The number of sexually transmitted microorganisms (e.g., bacteria, viruses, fungi, and parasites) and the spectrum of clinical syndromes or diseases associated with these agents are expanding with the nature of changing sexual orientations of people and increasing number of immunosuppressive population (people with HIV/AIDS, under immunosuppressive therapy, cancer conditions), etc.
While developed countries have access to testing methods, many populations in developing countries do not have access to STI information, testing, or preventative measures. Due to the asymptomatic nature of many STDs, those without this knowledge and access to medical testing and care are at higher risk of contracting and transmitting these infections.
Multiple pathogens can cause one particular disease, e.g., vaginitis can be caused by Neisseria gonorrhoeae, Chlamydia trachomatis, Trichomonas vaginalis, etc.
Table of Contents
Major Causes of Sexually Transmitted Diseases (STDs)
Neisseria gonorrhoeae
Neisseria gonorrhoeae is a Gram-negative, diplococci (kidney-bean shaped). It causes gonorrhea. Pili is one of the main virulence factors of N. gonorrhoeae, it helps in attachment, and non-piliated N. gonorrhoeae are non-pathogenic.
The presence of Gram-negative intracellular diplococci within neutrophils in the urethral discharge is diagnostic. Culture is important to isolate the pathogen and perform antimicrobial susceptibility testing, but molecular techniques are also available for diagnosis.
Treponema pallidum
Treponema pallidum is a spirochetes. This etiological agent of disease, syphilis, causes painless genital ulcers called a chancre. It can not be cultured, so diagnosis mostly relies on observing the spirochetes in the dark-field microscopy of the fluid from the painless genital lesion. Serological tests such as non-treponemal tests (VDRL and RPR) and treponemal tests (TPHA and FTA-ABS) are commonly used to diagnose syphilis.
Haemophilus ducreyi
Haemophilus ducreyi is a fastidious gram-negative rod. It causes a genital ulcer called chancroid (soft chancre). Chancroid is a sexually transmitted disease that begins with painful penile lesions (in contrast to syphilis, which is painless); nonindurated (soft) ulcers; and local lymphadenitis (bubo).
Pus aspirated from an ulcer of a lymph node is the sample for lab diagnosis. The organism can be isolated in chocolate agar. It requires factor X (heme) but not factor V (NAD).
Gardnerella vaginalis
Gardnerella vaginalis is a facultative gram-variable rod and one of the bacterial vaginosis’s etiological agents (BV) agents. Bacterial vaginosis is a polymicrobial disease involving G. vaginalis and other facultative and anaerobic organisms such as Prevotella spp. Mycoplasma spp. Mobiluncus spp. Prevotella spp. Bacteroides spp. and Prophyromonas spp.
The presence of “clue cells,” in the vaginal discharge, which are vaginal epithelial cells covered with G. vaginalis and other etiological agents, is diagnostic. A positive “whiff” test is another diagnostic test for bacterial vaginosis.
Trichomonas vaginalis
Trichomonas vaginalis is a protozoan parasite. It causes trichomoniasis (trich), a sexually transmitted infection (STI) mostly in women, though males are also affected.
Candida albicans
Candida albicans is a yeast. It can cause vaginitis and vulvovaginal candidiasis, especially in women with diabetes, using immunosuppressive agents, or having recently completed antibiotic therapy. Women heavily colonized with Candida albicans in the genital tract may pass this organism to newborns during delivery, causing thrush.
Chlamydia Trachomatis
Chlamydia trachomatis causes nongonococcal urethritis and cervicitis. Chlamydia trachomatis (L-1, L-2 and L-3 serovars) are responsible for causing lymphogranuloma venereum. Isolation of C. trachomatis from the sample is challenging as it can not be cultured on in-vitro culture media and requires living cells. Detection of chlamydial nucleic acids using nucleic acid amplification test (NAAT) is a commonly used diagnostic test for diagnosing sexually transmitted diseases caused by Chlamydia trachomatis.
Human immunodeficiency virus (HIV)
Human immunodeficiency virus (HIV) causes AIDS, which is the late stage of HIV infection when a person’s immune system is severely damaged and has difficulty fighting diseases and certain cancers. This retrovirus is transmitted by unprotected sex with HIV infected partner. The risk of transmission depends on the type of sexual activity, and the presence of other sexually transmitted infections increases the risk of transmission.
Serological tests such as P24 antigen detection, ELISA for antibody detection, and western-blot assay are used to diagnose HIV infection.
Herpes simplex virus (HSV)
Herpes simplex virus (HSV) 1 and 2 are members of the herpes virus family. HSV-2 is also responsible for causing genital herpes, neonatal herpes, and aseptic meningitis.
Primarily, HSV-2 and occasionally HSV-1 cause herpes genitalis (ulcers in the genital region). Apart from causing painful vesicular lesions on genital areas (genital herpes), HSV-2 also causes neonatal herpes and aseptic meningitis.
Molluscum contagiosum virus (MSV)
Molluscum contagiosum virus is a member of the poxvirus family. It is transmitted from close personal contact, including sexually, and causes small, pink, papular, wartlike benign tumors of skin or mucus membrane. Lesions have characteristic cup-shaped craters with a white core.
Human papillomaviruses (HPV)
Human papillomaviruses cause papillomas, benign tumors of squamous cells (warts on the skin of the genital area, mouth, and throats). This virus is transmitted primarily by skin-to-skin contact and genital contact. Genital warts (condylomata acuminata) are the most common sexually transmitted diseases. Nearly all sexually-active men and women get it at some point in their lives.
Others
Ectoparasites (such as scabies and lice) are transmitted from close personal contact, including sexual contact. Among the diseases caused by ectoparasites, scabies caused by Sarcoptes scabiei and pediculosis pubis or “crabs” infestation caused by Phthirus pubis are most common.
Ureaplasma urealyticum is one of the etiological agents of nongonococcal urethritis (NGU). Mycoplasma hominis has been implicated as an infrequent cause of the pelvic inflammatory disease (PID). Mycoplasma genitalium causes urethritis primarily in men. It is estimated to cause approximately 20% of nongonococcal urethritis. Ureaplasmas are distinguished from mycoplasmas by their ability to produce urease enzyme.
Klebsiella granulomatis causes granuloma inguinale. Which appear as single or multiple subcutaneous nodules in the genital regions. Among homosexual populations, intestinal protozoa such as Giardia lamblia, Entamoeba histolytica, and Cryptosporidium spp. are also significant causes of STDs.
Viruses such as hepatitis B (which causes acute and chronic hepatitis), adenovirus, human T-cell lymphotropic virus (HTLV), and coxsackie virus are also transmitted sexually.
| Disease | Name of the etiological agent(s) |
| Acquired Immunodeficiency Syndrome (AIDS) | Human immunodeficiency virus (HIV) |
| Bacterial vaginosis | Gardnerella vaginalis |
| Chancroid | Haemophilus ducreyi |
| Chlamydia trachomatis infections (such as cervicitis, urethritis, prepubertal vaginitis, proctitis, endometritis, etc) | Chlamydia trachomatis |
| Chlamydial lymphogranuloma | Chlamydia trachomatis serovars L1, L2 and L3 |
| Genital herpes | Herpes simplex virus type 2 (HSV-2) |
| Gonorrhea | Neisseria gonorrhoeae |
| Lymphogranuloma venereum | C. trachomatis |
| Nongonococcal urethritis | C. trachomatis, Ureaplasma urealyticum, or Trichomonas vaginalis |
| Pelvic inflammatory disease | N. gonorrhoeae or C. trachomatis |
| Syphilis | Treponema pallidum |
| Trichomoniasis | Trichomonas vaginalis |
| Vulvovaginal candidiasis | Candida albicans |
Testing of Sexually Transmitted Infections (STIs)
Laboratory diagnosis of some STIs requires expensive technologies or specialist laboratory facilities, while others can be diagnosed at home or in district laboratories.
STIs like gonorrhea, syphilis, trichomoniasis, candidiasis, G.vaginalis bacterial vaginosis, etc., can be diagnosed using microscopy and staining techniques. Culture can diagnose diseases like gonorrhea and chancroid using a selective enriched medium. Immunological methods (e.g., ELISA, ICT-based tests) are now available for diagnosing chlamydial infections, HIV, Genital herpes infection, syphilis, etc. PCR and molecular technologies have also been developed to diagnose some STIs like HIV.
References and further readings
- Bailey & Scott’s Diagnostic Microbiology, Forbes, 11th edition
- Review of Medical Microbiology and Immunology, Lange Medical Books, 13th edition
- Color Atlas and Textbook of Diagnostic Microbiology, Koneman, 5th edition
