**2. The most common bacterial STI pathogens**

*Chlamydia trachomatis* infections are the most reported STI worldwide. However, it is known that there are many infections caused by this bacterium that go undiagnosed and consequently remain untreated. *C. trachomatis* can persist for a long time in the genital tract in a resistant form, and symptoms of infection may go unnoticed in about 75–80% of women [4]. Since many cases are asymptomatic, the actual detection of the infection would require screening in the population [5]. It is very important to diagnose this bacterium in the early stages of infection and start treatment as quickly as possible to avoid complications that may occur in the long term, thus reducing the risk of reproductive tract sequelae.

Lymphogranuloma venereum (LGV) is a disease caused by *C. trachomatis* serotypes L1, L2, and L3. It is considered endemic to Asia, Africa, South America, and the southeastern United States. In Europe, LGV infection is found almost exclusively in men who have sex with men (MSM), presenting clinically as a proctocolitis commonly associated with HIV [6].

*Neisseria gonorrhoeae* is the etiological agent of gonorrhea, one of the most common bacterial STIs, which is characterized by purulent inflammation of the mucous membranes of the genitourinary system, producing over 82 million new infections worldwide every year. Since 2008, the total number of gonorrhea cases has increased by 79%, especially among men. This increase appears to be related to the increase in cases among MSM [1]. Being a bacterium with remarkable genetic variability has led to the emergence and spread of multidrug-resistant strains of *N. gonorrhoeae*. This antibiotic resistance and the absence of an effective vaccine are major problems worldwide, highlighting the need for routine surveillance, prevention, and control measures [7].

*Treponema pallidum* is the etiological agent of syphilis, one of the bacterial STIs known for centuries [8]. Syphilis is a disease characterized by a decades-long clinical course that may include four different stages, without adequate treatment leading to either death or spontaneous resolution after the secondary stage. The overall rate of syphilis has increased since 2010, especially among men. In Europe, the incidence of syphilis has been reported to be five times higher in men than in women, and most cases occur in people over 25 years of age [1]. It is a serious disease because treponemes can cross the hemato-encephalic barrier and trigger neurological signs and symptoms, the risk being higher in patients with immunodeficiency or HIV. Syphilis can be transmitted from mother to unborn child, the stage of the mother's disease during pregnancy has an important role. If the mother's infection is massive it will lead to spontaneous abortion, but if there are few pathogens in the mother's blood, the child will develop congenital syphilis [6]. The upward trend in syphilis rates in many EU countries may be partly explained by increased case finding due to more testing among HIV-positive MSM, as recommended in current HIV management guidelines, or by an improved and more efficient reporting of detected cases.

Mycoplasmas are the smallest free-living microorganisms that lack a cell wall. Genital mycoplasmas are represented by species frequently found in the lower genitourinary tract of sexually active people, the most widespread being *Mycoplasma*  *Introductory Chapter: Bacterial Sexually Transmitted Infections – New Perspectives DOI: http://dx.doi.org/10.5772/intechopen.108665*

*hominis*, *Ureaplasma urealyticum,* and *Mycoplasma genitalium*. Data suggest that infection with *U. urealyticum* occurs in 10–50% of women and that with *M. hominis* in less than 20% of them [9]. Since they have a high prevalence among asymptomatic women, their role in sexually transmitted diseases should be well evaluated [10]. These species of microorganisms are considered to induce a wide spectrum of pathological conditions in the lower urinary tract and genital organs, and *M. genitalium* would be implicated as a causative agent in conditions with significant sequelae. Some studies have reported that these organisms are involved in female infertility or premature birth [11]. In addition, some studies present mycoplasmas as causative agents of male infertility by changing some sperm characteristics [12].

*Klebsiella granulomatis* (formerly known as *Calymmatobacterium granulomatis*) is a gram-negative bacterium, the causative agent of donovanosis, a chronic ulcerative genital disease. The disease rarely occurs in Western European countries but is common in parts of Africa and South America. Transmission is mostly sexual, so infection with this bacterium can be considered an STI.
