A Selective Medium for the Cultivation of N. gonorrhoeae and N. meningitidis (1964)

A SIGNIFICANT complication in the control of gonorrhea is the large number of women who harbor undiagnosed, asymptomatic infection. Development of better and more rapid laboratory procedures for isolation and identification of the gonococcus would therefore help to control this source of the disease. For women, diagnosis by culture has always been found greatly superior to gram-stained smears. The culture method, however, becomes much less effective when specimens aLre contaminated with bacteria that overgrow the more slowly developing gonococci. Complete overgrowth may occur in 10 percent of cervical cultures. It is not known to what extent cultures are lost by contamination when few gonococcal cells are present, as in chronic or asymptomatic gonorrhea or early in the detection of therapeutic failure. Selective media for the isolation of a single bacterial species from contaminated specimens are seldom absolute; however, growth of the desired organisms may be favored or the undesired contaminants inhibited to the point where pure cultures may be easily isolated. Since Neisseria gonorrhoeae of ne.cessity requires highly enriched media favorable also to bacterial and yeast contaminants, previous attempts to prepare a selective medium have aimed at reducing the contaminating organisms by the addition of inhibitory substances such as thallium acetate (1), crystal violet (2), Nile Blue A (3), tyrothricin (4), aerosporin (5), boric acid (6), and chloiral hydrate (6). Use of these agents was not successful, primarily because some sensitive gonococcal strains were intolerant to concentrations required to inhibit contaminants. To accomplish the desired end, the full impact of the inhibiting agent may need to be compromised to allow growth of all or nearly all sensitive strains. Further, when two or more inhibitory agents are used together, antagonistic or synergistic action for the desired and unwanted bacteria must be considered. In women suspected of gonorrheal infection, cultures are taken routinely from only two sites, the endocervical canal and urethra, although gonococci are known to reside in the posterior fornix of the vagina and in the rectum in many cases. In an attempt to raise to the highest possible degree the standards of bacteriological investigation employed for diagnosis and test of cure, two studies in England used cultures from the vagina and rectum in addition to those from the cervix and urethra. Of 229 patients, Nicol (7) found 19.6 percent positive by vaginal culture, 7.5 percent being positive only from this site. Wilkinson (8) examined 224 patients with gonorrhea for rectal infection and found 17.8 percent positive by culture. Gram-stained smears gave a higher proportion, 28 percent, of positive results. Overgrowth of rectal cultures by contaminants resulted in more positive findings by microscopic examination. Obviously, culture procedures would be improved greatly if a highly selective medium could be devised for the gonococcus. Such a Dr. Thayer is chief and Mr. Martin is a bacteriologist, Antibiotic Surveillance and Methodology Laboratory, Venereal Disease Branch, Communicable Disease Center, Public Health Service, Atlanta, Ga.