Mycoplasma and Prions

 

Introduction

Among the tiniest biological entities known to science, Mycoplasma and Prions represent two fascinating extremes of life. While Mycoplasma are the smallest self-replicating prokaryotes, Prions are infectious protein particles that challenge our definition of life itself. Understanding these two helps bridge concepts between microbiology, molecular biology, and infectious disease research — vital in both medicine and biotechnology.

Mycoplasma: The Cell Wall-Less Bacteria

Mycoplasma are a group of wall-less bacteria belonging to the class Mollicutes. They are the smallest free-living organisms capable of independent replication.

Key Features

1. No Cell Wall: Instead of peptidoglycan, they have a triple-layered cell membrane containing sterols.

2. Size: Ranges from 0.2 to 0.3 µm, making them filterable like viruses.

3. Genome: One of the smallest known bacterial genomes (~0.6–1.3 Mb).

4. Shape: Pleomorphic – can appear as spherical, filamentous, or flask-shaped forms.

5. Habitat: Commonly found in mucosal surfaces of humans, animals, and plants.

6. Pathogenicity: Causes atypical pneumonia, urethritis, and pelvic infections.

Reproduction

They reproduce by binary fission, similar to bacteria, though replication is slower due to their reduced genome.

Importance

Mycoplasma holds importance in several fields of biology and medicine. Medically, it is known to cause various respiratory and urogenital infections in humans, such as atypical pneumonia and urethritis. From a biotechnological perspective, Mycoplasma contamination poses a serious problem in cell culture laboratories, as it can alter cellular behavior and compromise experimental results. In terms of evolutionary significance, Mycoplasma serves as a remarkable example of a minimalist cell, offering insights into how life can function with the bare minimum of genetic and structural components.

Prions:

Prions (Proteinaceous Infectious Particles) are misfolded proteins that cause neurodegenerative diseases in animals and humans, without containing DNA or RNA.

Key Features

a) Acellular infectious agents – no nucleic acid.

b) Resistant to heat, radiation, and disinfectants.

c) Cause spongiform encephalopathies such as Scrapie, BSE (Mad Cow Disease), and Creutzfeldt–Jakob Disease.

Mechanism of Infection

Prions convert normal cellular proteins (PrPc) into abnormal misfolded forms (PrPsc). This accumulation of misfolded proteins causes neuronal damage and brain tissue degeneration.

Difference Between Mycoplasma and Prions

Feature	Mycoplasma	Prions
Nature	Cellular (Prokaryote)	Acellular (Protein only)
Cell Wall	Absent	Absent
Genetic Material	DNA & RNA present	No DNA or RNA
Reproduction	Binary fission	Induces protein misfolding
Size	0.2–0.3 µm	~5 nm
Diseases	Pneumonia, Urethritis	Mad Cow, CJD, Scrapie
Sensitivity	Killed by antibiotics	Resistant to antibiotics, radiation

Frequently Asked Questions (FAQs)

1. Q: What makes Mycoplasma unique among bacteria?

A: They lack a cell wall and contain sterols in their plasma membrane.

2. Q: Can Prions be destroyed by boiling or radiation?

A: No. Prions are extremely resistant and require autoclaving at 134°C for 18 minutes.

3. Q: Are Prions living organisms?

A: No, they lack nucleic acids and cannot reproduce independently — they are infectious proteins.

4. Q: Which Mycoplasma causes pneumonia in humans?

A: Mycoplasma pneumoniae causes atypical (walking) pneumonia.

5. Q: Why are Mycoplasma important in cell culture?

A: They can contaminate cultures and alter gene expression, leading to misleading experimental results.

Multiple Choice Questions (MCQs)

1. Mycoplasma lack which structure?
A) Nucleus
B) Ribosome
C) Cell Wall ✅
D) Plasma membrane

2. The causative agent of Mad Cow Disease is:
A) Virus
B) Bacterium
C) Fungus
D) Prion ✅

3. Mycoplasma are resistant to penicillin because:
A) They are eukaryotic
B) They have no cell wall ✅
C) They are endospore-forming
D) They are anaerobic

4. The normal form of prion protein in humans is:
A) PrPᶜ ✅
B) PrPˢᶜ
C) PrPᴺ
D) PrPʀ

5. Who discovered prions?
A) Robert Koch
B) Louis Pasteur
C) Stanley Prusiner ✅
D) Anton van Leeuwenhoek

References

1. Prusiner, S. B. (1998). Prions. PNAS, 95(23), 13363–13383.

2. Razin, S., & Hayflick, L. (2010). Mycoplasmas: Molecular Biology, Pathogenicity, and Strategies for Control. ASM Press.

3. Tortora, G. J., Funke, B. R., & Case, C. L. (2021). Microbiology: An Introduction, 13th Edition. Pearson.

4. Madigan, M. T., et al. (2022). Brock Biology of Microorganisms, 16th Edition. Pearson.

5. WHO: Infection Control Guidelines for Transmissible Spongiform Encephalopathies (2020).