**1. Introduction**

Urethritis is an inflammation of the urethra, a fibromuscular tube through which urine and semen pass. The main cause of urethral inflammation is infection by sexually transmitted bacteria. Bacterial agents associated with urethritis are classified as gonococcal and nongonococcal [1]. Nonspecific urethritis is a term used for urethritis caused by nongonococcal and nonchlamydial pathogens [2]. *Chlamydia trachomatis* is one of the leading causes of nongonococcal urethritis [3]. Together with the gonococcal pathogen *Neisseria gonorrhoeae*, they are the most common causes of sexually transmitted infections in men and women [4].

Other common pathogens of nongonococcal urethritis are *Mycoplasma genitalium*, *Ureaplasma urealyticum*, and *Trichomonas vaginalis* [5]. The role of *Ureaplasma parvum* and *Mycoplasma hominis* in urethritis is controversial. There is strong evidence that *M. hominis* can cause urethritis at high microbial loads. However, *U. parvum* is regarded more as a commensal bacterium of the normal microflora and is not considered a urethritis pathogen [6].

Pathogens that are occasionally detected in patients with urethritis but rarely identified as causes of infectious urethritis include herpes simplex virus, adenoviruses, *Treponema pallidum* (endourethral chancre), *Haemophilus influenzae*, *Candida*, *Neisseria meningitis*, *Escherichia coli*, and streptococci. These are transmitted by direct contact *via* the oral, anal, or vaginal route, depending on their location [7].

In some men with urethritis, no known pathogen can be detected. Some of these cases are noninfective. However, there may not be sufficient clinical and laboratory findings to clearly distinguish them from possible infective urethritis [8].

Rare causes of noninfectious urethritis include trauma caused by urethral catheterization and instrumentation, foreign body insertion, or cycling; friction due to tight clothing or sexual intercourse; and exposure to irritants such as soap, powder, and spermicide [9].

*C. trachomatis* is a small, gram-negative obligate intracellular microorganism in the chlamydial bacterial family. It preferentially infects squamocolumnar epithelial cells. *C. trachomatis* has many serotypes, and serovars D-K infect the genital tract [10].

Unlike other bacteria, *C. trachomatis* has a biphasic life cycle. During this life cycle, they assume two forms called the elementary body (EB) and reticular body (RB). The EB is a metabolically inactive, infective form that is resistant to environmental conditions. In contrast, the RB is a metabolically active form that is not infective but has the ability to multiply within host cells. Between 48 and 72 h after infection, the host cell ruptures and *C. trachomatis* returns to the EB form to infect other cells. Once inside the epithelial cells, there is a neutrophilic response followed by lymphocyte, macrophage, plasma cell, and eosinophilic invasion. Infected epithelial cells release cytokines and interferon, initiating an inflammatory cascade. Thus, *C. trachomatis* induces a humoral and cellular response and causes the symptoms of urethritis [10].
