Mycoplasma: Structure, Habitat, Morphology, and Clinical Significance

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.

Habitat of Mycoplasma

• 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.

Morphology of Mycoplasma

• 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.

Cultural and Biochemical Characteristics of Mycoplasma

• 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).

Clinical Significance of Mycoplasma

• 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.