**2. Definition - background**

"Chlamydiae", originate from the Greek word "chlamyda", meaning "cloak". They are Gram-negative obligate intracellular bacteria pathogenic to humans or animals and mainly transmitted through direct contact with infected tissue, including vaginal, anal or oral sex and can even be passed from an infected mother to the newborn during childbirth. Some species though, pathogenic to animals can be transmitted to humans also [3].

Chlamydiae have a unique biphasic developmental cycle which involves transition between two major morphologic forms: (1) the infectious forms, called elementary bodies convert into (2) non-infectious, reproductive forms, called reticulate bodies. This transition takes place within the host cell and differentiates the metabolically inactive elementary body into the metabolically active reticulate body. The reticulate body contains no cell wall and is detected as an inclusion in the cell. Chlamydiae contain DNA, RNA and ribosomes [4, 5].

There are four recognized species of Chlamydiae: Chlamydia trachomatis, Chlamydia psittaci, Chlamydia pneumoniae, and Chlamydia pecorum. Humans are the only natural host for *Chlamydia trachomatis (C. trachomatis)*. This bacterium has a distinct developmental cycle, approximately 48–72 hours, which only replicates inside eukaryotic host cells. It uses energy phosphate compounds from the host cell during its growth and replication [6]. Preferentially infects squamo-columnar epithelial cells of the eye and the genital tract. Its genome size is 1000 kB [7].
