**1. Introduction**

Among the diseases caused by chlamydia, trachoma is the most widely known. The earliest known information about trachoma is found in Egyptian papyrus and Ancient Chinese inscriptions [1–3]. Intracytoplasmic inclusion bodies of *Chlamydia trachomatis* were first demonstrated by the researchers Halberstaedter and Von Prowazek [4]. In 1907, Halberstaedter and Von Prowazek applied a sample from a patient with trachoma to the conjunctiva of a monkey and, after the infection emerged, were able to show the intracytoplasmic inclusion bodies by staining the mucopurulent exudate sample with Giemsa. They defined these bodies as protozoa and named them Chlamidozoa [5]. The role of *C. trachomatis* in genital infections was evidenced by the same researchers in 1909 when they showed inclusion bodies in the conjunctival cells of babies with non-gonococcal ophthalmia neonatorum, in the cervical epithelial cells of the mothers of babies, and in the urethral epithelial cells of male patients with non-gonococcal urethritis. In 1910, Lindner showed how the mother and father of a baby with inclusion conjunctivitis showed inclusion bodies in the mother's cervical

and father's urethral specimens [5]. Lymphogranuloma venereum (LGV) was first described in the late 1700s [3] and was reported by Durand, Nicolas, and Favre in 1913. Gamma and Favre showed inclusion bodies in the cytoplasm of mononuclear cells in an infected lymph node in 1924 [5]. Psittacosis was first described in humans by Ritter in 1879. Later, outbreaks were reported in many European countries. In the 1950s, attention was drawn to contamination, especially from poultry [3]. *Chlamydia pneumonia* was first isolated in 1965 during trachoma vaccine studies in Taiwan. It was later isolated from throat cultures of children with pharyngitis in the United States in 1983 [3, 6]. *C. trachomatis* was produced in the yolk sac of embryonated eggs by T'ang et al. in 1957 and isolated from cell culture by Gordon and Quan in 1965. In 1970, the microimmunofluorescence technique started being used in diagnosis, and Direct Fluorescent Antibody (DFA) and Enzyme Immunoassay (EIA) tests were developed in the 1980s [3, 7]. After the 1990s, polymerase chain reaction (PCR), ligase chain reaction (LCR), and nucleic acid amplification tests (NAAT) have been used in diagnosis [8, 9].

Chlamydiae are gram-negative, cocci-shaped, immobile, and obligate intracellular microorganisms that cause many diseases in humans. Although they have many enzymes and limited metabolic activities, they have been studied among viruses for many years due to their lack of mechanisms for providing metabolic energy and their inability to generate their own energy (Adenosine Triphosphate). However, they are distinguished from viruses by the fact that they contain both DNA and RNA, reproduce by dividing in the middle, have a similar cell wall structure to gram-negative bacteria, have ribosomes, have various enzymes that provide metabolic activity, and are sensitive to various antibiotics [1]. Unlike other bacteria, chlamydia has a biphasic life cycle. During their life cycles, they appear in two forms, which are called elementary bodies and reticular bodies, with different metabolic activity characteristics, sizes, and morphological appearances. Elementary body is the infective form that is resistant to environmental conditions and is not metabolically active. Reticular body, on the other hand, is the metabolically active form that has no infective properties but has the ability to proliferate within the host cell. Because chlamydia are compulsory intracellular parasites, they cannot be grown in artificial media [10]. They require live cell environments for their reproduction [2].

Chlamydias belong to the *Chlamydiaceae* family, which is under the order Chlamydiales. In the taxonomic classification according to their phenotypic characteristics, there are four species in the genus *Chlamydia*, namely *C. trachomatis*, *C. psittaci*, *C. pneumoniae*, and *C. pecorum*. Among these four species, only *C. pecorum* does not cause disease in humans [5]. Apart from this classification, two different genera were proposed based on 16S rRNA and 23S rRNA sequence analyses. There are three species within the genus *Chlamydia*, *C. trachomatis*, *C. suis*, and *Chlamydia muridarum*, and six species within the genus *Chlamydophila*, *C. pneumonia*, *C. psittaci*, *C. pecorum*, *C. felis*, *Chlamydophila caviae*, and *C. abortus*. However, since this classification is not accepted by many researchers, both classifications are used in the literature today [1, 8, 11, 12]. There are three biovars within the species *C. trachomatis*. These are Trachoma biovar associated with oculogenital diseases, Lymphogranuloma biovar, and rat pneumonia causative biovar. Wang and Grayston determined 15 serotypes according to antigenic structures in trachoma and LGV agents [13]. Three of these serotypes (L1–3) determined by the microimmunofluorescence method were associated with LGV, and the other 12 were associated with oculogenital diseases. The rat pneumonia-causing agent does not infect humans [8, 13, 14]. The L1, L2, and L3 serotypes of *C. trachomatis* are associated with Lymphogranuloma venereum (LGV)

#### *Chlamydia and the Gastrointestinal System DOI: http://dx.doi.org/10.5772/intechopen.110485*

and are sexually transmitted. The A, B, Ba, and C serotypes cause endemic trachoma and are transmitted by hand-eye contact and flies. The D, E, F, G, H, I, J, and K serotypes cause inclusion conjunctivitis, nongonococcal urethritis, cervicitis, salpingitis, proctitis, epididymitis, neonatal pneumonia, and conjunctivitis and are transmitted sexually, perinatally, and by hand-eye contact. Today, *C. trachomatis* is one of the most important causes of sexually transmitted diseases [15]. *C. trachomatis* usually infects epithelial cells lining the mucous membranes. These are columnar cells in the cervix, cells of the urethra, rectum, conjunctiva, and cells of the newborn's respiratory system [16]. *C. trachomatis* is the cardinal pathogen of non-gonococcal urethritis [17]. Sexually transmitted *C. trachomatis* infection can sometimes be asymptomatic [18]. In this case, accurate and early diagnosis is important since patients will continue to transmit the infection [19]. Undiagnosed and untreated *C. trachomatis* infections cause diseases such as pelvic inflammatory disease and may result in ectopic pregnancy and tubal infertility. It has also been reported that *C. trachomatis* infections seen during pregnancy may be associated with pregnancy complications such as postpartum endometritis, premature rupture of membranes, premature birth, stillbirth, and low birthweight. *C. trachomatis* is also one of the important causes of reactive arthritis [8].

Cytological examination, cell culture, antigen determination, DFA, EIA, and NAAT are used in the diagnosis of *C. trachomatis*. The sensitivity of the cell culture method varies from 50 to 85%. For DFA and EIA, the sensitivity is given as 45.5–85% and 52–84.4%, respectively. Various studies have shown that the sensitivity for NAAT is from 80 to 100%. The specificity of all tests varies from 90 to 100% [20].
