What are the main sexually transmitted diseases caused by bacteria? Describe the characteristics of the organisms that cause them and how you would detect them in the laboratory.

Outline:

·        Syphilis: dark ground illumination, VDRL, TPHA, FTA-abs

·        Chancroid: Gram stain

·        Lymphogranuloma venereum: inclusion bodies, serology

·        Granuloma inguinale: Donovan bodies

·        Gonorrhoea: Gram stain, culture, serology

Suggested Answer:

            Syphilis is a primary, secondary or latent infection caused by Treponema pallidum up to 1 year after infection. Infection is spread by sexual or other mucosal contact. The bacterium enters the tissues via microscopic trauma and reaches other parts of the body via the bloodstream. A painless chancre appears at the site of primary inoculation after 2 to 3 weeks. The chancre heals spontaneously at about the same time lesions of secondary syphilis appear. These include a symmetrical, erythematous, maculopapular rash that starts centrally and spread to palms and soles. After secondary syphilis, the disease becomes latent until years later, when late manifestations of neurosyphilis or gummas occur in around 25% of untreated patients. Treponema pallidum are thin-walled, flexible spiral rods which are motile. They cannot be grown on bacterial media and detection in the laboratory involves visualization of the spirochaetes using dark ground illumination microscopy. Nontreponemal tests such as VDRL detects antibodies to nontreponemal antigens (e.g. cardiolipin) and are less specific with false-positive reactions in infections such as leprosy. Treponemal antibody tests (TPHA / FTA-abs) are more specific, detecting antibodies to Treponema. During secondary syphilis, both the tests are positive in all cases.

            Chancroid is an ulcerative condition of the external genitalia caused by Haemophilus ducreyi infection. The initial lesion is a papule, which breaks down to form a painful ulcer. The most common sites are on the penis or on the thighs adjacent to penile lesions. There may also be painful inguinal lymphadenopathy. Gram stain and culture of exudate from the ulcer base forms the basis of diagnosis. The typical appearance of H. ducreyi is chains of Gram-negative bacilli.

            Lymphogranuloma venereum is a genital infection in which lymphadenopathy is the predominant feature, caused by Chlamydia trachomatis L1, L2 and L3. Chlamydia is an obligate intracellular bacteria which can grow only inside host cells. They have a rigid cell wall but no peptidoglycan layer. It shares a group-specific lipopolysaccharide antigen, detected by complement fixation. It infects only humans and is transmitted by close personal contact: sexually or passage through birth canal. Chlamydia forms cytoplasmic inclusions seen by Giemsa’s stain or by immunofluorescence. In exudates, organisms can be identified within epithelial cells by fluorescent antibody staining, ELISA or hybridization with a DNA probe. Though serologic tests are rarely helpful, a high titre (complement fixation) is specific for lymphogranuloma venereum.

            Granuloma inguinale is a sexually transmitted infection caused by an uncultivatable Gram-negative bacterium Calymattobacterium granulomatis. This condition is prevalent in parts of Asia and the Caribbean and causes a painless papule that enlarges slowly. Inguinal lymphadenopathy is rare, but local extension can produce pseudobuboes. The diagnosis can be confirmed by Giemsa stain of a tissue smear, which reveals the black, intracellular Donovan bodies.

            Gonorrhoea is a sexually transmitted disease which causes localized lesions in the genital tract and disseminated infections with seeding of various organs. Gonorrhoea in man manifests clinically as a acute urethritis, dysuria and purulent discharge from the penis. In women, symptoms include a purulent vaginal discharge, intermenstrual bleeding, acute salpingitis which can spread further to cause acute pelvic inflammatory diseases. Pharyngitis is common in both sexes. Disseminated infection leads to fever, joint pain/infection and small pustular skin lesions. The offending bacteria is Nesseria gonorrhoea. Neisseria gonorrhoea is a gram-negative cocci which is seen in pairs  (diplococci) under the microscope. Diagnosis of gonorrhoea is by microscopy, culture, and antigen detection. A Gram stain performed on a smear of purulent urethral discharge is a good means of detecting N. gnorrhoeae, which appear intracellular Gram-negative diplococci. The bacteria can be grown on Thayer-Martin medium producing a oxidase-positive colony composed of Gram-negative diplococci. ELISA detects gonococcal antigens while a DNA probe assay detects gonococcal ribosomal genes.

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