Table of Contents

Mycoplasmatales

Evolutionary Context

Mycoplasmas and Ureaplasmas belong to the class Mollicutes (derived from the Latin mollis meaning "soft" and cutis meaning "skin"). These organisms are believed to have evolved through "regressive evolution" from Gram-positive ancestors (like Clostridia) by shedding their cell walls and significantly reducing their genome size to become the smallest free-living organisms capable of self-replication.

I. The Biological Paradigm: Absence of Peptidoglycan

The most defining characteristic of the order Mycoplasmatales is the absolute lack of a cell wall. This structural absence has profound clinical and diagnostic implications:

Intrinsic Antibiotic Resistance: Because they lack a peptidoglycan target, they are inherently and 100% resistant to all β-lactam antibiotics (Penicillins, Cephalosporins, Carbapenems) and Glycopeptides (Vancomycin).
Gram Stain Invisibility: They do not take up Gram stain dyes because there is no cell wall to trap the crystal violet-iodine complex. They are effectively "invisible" under standard light microscopy using routine staining methods.
Pleomorphism: Lacking a rigid murein sacculus, they are highly plastic and pleomorphic. They can appear as coccoid bodies (0.2 μm), swollen ring shapes, or long branching filaments.

II. General Characteristics & Physiology

1. Ultrastructure and Size

Measuring only 0.2 to 0.3 micrometers in diameter, they are the only bacteria that can pass through standard 0.45-μm filters used to "sterilize" liquids from bacteria. Historically, this led to them being misclassified as "filterable viruses."

2. The Sterol-Reinforced Membrane

To compensate for the lack of a cell wall, Mycoplasmatales have evolved a unique triple-layered cell membrane that contains sterols (cholesterol).

The Requirement: They cannot synthesize sterols themselves due to their limited genome. They must scavenge cholesterol from the host's serum or mucosal surfaces.
Laboratory Impact: To grow these organisms in vitro, the culture media (such as PPLO broth or SP-4 agar) must be supplemented with 10% to 20% animal serum (usually horse or fetal bovine serum).

3. Genomic and Growth Profiles

Genome: They possess the smallest known bacterial genomes (0.58 to 1.38 Megabases). This results in a "minimalist" metabolism with no ability to synthesize amino acids, fatty acids, or nucleotides.
Growth Rate: They are extremely slow-growing. A single colony may take up to 3 weeks to appear.
"Fried Egg" Colonies: On specialized agar, most species produce colonies with a dense central zone that grows into the agar and a lighter peripheral zone, resembling a fried egg. Note: Mycoplasma pneumoniae is an exception and often produces a granular, mulberry-like appearance.

III. Clinical Classification of Pathogenic Species

Primary System	Pathogen	Associated Clinical Syndromes
Respiratory	Mycoplasma pneumoniae	Walking pneumonia, tracheobronchitis, bullous myringitis.
Urogenital	Mycoplasma genitalium	Nongonococcal urethritis (NGU), cervicitis, PID.
Urogenital	Ureaplasma urealyticum	NGU, chorioamnionitis, struvite kidney stones.
Opportunistic	Mycoplasma hominis	Postpartum fever, pyelonephritis, neonatal sepsis.

IV. Mycoplasma pneumoniae: The Primary Respiratory Pathogen

M. pneumoniae is a strictly human pathogen, causing widespread respiratory disease, particularly in temperate climates.

A. Virulence Factors & Pathogenesis

P1 Adhesin (The Anchor): A specialized attachment organelle at the tip of the bacterium. It binds specifically to sialic acid receptors on the base of the cilia of respiratory epithelial cells. This attachment is so firm that it prevents the bacteria from being cleared by the "mucociliary escalator."
CARDS Toxin: Stands for Community-Acquired Respiratory Distress Syndrome toxin. It is an ADP-ribosylating toxin similar to Pertussis toxin. It causes ciliostasis (paralysis of the cilia) and subsequent desquamation of the respiratory epithelium.
Oxidative Stress: The organism produces hydrogen peroxide (H₂O₂) and superoxide radicals, which cause direct necrosis of host cell membranes through lipid peroxidation.

B. Clinical Manifestations

Primary Infection

"Walking Pneumonia"

Clinically termed Primary Atypical Pneumonia. It has an insidious (creeping) onset over 1–3 weeks.

Presentation: Low-grade fever, severe headache, and a dry, hacking, non-productive cough that is worse at night.
The CXR Paradox: Chest X-ray often shows extensive patchy or reticulonodular infiltrates that look "terrifying," yet the patient appears relatively well and is "walking" around the clinic.

Autoimmune Complications

The "Molecular Mimicry" Effect

Because Mycoplasma antigens resemble human tissue, the immune system may accidentally attack the host (Type II Hypersensitivity).

Cold Agglutinins: Up to 70% of patients produce IgM antibodies that agglutinate Red Blood Cells at 4°C. This can lead to transient hemolytic anemia.
Bullous Myringitis: A highly specific (though rare) sign where fluid-filled blisters form on the eardrum.
Stevens-Johnson Syndrome (SJS): Severe, life-threatening blistering of the skin and mucous membranes.

Mnemonic

"Myco Makes Cold Erythema And Brains Ache"

Myco: Myocarditis / Myringitis (bullous)
Makes: Maculopapular rash
Cold: Cold agglutinins (Hemolytic anemia)
Erythema: Erythema multiforme / Stevens-Johnson Syndrome
Brains: Neurological (Guillain-Barré, Encephalitis)
Ache: Arthritis / Arthralgia

V. Urogenital Pathogens: Ureaplasma & M. genitalium

1. Ureaplasma urealyticum

Distinguished by its ability to produce the enzyme Urease. It hydrolyzes urea into ammonia and CO₂.

Kidney Stones: The production of ammonia raises urinary pH (alkalinization). In an alkaline environment, calcium and magnesium phosphate precipitate, leading to the formation of Struvite (Staghorn) calculi.
Neonatal Impact: Can be transmitted vertically during birth, leading to neonatal pneumonia or chronic lung disease (bronchopulmonary dysplasia) in very low birth weight infants.

2. Mycoplasma genitalium

A sexually transmitted pathogen now recognized as a major cause of NGU that does not respond to standard therapy.

Clinical Focus: It is highly associated with pelvic inflammatory disease (PID) and infertility in women.
Resistance Warning: Global rates of resistance to Macrolides (Azithromycin) are skyrocketing, often requiring the use of Moxifloxacin.

VI. Laboratory Diagnosis

NAAT (PCR): The modern "Gold Standard." It is the most sensitive and rapid method for detecting Mycoplasma DNA in respiratory or urogenital swabs.
Serology: Looking for a 4-fold rise in IgG titers between acute and convalescent sera (taken 2-4 weeks apart). A single high IgM titer is suggestive of acute infection in children.
Culture: Rarely used in clinics. Requires PPLO agar and several weeks of incubation.

VII. Pharmacological Management

Treatment Restrictions

NEVER use Penicillins or Cephalosporins. They are clinically useless against these organisms.

Macrolides

(Azithromycin, Clarithromycin)

Mechanism: Targets the 50S Ribosome. Drug of choice for children and pregnant women.

Tetracyclines

(Doxycycline)

Mechanism: Targets the 30S Ribosome. Primary choice for adults and for urogenital infections.

Fluoroquinolones

(Levofloxacin, Moxifloxacin)

Mechanism: Inhibits DNA Gyrase. Reserved for refractory cases or macrolide-resistant M. genitalium.

VIII. References

1. Murray, P. R., Rosenthal, K. S., & Pfaller, M. A. (2020). Medical Microbiology (9th ed.). Elsevier.
2. Waites, K. B., & Talkington, D. F. (2004). Mycoplasma pneumoniae and its role as a human pathogen. Clinical Microbiology Reviews.
3. Brooks, G., Carroll, K. C., Butel, J., & Morse, S. (2019). Jawetz, Melnick, & Adelberg's Medical Microbiology (28th ed.). McGraw-Hill.
4. World Health Organization (WHO). (2023). Global guidance on urogenital mycoplasmas and antimicrobial resistance.