**1. Introduction**

The most important characteristic that distinguishes *Chlamydia* genus bacteria from other bacteria is their biphasic growth cycle and their status as obligate intracellular pathogens. The *Chlamydia* genus belongs to the Chlamdiaceae family within the Chylamdiades order. In classification, their antigenic structures, intracellular inclusion bodies, sulfonamide sensitivity, and disease presentations are taken into account. Their three-layered outer membrane makes them resemble Gram-negative bacteria because they have a non-peptidoglycan envelope. The proteins they encode are referred to as major outer membrane proteins, which all *Chlamydia* species produce. *Chlamydia*'s reproduction is unique to itself, and in its growth cycle, it has infectious and reproductive forms known as EC: Elementary Body and RC: Reticulate Body. The *Chlamydia* genus includes *C. pneumonia*, *C. Trachomatis*, *C. pisittaci*, and *C. pecorum* [1, 2].

In previous years, the Chlamydiaceae family was divided into several classes, but recently, it has been assessed that it may be separated into two possible genera. Genus 1 includes *Chlamydia trachomatis*, *Chlamydia suis*, and *Chlamydia muridarum*. Genus 2 includes *Chlamydophila abortus* (*Chlamydia psittaci* serotype 1), *Chlamydophila caviae*, *Chlamydophila felis*, *Chlamydophila pecorum*, *Chlamydophila pneumoniae*, and *Chlamydophila psittaci* [3].

*C. pneumoniae* is involved in respiratory tract infections such as sinusitis, pharyngitis, bronchitis, asthma, pneumonia, and atherosclerotic diseases in the heart, brain, and peripheral arterial system. It also plays a role in most cases of acute ischemia [4]. The bacteria *C. trachomatis* causes psittacosis/ornithosis in humans, urogenital infections, and lymphogranuloma venereum (LGV) [5].

Serological tests are often used for the diagnosis of the disease because the causative agent can only be produced in vitro environments. This requires living environments such as cell culture and embryonated eggs, which are difficult and time-consuming. This is the main reason why serological tests are used in the diagnosis of the disease [3, 6].

The tests used in the diagnosis of the disease are immunofluorescence assay (IFA), enzyme-linked immunosorbent assay (ELISA), and complement fixation test (CFT). The IFA is used to diagnose and differentiate *Chlamydia* species, as its sensitivity and specificity are high. However, ELISA and CFT have limitations in diagnosis and species identification [3, 6].
