Chlamydia trachomatis
To learn about other chlamydial infections caused by species other than C. trachomatis, click here.

# Overview
- Chlamydia trachomatis, an obligate intracellular human pathogen, is one of four bacterial species in the genus Chlamydia.
- C. trachomatis is a Gram-negative bacterium, therefore its cell wall components retain the counter-stain safranin and appear pink under a light microscope. It is ovoid in shape.
- C. trachomatis includes three human biovars, based on variations in the major outer membrane protein (MOMP):
- serovars Ab, B, Ba, or C  cause trachoma: infection of the eyes, which can lead to blindness
- serovars D-K  cause urethritis, pelvic inflammatory disease, ectopic pregnancy, neonatal pneumonia, and neonatal conjunctivitis
- serovars L1, L2 and L3  lymphogranuloma venereum (LGV)
- The L2 serovar can be further differentiated into L2, L2', L2a, and L2b based on significant amino acid differences
- Many, but not all, C. trachomatis strains have an extrachromosomal plasmid.
- Chlamydia can exchange DNA between its different strains, thus the evolution of new strains is common.

# Identification
Chlamydia species are readily identified and distinguished from other Chlamydia species using DNA-based tests.
Most strains of C. trachomatis are recognized by monoclonal antibodies (mAbs) to epitopes in the VS4 region of MOMP. However, these mAbs may also cross-react with two other Chlamydia species, C. suis and C. muridarum.

# Life-cycle
Chlamydiae are obligate intracellular bacterial pathogens, which means they are unable to replicate outside of a host cell. However, to facilitate effective dissemination, these pathogens have evolved a distinct biphasic life cycle wherein they alternate between two functionally and morphologically distinct forms.
- The elementary body (EB) is infectious, but metabolically inert (much like a spore), and can survive for limited amounts of time in the extracellular milieu. Once the EB attaches to a susceptible host cell, it mediates its own internalization through pathogen-specified mechanisms (via type III secretion system) that allows for the recruitment of actin with subsequent engulfment of the bacterium.
- The internalized EB, within a membrane-bound compartment, immediately begins differentiation into the reticulate body (RB). RBs are metabolically active but non-infectious, and in many regards, resemble normal replicating bacteria. The intracellular bacteria rapidly modifies its membrane-bound compartment into the so-called chlamydial inclusion so as to prevent phagosome-lysosome fusion. The inclusion is thought to have no interactions with the endocytic pathway and apparently inserts itself into the exocytic pathway as it retains the ability to intercept sphingomyelin-containing vesicles.
- The mechanism by which the host cell protein is trafficked to the inclusion through the exocytic pathway is not fully understood. As the RBs replicate, the inclusion grows as well to accommodate the increasing numbers of organisms. Through unknown mechanisms, RBs begin a differentiation program back to the infectious EBs, which are released from the host cell to initiate a new round of infection. Because of their obligate intracellular nature, Chlamydiae have no tractable genetic system, unlike E. coli, which makes Chlamydiae and related organisms difficult to investigate.

## Diseases caused by Chlamydia trachomatis
Chlamydia trachomatis can cause the following conditions:
- Cervicitis
- Conjunctivitis
- Fitz-Hugh-Curtis syndrome
- Lymphogranuloma venereum
- Pelvic inflammatory disease
- Pneumonia in infants
- Reactive arthritis
- Urethritis
- Rectal infection (proctitis)
- Prostatitis
- Ectopic pregnancy

# Gallery
- Photomicrograph of Chlamydia trachomatis taken from a urethral scrape. From Public Health Image Library (PHIL). 
- McCoy cell monolayers with Chlamydia trachomatis inclusion bodies (200X mag). From Public Health Image Library (PHIL). 
- McCoy cell monolayers with Chlamydia trachomatis inclusion bodies (50X mag). From Public Health Image Library (PHIL). 
- Photomicrograph depicts HeLa cells infected with Type-A Chlamydia trachomatis (400X mag). From Public Health Image Library (PHIL). 
- Patients left eye with the upper lid retracted in order to reveal the inflamed conjunctival membrane lining the inside of both the upper and lower lids, due to what was determined to be a case of inclusion conjunctivitis caused by the bacterium, Chlamydia trachomatis. From Public Health Image Library (PHIL). 
- From Public Health Image Library (PHIL).