11.1 Morphological Characterization of Bacteria - MicroCore RC

Which of the following is a gelatinous polymer of polysaccharides that forms either as a capsule or slime layer?

• exopolysaccharide

• metachromatic granules

• glycolipid

• glycocalyx

Answer explanation

Answer: glycocalyx

A glycocalyx, literally meaning "sugar coat" (glykys = sweet, kalyx = husk), is a network of polysaccharides that project from cellular surfaces of bacteria, which classifies it as a universal surface component of a bacterial cell, found just outside the bacterial cell wall.

Reference: Madigan, M. T., Bender, K. S., Buckley, D. H., Sattley, W. M., & Stahl, D. A. (2018). Brock Biology of Microorganisms. 15^th ed. Pearson Higher Education.

Which of the following is not found in Gram-positive bacteria?

• N-acetylglucosamine

• Lipid A

• Murein

• Teichoic acid

• Cell membrane

Answer explanation

Answer: Lipid A.

Lipid A is a lipid component of an endotoxin held responsible for the toxicity of gram-negative bacteria. It is the innermost of the three regions of the lipopolysaccharide (LPS), also called endotoxin molecule, and its hydrophobic nature allows it to anchor the LPS to the outer membrane.

 

Reference: Madigan, M. T., Bender, K. S., Buckley, D. H., Sattley, W. M., & Stahl, D. A. (2018). Brock Biology of Microorganisms. 15^th ed. Pearson Higher Education.

Which term describes a bacterial species that alter its shape in response to varying environmental conditions?

• Amorphous

• Unusual

• Pleiomorphic

• Stalked

Answer explanation

Answer: Pleiomorphic

Pleomorphism refers to the ability of an organism to alter its morphology and reproductive and other metabolic behavior in response to environmental changes.

 

Reference: Madigan, M. T., Bender, K. S., Buckley, D. H., Sattley, W. M., & Stahl, D. A. (2018). Brock Biology of Microorganisms. 15^th ed. Pearson Higher Education.

McLaughlin, R. W., Vali, H., Lau, P. C. K., Palfree, R. G. E., De Ciccio, A., Sirois, M., Ahmad, D., Villemur, R., Desrosiers, M., & Chan, E. C. S. (2002). Are there naturally occurring pleomorphic bacteria in the blood of healthy humans? Journal of Clinical Microbiology, 40(12), 4771–4775. https://doi.org/10.1128/jcm.40.12.4771-4775.2002

Which of the following staining methods is NOT used to observe bacterial capsules?

• Gregersen's Method

• Anthony's method

• Maneval's staining method

• Negative staining

Answer explanation

Answer: Gregersen's method

In Gregersen's method, 3% KOH lyses the cell wall by dissolving lipids. Gram-negative bacteria are more susceptible (due to the OM), releasing DNA that produces a sticky, slimy thread.

 

References: Reyes, A. T. (2018). Morpho-Biochemical Aided Identification of Bacterial Isolates from Philippine Native Pig. Advances in Pharmacology & Clinical Trials, 3(5). https://doi.org/10.23880/apct-16000148

https://microbiologyinfo.com/potassium-hydroxide-test/

Which of the following structures are pentacyclic triterpenoids structurally similar to cholesterol but found in bacterial membranes?

• Teichoic acids

• Integral proteins

• Holdfast

• Hopanoids

Answer explanation

Answer: Hopanoids.

Hopanoids usually comprise around 1 – 5% of the cells' total lipids, but in some cases their proportion raises considerably in response to stress. Just like cholesterol in eukaryotic membranes, they insert in bacterial membranes where they contri­bute to their stability. Both hopanoid and steroid mole­cules are almost planar, thus can intercalate into the lipid bilayers with relative ease and there interact with the adjacent fatty acid chains to enhance stiff­ness.

References: Belin BJ, Busset N, Giraud E, Molinaro A, Silipo A, Newman DK. Hopanoid lipids: from membranes to plant-bacteria interactions. Nat Rev Microbiol. 2018 May;16(5):304-315. doi: 10.1038/nrmicro.2017.173. Epub 2018 Feb 19. PMID: 29456243; PMCID: PMC6087623.

https://schaechter.asmblog.org/schaechter/2014/09/bacterial-hopanoids-the-lipids-that-last-forever.html

What happens when you treat Bacillus megaterium with lysozyme?

• It will not be killed since being a Gram positive bacteria, it has a thick peptidoglycan layer.

• It will be killed because it blocks the activity of the transpeptidase thereby interrupting the cross-linking & CW synthesis.

• It will be killed because it hydrolyses the β-1, 4 bond connecting NAM & NAG residues.

• It will not be killed because it has mycolic acids similar to that of Mycobacterium tuberculosis.

Answer explanation

Answer: It will be killed because it hydrolyses the β-1, 4 bond connecting NAM & NAG residues.

Glycosidic bond (beta 1, 4) connecting NAM and NAG is susceptible to lysozyme. Lysozyme enzymatically cleaves the bonds in existing peptidoglycan, degrading the cell wall, while penicillin inhibits the cross-linking of peptidoglycan during its synthesis by binding to penicillin-binding proteins (PBPs).

References:
Nawaz N, Wen S, Wang F, Nawaz S, Raza J, Iftikhar M, Usman M. Lysozyme and Its Application as Antibacterial Agent in Food Industry. Molecules. 2022 Sep 24;27(19):6305. doi: 10.3390/molecules27196305. PMID: 36234848; PMCID: PMC9572377.

https://chemistrytalk.org/lysozyme/

Which of the following is not a function of the bacterial capsule?

• For resisting phagocytosis

• Attachment

• Energy reserve

• Excludes bacteriophages

• None of the choices

Answer explanation

Answer: None of the choices.

All of the choices are functions of the bacterial capsule. Here are the different functions:

-for resisting phagocytosis

-protection against desiccation

-exclusion of bacterial viruses and hydrophobic toxic materials

-attachment to surfaces

-reserve during starvation

References:

Rautemaa, R., & Meri, S. (1999). Complement-resistance mechanisms of bacteria. Microbes and Infection, 1(10), 785–794. https://doi.org/10.1016/s1286-4579(99)80081-1

https://open.oregonstate.education/generalmicrobiology/chapter/bacteria-surface-structures/#:~:text=A%20bacterial%20capsule%20is%20a,help%20in%20attachment%20to%20surfaces.

What is this structure that is defined as an amorphous material, a polysaccharide-based adhesin, that is attached at one pole of the cell?

• Sheath

• Flagella

• Holdfast

• Capsule

• Hopanoid

Answer explanation

Answer: Holdfast

Caulobacter crescentus synthesizes a polysaccharide-based adhesin known as the holdfast at one of its cell poles, which enables tight attachment to exogenous surfaces.

References:
Hershey DM, Porfírio S, Black I, Jaehrig B, Heiss C, Azadi P, Fiebig A, Crosson S. Composition of the Holdfast Polysaccharide from Caulobacter crescentus. J Bacteriol. 2019 Aug 8;201(17):e00276-19. doi: 10.1128/JB.00276-19. PMID: 31209074; PMCID: PMC6689307.

Which of the following is NOT true about the cell membrane?

• The lipid bilayer component is hydrophobic.

• It contains both lipids and proteins.

• Majority of the cell membrane is made up of proteins.

• It is semi-permeable.

Answer explanation

Answer: The lipid bilayer component is hydrophobic.

All of the lipid molecules in cell membranes are amphipathic (or amphiphilic)—that is, they have a hydrophilic (“water-loving”) or polar end and a hydrophobic (“water-fearing”) or nonpolar end.

Reference:

Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. The Lipid Bilayer. Available from: https://www.ncbi.nlm.nih.gov/books/NBK26871/

What happens when you Gram stain a Mycoplasma?

• The cells are colored blue/violet.

• The cells are colored pink/red.

• You cannot stain them since they have no cell wall.

• The cells remain colorless.

Answer explanation

Answer: The cells are colored pink/red.

Because Mycoplasma lacks a cell wall, it does not retain the crystal violet stain during the Gram staining process. As a result, Mycoplasma appears Gram-negative. However, this result does not reflect the true Gram-negative nature of Mycoplasma because the absence of a cell wall is the key factor causing this result, not the typical characteristics of Gram-negative bacteria.

References:

Tripathi N, Zubair M, Sapra A. Gram Staining. [Updated 2025 Mar 28]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK562156/

https://www.letstalkacademy.com/mycoplasma-gram-staining-bacteria-without-cell-wall/