%3A_Microbiology_(Boundless)/6%3A_Culturing_Microorganisms/6.3%3A_Culturing_Bacteria/6.3C%3A_Selective_and_Differential_Media

Which of the following statements is incorrect?

• EMBA is both selective and differential.

• TCBSA is both selective and differential

• TCBSA cannot differentiate between species of Vibrio.

• EMBA can differentiate between strong and weak lactose fermenters.

Answer explanation

Bile, a derivative of bile salts and sodium citrate inhibits gram-positive bacteria and coliforms. If the sucrose is fermented, yellow colonies are produced.

Vibrio cholerae produces yellow colonies in TCBS whereas Vibrio parahaemolyticus produces green colonies.. Vibrio cholerae produces yellow colonies in TCBS whereas Vibrio parahaemolyticus produces green colonies.

Eosin Methylene Blue Agar (EMBA) contains digested meat proteins as a source of organic nutrients. Two indicator dyes, eosin and methylene blue, inhibit the growth of gram-positive bacteria and distinguish between lactose fermenting and non-lactose fermenting organisms. Lactose fermenters form metallic green or deep purple colonies, whereas non-lactose fermenters form completely colorless colonies. EMB agar is an example of which of the following?

• Selective media

• Differential Media

• Selective and Differential

• Selective, Differential, and Complex

• Selective, Differential, and Defined

Answer explanation

Eosin-methylene blue agar is selective for gram-negative bacteria against gram-positive bacteria. In addition, EMB agar is useful in the isolation and differentiation of the various gram-negative bacilli and enteric bacilli, generally known as coliforms and fecal coliforms respectively.

Which isolation technique is best to use for samples with a low population?

• Membrane filtration

• Serial dilution

• Isolation is impossible

• Both membrane filtration and serial dilution

Answer explanation

The membrane filter functions like a sieve and traps particles in the filter surface. These filters are only 0.1 mm thick. The pore size is 0.22-0.45 micrometers intended for bacteria. Image from: https://plantlet.org/general-methods-of-microbial-isolation/

Which of the following preservation methods is best used for the long-term storage of microbial cultures?

• Refrigeration

• Subculturing

• Mineral Oil Overlay

• Lyophilization

Answer explanation

Freeze-drying bacteria (lyophilization) is a very well-established method for archiving and long-term storage. It is used to preserve bacteria, fungi, yeasts, proteins, nucleic acids, and any other molecules which may be degraded due to the presence of water. Image from: https://stock.adobe.com/ph/images/medical-illustration-of-the-lyophilization-process-of-probiotic-bacteria/189955700

Which culture preservation method is based on the principle of limiting the availability of oxygen, thereby decreasing the metabolic rate of the organism?

• Subculturing

• Mineral Oil Overlay

• Freeze-drying

• Freezing with liquid nitrogen

Answer explanation

Mineral oil-covered cultures are stored preferably at 0-5°C.