Sexually Transmitted Diseases... Health Article

Gonorrhea

Neisseria gonorrhoeae, also called gonococcus or GC, causes gonorrhea. It infects the mucosal surfaces of the genitourinary tract, primarily the urethra in males and the cervix in females. When seen on Gram stain, Neisseria gonorrhoeae are gram-negative diplococci (pairs of round or bean-shaped bacteria) often located inside white blood cells. The best specimen from which to culture Neisseria gonorrhoeae is a swab of the urethra in a male or the cervix in a female. Other possible specimens include the mouth, anus, or a swab of a genital lesion. All swabs are plated on modified Thayer-Martin (MTM) agar or New York City (NYC) agar. These media are selective for the growth of N. gonorrhoeae. MTM is chocolate agar (heated sheep blood agar) containing colistin to inhibit the growth of gram negative bacilli, nystatin or anisomycin to inhibit yeast, vancomycin to inhibit growth of gram-positive bacteria, and trimethoprim to inhibit Proteus spp. NYC agar contains amphotericin B instead of nystatin and consists of clear proteose-peptone supplemented agar. In addition, the sample is plated on either 5% sheep blood agar or Columbia agar with 5% sheep blood and colistin and nalidixic acid (CNA) to isolate Candida albicans which causes a yeast infection in the vagina and Gardnerella vaginalis which causes vaginosis as well. Plates are incubated at 96.8°F (36°C) in 5–10% carbon dioxide. MTM or NYC agar are examined for growth at 24 hours and if negative again at 48 hours. After 24 hours, any suspicious colonies are Gram-stained and tested for oxidase which provides presumptive identification of Neisseria if positive. The physician can be notified at this point by a preliminary report that gonococcus has been identified presumptively. Further biochemical testing may be performed to differentiate N. gonorrhoeae from N. meningitides which is sometimes isolated from homosexual males. Isolated colonies should also be tested for penicillin resistance. Plates may be discarded at 48 hours if no growth is seen. Rapid non-culture DNA amplification and enzyme immunoassay tests are available to test for Neisseria gonorrhoeae and provide results on the same day.

Microscopic analysis should always be included with genital culture. Wet preparations can identify yeast, Trichomonas vaginalis, and G. vaginalis. The latter can be seen as rods attached to large squamous epithelial cells called "clue cells." A Gram stain of the swab material can identify gram-negative diplococci which is presumptive evidence of gonococcal infection. In males, a positive finding on the Gram stain obviates the need for culture and the patient can begin antibiotic treatment. In females, the diplocicci must be located intracellularly in order to make a presumptive diagnosis of gonorrhea infection, and culture must be performed to confirm the diagnosis. The presence of clue cells, epithelial cells containing gram-negative or gram-variable coccobacilli, can signal the presence of Gardnerella vaginalis.

Chancroid

Chancroid is caused by Haemophilus ducreyi. It is characterized by genital ulcers with nearby swollen lymph nodes. The specimen is collected by swabbing one of these pus-filled ulcers. The Gram stain cannot differentiate Haemophilus ducreyi from other Haemophilus species. The physician must request a specific culture for a person who has symptoms of chancroid. Even using special culture, Haemophilus ducreyi is isolated from less than 80% of the ulcers it infects. If a culture is negative, the physician must diagnose chancroid based on the person's symptoms, and by ruling out other possible causes of these symptoms, such as syphilis (which is diagnosed by a blood test for antibodies).

H. ducreyi is fastidious and culture media should be inoculated within 10 minutes of sample collection. The swab should be spread over a chocolate (heated sheep blood agar) plate and incubated at 96.8°F (36°C) in 5–10% carbon dioxide. Isolated colonies are Gram-stained to identify the bacteria as small gram-negative bacilli, and a colony is transferred to trypticase soy broth and a suspension is made. This is plated onto Mueller-Hinton or trypti-case soy agar and strips of factor X (hemin) and factor V (NAD) are applied. Haemophilus ducreyi requires X factor but not V factor for growth. Like other Haemophilus species the organism is oxidase positive and reduces nitrate. Unlike most other Haemophilus species it does not produce catalase and does not ferment glucose, and these tests can be used for positive identification.