Chlamydia: Types, Morphology & Infections

• Chlamydia is caused by obligate intracellular bacteria affecting humans, animals, and birds.
• Chlamydia exhibits tropism for squamous epithelial cells and macrophages in the respiratory and gastrointestinal tracts.
• It can cause ocular infections, a range of sexually transmitted infections, like urethritis, cervicitis, and lymphogranuloma venereum.
• Unlike bacteria, Chlamydia doesn't have peptidoglycan in their cell walls
• It lacks enzymes for electron transport so can only survive in an intracellular environment and cannot grow in an artificial membrane.

Classification

• Chlamydiae have been divided into two genera: Chlamydia (including Chlamydia trachomatis) and Chlamydophila (including Chlamydophila pneumoniae and Chlamydophila psittaci).

Morphology

• Chlamydiae occur in two forms: elementary bodies and reticulate bodies.
• The Elementary body is the extracellular, infective form.
• It is a spherical particle, 200-300nm in diameter, with a rigid trilaminar cell wall

Reticulate body

• Formerly called the initial body.
• It is the intracellular, growing, and replicative form, 500-1,000 nm in size with a fragile and pliable cell wall.

Growth Cycle

• Infection starts with the elementary body attaching to the surface of susceptible epithelial cells, followed by endocytosis.
• Inside the host cell, the elementary body is in the endosome throughout the active growth cycle.
• The elementary body in the endosome converts to a large reticulate body in about 8 hours
• Binary fission starts in 12 hours, and by 20-24 hours, the progeny are converted to elementary bodies.
• The developing chlamydial microcolony within the host is called an inclusion body.

Transmission

• Direct contact such as sexual transmission of Chlamydia trachomatis is a method of transmission
• Vertical transmission of Chlamydia trachomatis from mother to child during delivery is another method of transmission
• Inhalation is a method of transmission for aerosolized particles of Chlamydia pneumoniae and Chlamydia psittaci.
• Transmission via contact with infected birds can spread Chlamydia psittaci.

Antigenic Properties

• The Major Outer Membrane Protein (MOMP) exhibits species-specific and serovar-specific epitopes.
• Variations in the MOMP gene determine the serovars, particularly in Chlamydia trachomatis.
• Chlamydial LPS is a genus-specific antigen, less endotoxic than other Gram-negative bacteria.
• Polymorphic Membrane Proteins (PMPs) are highly immunogenic and involved in adhesion and immune evasion, playing a role in antigenic variation.
• Chlamydia produces Hsp60 and Hsp70, which are highly conserved and immunogenic, and may be implicated in chronic inflammation and immunopathology.

Pathogenesis

• Chlamydia infections adhere, invade, replicate intracellularly, and evade the immune system.
• Chlamydia enters the host through mucosal surfaces in the genital, respiratory, or conjunctival areas.
• The elementary body (EB), attaches to specific receptors on epithelial cells with outer membrane proteins.
• Elementary bodies encourage their uptake via endocytosis or phagocytosis and Inside the host cell, elementary bodies prevent lysosomal fusion
• It creates a specialized vacuole called an inclusion for residence and multiplication.
• Elementary bodies differentiate into reticulate bodies (RBs), which divide and exploit host cell resources for growth.

Immune Response and Tissue Damage

• Mature elementary bodies release via cell lysis or extrusion, ready to infect new cells
• Infected cells release pro-inflammatory cytokines, leading to recruitment of neutrophils and macrophages, causing inflammation and damage
• T-cell-mediated immunity involves the production of antibodies against Chlamydia antigens
• Chlamydia can enter a non-replicative, persistent state to evade immune responses causing further harm
• Direct damage: caused by cell lysis during elementary body release
• Immune-mediated damage:Chronic inflammation, fibrosis, and scarring due to prolonged immune activation.
• Trachoma: Scarring of the conjunctiva, leading to blindness.
• Pelvic Inflammatory Disease (PID): Can lead to infertility because of damage to fallopian tubes.

Chlamydia Trachomatis Serotypes

• Serotypes A-C are highly infectious and transmitted through direct contact, clothing and insects can cause leading cause of infection blindness worldwide and Corneal scarring
• Serotypes D-K are the most commonly sexually transmitted infection, cause urethritis, prostatitis, cervicitis and PID, includes symptoms discharge and dysuria.
• Serotypes L1-L3 primary infection after one week a patient presents small painless genital ulcers and Secondary infection-occurs after 3 weeks: patient presents painful swelling of inguinal lymph node(BUBOS)

Laboratory Diagnosis

• Nucleic acid amplification testing is used
• Culture and gram staining not done routinely
• Giemsa stain is useful for visualization-chlamydial inclusion in cytoplasm of epithelial cells
• Serology technique can be used.

Treatment

• Chlamydia cell walls lack muramic acids,so antibiotics inhibiting cell wall synthesis like penicillin aren't effective.
• They are treated with antibiotics that inhibit protein synthesis like macrolides such as azithromycin
• For genital infections associated with coinfection of N.gonorrhea Ceftriaxone is given along with a chlamydia regime.

Prevention

• Regular screening helps
• Avoid douching, to retain good bacteria in the vagina

Chlamydophola Pneumoniae

• Transmitted by aerosols
• Results in atypical pneumonia: fever, non-reproductive cough, headache and myalgias.
• Can be treated with macrolides such as azithromycin, or clarithromycin or with second line treatment doxycycline.

Chlamydophila Psittaci

• It results in psittacosis (parrot fever), a zoonotic infection
• Symptoms always have flu like symptoms such as fever, headache, arthralgia
• Can be treated with first line treatment doxycycline or second line treatment macrolides