The diagnosis of urogenital infections depends on Gram staining and culture of the discharge (see Figure 1). However, nucleic acid amplification tests are widely used as screening tests.
Fig1. Neisseria gonorrhoeae—Gram stain. Blue arrow points to a typical gram-negative diplococcus within a neutrophil. Many other gram-negative diplococci can be seen within the neutrophils. (Used with permission from Dr. Bill Schwartz, Centers for Disease Control and Prevention.)
The finding of gram-negative diplococci within PMNs in a urethral discharge specimen is sufficient for diagnosis (see Figure 1). Gram stains on cervical specimens can be falsely positive because of the presence of gram-negative diplococci in the normal flora and can be falsely negative because of the inability to see small numbers of gonococci when using the oil immersion lens. Cultures must also be used in diagnosing suspected pharyngitis or anorectal infections.
Specimens from mucosal sites, such as the urethra and cervix, are cultured on Thayer-Martin medium, which is a chocolate agar containing antibiotics (vancomycin, colistin, trimethoprim, and nystatin) to suppress the normal flora. The finding of an oxidase-positive colony (see Figure 2) com posed of gram-negative diplococci is sufficient to identify the isolate as a member of the genus Neisseria. Specific identification of the gonococcus can be made either by its fermentation of glucose (but not maltose) or by fluorescent antibody staining. Note that specimens from sterile sites, such as blood or joint fluid, can be cultured on chocolate agar without antibiotics because there is no competing normal flora.
Fig2. Oxidase test. A drop of the oxidase reagent was placed on the left and right side of the filter paper. Bacteria from a colony of Neisseria gonorrhoeae were rubbed on the drop on the left, and the purple color indicates a positive test (i.e., the organism is oxidase-positive). Bacteria from a colony of Escherichia coli were rubbed on the drop on the right, and the absence of a purple color indicates a negative test (i.e., the organism is oxidase-negative). (Used with permission from Professor Shirley Lowe, University of California, San Francisco School of Medicine.)
Nucleic acid amplification tests, often abbreviated NAAT, detect the presence of gonococcal nucleic acids in patient specimens. These tests are widely used for screening purposes, produce results rapidly, and are highly sensitive and specific. They can be used on urine samples, obviating the need for more invasive collection techniques. Note that serologic tests to determine the presence of antibody to gonococci in the patient’s serum are not useful for diagnosis.
