Mycoplasma: Structure, Habitat, Morphology, and Clinical Significance

Table of Contents

Mycoplasma

Mycoplasma are the smallest known free-living organisms without a cell wall.
They are unicellular, prokaryotic (Gram -ve), and independently replicating (i.e., they contain metabolic machinery to grow and multiply in the absence of other cells, thus showing autonomous growth).
Due to the absence of a cell wall, they are resistant to beta-lactam antibiotics (e.g., penicillin and cephalosporins).
The first Mycoplasma was isolated by E. Nocard and R. Roux in 1898, from bovine pleuropneumonia. It is now known as Mycoplasma mycoides subsp. mycoides.
This bacterium was later identified as pleuropneumonia-like organisms (PPLO) and is found in various animals.

Ubiquitous in nature – found in plants, insects, animals, and humans.
Reservoirs:
- Plants, insects, and animals serve as natural hosts.
- Human mucosa – Mycoplasma is mainly found in the upper respiratory tract (URT) and urogenital tract.
Medically important species:
- Mycoplasma pneumoniae – Causes atypical pneumonia (walking pneumonia).
- Mycoplasma hominis – Associated with genital infections.
- Mycoplasma genitalium – Causes nongonococcal urethritis (NGU).
- Mycoplasma mycoides – Cattle pathogen causing pleuropneumonia.

Size: About 0.3 µm in diameter (smallest known self-replicating organism).
Shape: Highly pleomorphic (can be coccoid, filamentous, or irregular).
They lack a cell wall, making them resistant to cell wall-targeting antibiotics.
Do not stain with Gram stain due to the absence of peptidoglycan.
All members of Mycoplasma contain cholesterol-containing cell membranes (essential for their structure).
Electron microscopy reveals that M. pneumoniae has a specialized terminal organelle that helps in attachment to host cells.
Genome size: 580 kbp (one of the smallest bacterial genomes).
Ultra-filterable – Can pass through bacteria-proof filters used to remove bacterial contamination.

Oxygen Requirement:
- Most Mycoplasma spp. are facultative anaerobes.
- Mycoplasma pneumoniae is a strict aerobe.
Growth Characteristics:
- Fastidious – Requires specialized media with sterols and nucleic acids for growth.
- Slow-growing – Colonies form a “fried egg” appearance on agar plates.
Nutritional Requirements:
- All mycoplasmas except Acholeplasma require cholesterol or sterol for growth.
Antibiotic Resistance and Sensitivity:
- Resistant to: Penicillin, cephaloridine, vancomycin (due to lack of a cell wall).
- Sensitive to: Tetracycline and macrolides (e.g., erythromycin, azithromycin).
Unique Features:
- Mycoplasma can survive without a host cell, unlike viruses.
- Highly adaptive – Can persist in harsh conditions and evade immune responses.
- Acts as a bridge between viruses and bacteria (small genome, lacks cell wall, but can replicate independently).

Mycoplasma pneumoniae – Causes atypical pneumonia (mild, persistent cough, fever, headache).
Mycoplasma hominis – Pelvic inflammatory disease (PID), postpartum fever.
Mycoplasma genitalium – Sexually transmitted infections (STIs), urethritis.
Mycoplasma mycoides – Pleuropneumonia in cattle, economic loss in livestock.