Overview
Chlamydiae are obligate, aerobic, intracellular parasites of eukaryotic cells, showing characteristics intermediate between bacteria and viruses. They are small Gram-negative coccoid or rod-shaped, non-motile organisms with tropism for squamous epithelial cells and macrophages of the respiratory and gastrointestinal tract.
Despite viral-like obligate intracellular dependence, Chlamydiae are classified as bacteria because they possess both DNA and RNA, a Gram-negative-type cell wall and ribosomes, replicate by binary fission, and are susceptible to antibiotics. Chlamydia trachomatis is globally significant as the leading cause of both preventable blindness and bacterial sexually transmitted infection.
Learning Objectives
After this lesson you will be able to…- Distinguish the key characteristics that place Chlamydiae as bacteria despite their viral-like features.
- Describe the elementary body and reticulate body forms and the chlamydial life cycle.
- Enumerate the diseases produced by Chlamydia trachomatis, C. psittaci and C. pneumoniae.
- Explain the methods used to culture and isolate Chlamydia.
- Describe the rapid diagnostic methods for detecting chlamydial antigen.
Clinical Story
Why This MattersA 24-year-old woman presents with a mucopurulent cervical discharge and mild lower abdominal discomfort. Gonorrhea testing is negative. Suspecting a non-gonococcal cause, the physician requests Chlamydia trachomatis testing. The lab performs a direct fluorescent antibody test on the cervical scraping, detecting major outer membrane proteins — confirming chlamydial cervicitis. Prompt treatment with a single dose of azithromycin is initiated, along with partner notification, to prevent long-term complications such as pelvic inflammatory disease and infertility.
Core Concepts
Although Chlamydiae exhibit intermediate viral/bacterial characteristics, they are recognized as bacteria because they: (1) possess both DNA and RNA, (2) have a cell wall resembling that of Gram-negative bacteria and ribosomes, (3) replicate by binary fission, and (4) are susceptible to antibiotics.
Order Chlamydiales, Family Chlamydiaceae, genera Chlamydia and Chlamydophila. Three species are of medical importance: C. trachomatis (with two biovars — TRIC and LGV), Chlamydophila psittaci, and Chlamydophila pneumoniae.
Chlamydiae have a cytoplasmic membrane and outer membrane similar to Gram-negative bacteria but lack a peptidoglycan cell wall, and cannot synthesize their own ATP, requiring an intracellular habitat for viability. They stain better with Castaneda, Machiavello or Gimenez stains despite being Gram-negative.
The elementary body (EB) is the dispersal, extracellular infective form (~0.3 µm/200–300 nm), analogous to a spore, which induces its own endocytosis on contact with a target cell. The reticulate body (RB) is the intracellular, multiplicative, non-infectious growing form (500–1000 nm).
After endocytosis, the EB prevents phagolysosomal fusion, enabling intracellular survival, and transforms into the larger RB within the endosome. The RB is the reproductive form, dividing by binary fission every 2–3 hours and forming a basophilic inclusion body around the nucleus (stainable with Lugol's iodine due to a glycogen matrix). After division, the RB transforms back into infectious EBs, released by exocytosis — one phagolysosome typically produces 100–1000 EBs. The whole cycle takes 24–48 hours, and released EBs can infect new cells.
Chlamydia antigens are divided into three groups: genus-specific antigen, species-specific protein antigen, and serotype-specific antigen (located on the major outer membrane protein, MOMP), which forms the basis for classification into multiple serovars/serotypes.
Isolation methods: animal inoculation (mice, intranasal/intraperitoneal/intracerebral routes), egg yolk sac inoculation, and tissue culture (McCoy cells treated with cycloheximide is the standard method).
Diseases: Ocular — trachoma (serotypes A, B, Ba, C; leading cause of preventable blindness), inclusion conjunctivitis (serotypes D–K), ophthalmia neonatorum. Genital — non-gonococcal urethritis and lymphogranuloma venereum (serotypes L1, L2, L3), pelvic inflammatory disease and infertility in women. Respiratory — C. pneumoniae causes pneumonia; C. psittaci causes psittacosis.
Elementary Body vs Reticulate Body
Key Comparison⚠️ Values summarised from standard microbiology references; always confirm with your laboratory's SOPs.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Inclusion bodies staining with Lugol's iodine | Presence of glycogen matrix within reticulate body inclusions | Supports presumptive C. trachomatis identification in ocular/cervical scraping |
| Positive direct fluorescent antibody (DFA) test | Detects major outer membrane proteins directly in the specimen | Confirms chlamydial infection; consider treatment and partner notification |
| Mucopurulent cervical discharge with negative gonorrhea test | Suggests non-gonococcal cervicitis, often chlamydial | Send specific chlamydia testing (DFA, ELISA or NAAT/PCR) |
| Follicular conjunctivitis with corneal scarring in an endemic region | Suggestive of trachoma | Refer for serotyping and long-term ophthalmologic follow-up |
Common Errors & How to Avoid Them
Cause: Chlamydia is intracellular; free discharge contains few organisms compared to epithelial cells.
Prevention: Always collect an adequate scraping of the mucosa (cervical, urethral, ocular) rather than just discharge.
Cause: Chlamydiae are Gram-negative but stain poorly and inconsistently with standard Gram technique.
Prevention: Use Giemsa, Castaneda, Machiavello or Gimenez stains, or preferably direct fluorescent antibody testing, for reliable detection.
Cause: Not all cell lines support chlamydial growth efficiently.
Prevention: Use cycloheximide-treated or irradiated McCoy cells, the preferred isolation method for Chlamydia trachomatis.
Laboratory Tips from the Bench
Always collect a scraping (not just discharge) when chlamydia is suspected — because Chlamydia is obligately intracellular, adequate epithelial cell sampling is critical for test sensitivity.
Direct fluorescent antibody testing is considered by many the test of choice for rapid chlamydial diagnosis, offering both sensitivity and speed compared to culture.
Remember the life cycle sequence with 'E-R-E': Elementary body infects → becomes Reticulate body (multiplies) → reverts to Elementary body (released to infect anew).
Important Notes
Chlamydia trachomatis remains the leading cause of preventable blindness (trachoma) and one of the most common bacterial sexually transmitted infections worldwide, making early detection and partner treatment essential.
Single-dose azithromycin is the drug of choice for non-gonococcal urethritis due to its excellent compliance profile; sulphonamides and tetracycline are also effective.
Interactive Quiz
Test Your KnowledgeFlashcards
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Clinical Case Study
Apply Your KnowledgeMucopurulent cervical discharge and mild lower abdominal discomfort for one week. Sexually active, new partner in past two months. Gonorrhea culture negative.
A negative gonorrhea culture combined with a positive direct fluorescent antibody test for chlamydial major outer membrane proteins confirms non-gonococcal cervicitis due to Chlamydia trachomatis.
- →Chlamydia is a leading cause of non-gonococcal genital infection and must be actively tested for, not assumed absent.
- →DFA testing on an adequate cervical scraping offers rapid, sensitive diagnosis.
- →Partner notification and treatment are essential to prevent reinfection and complications like PID and infertility.
Frequently Asked Questions
Because it cannot synthesize its own ATP and must obtain energy from the host cell, making it entirely dependent on intracellular life, similar in this respect to viruses despite being a true bacterium.
The TRIC biovar (trachoma-inclusion conjunctivitis) causes ocular and genital infections associated with serotypes A–K, while the LGV biovar (serotypes L1–L3) causes the more invasive lymphogranuloma venereum.
Chlamydial inclusion bodies contain a glycogen matrix, which stains with Lugol's iodine, providing a simple though less sensitive method compared to immunofluorescence or molecular testing.
Quick Revision
10-Minute ReviewKey Takeaways
- Chlamydiae are obligate intracellular bacteria with a distinctive biphasic life cycle of elementary and reticulate bodies.
- Chlamydia trachomatis is the leading cause of preventable blindness (trachoma) and a major cause of sexually transmitted infection worldwide.
- Direct fluorescent antibody testing is considered the test of choice for rapid, sensitive diagnosis.
- McCoy cell culture with cycloheximide treatment remains the gold-standard isolation method.
- Different C. trachomatis serotypes cause distinct clinical syndromes — ocular, genital and lymphogranuloma venereum.
- Azithromycin single-dose therapy has revolutionised treatment compliance for chlamydial genital infections.
Competency Checklist
Track Your MasteryReferences
- National Institute of Open Schooling. Microbiology Module — Lesson 33: Chlamydia.
- Collee JG, et al. Mackie & McCartney Practical Medical Microbiology, 14th ed.
- World Health Organization. Trachoma and Sexually Transmitted Infections Fact Sheets.