Trimethoprim as an additional selective agent in media for the isolation ofN. gonorrhoeae

Abstract

SYNOPSISThe incorporation of trimethoprim into existing selective media using either saponin-lysed blood agar or chocolate agar as the base is suggested for the routine isolation of the gonococcus. The principal advantage over other selective media is the inhibition ofProteus mirabilis and other commensal organisms as a consequence of which multiple swabs can be plated out onto a single selective plate irrespective of source, while the recovery rate of the gonococcus from these specimens is increased. The relative merits of both basic media are briefly discussed.The isolation of N. gonorrhoeae on selective media is still hampered by their failure to suppress the growth of some commensal organisms, notably Proteus mirabilis, spreading members of which overgrow other organisms and render isolation of the gonococcus impossible. Also, if multiple swabs are inoculated onto a single plate, one colony of this organism will spread and invalidate all results. The incorporation of first ristocetin and polymyxin (Thayer and Martin, 1964), and, more recently, vancomycin, sodiumcolistimethate, and nystatin (Martin, Billings, Hackney, and Thayer, 1967) incorporated into the VCN medium of Thayer and Martin. The lowest amount of trimethoprim which would inhibit the growth of 10 spreading strains of Pr. mirabilis was 1-5 ,tg/ml. Six times this amount did not affect the recovery of N. gonorrhoeae and 3 ,ug/ml was selected as the amount which would safely inhibit Pr. mirabilis while not affecting the gonococcus. Vancomycin was then omitted from the above mixture but had to be reintroduced as certain commensals, notably staphylococci, were insufficiently suppressed.A comparison was made in the first part of the study of the isolation rates of N. gonorrhoeae on plain chocolate agar, the VCN medium and the VCN medium with trimethoprim 3,ug/ml. In the latter medium the vancomycin was increased fronP 3,ug/ml to 5,ug/ml, a previous experiment showing this concentration to be more effective against Gram-positive cocci without reducing the yield of gonococci. A similar study was made in the second part but using saponin-lysed blood agar as the basal medium. Vancomycin was used at a concentration of 3,ug/ml as in the original Thayer and Martin medium, but in the trimethoprimcontaining medium nystatin was replaced by an

Diagnosis of gonorrhoea by culture on a selective medium containing vancomycin, colistin, nystatin and trimethoprim (VCNT). A comparison with Gram-staining and immunofluorescence.

“…of trimethoprim suppressed the growth of Proteus in 57 out of 58 samples, revealing the otherwise undetected gonococcus in seven instances. Riddell and Buck (1970) found that 3 Vg./ml. trimethoprim reduced the viable counts of Proteus by 105 to 106 organisms per ml.…”

Acquired and natural immunity to gonococcal infection in chimpanzees.

“…All specimens were plated onto selective medium (16) and vancomycin, colistin, nystatin, and trimethoprim (VCNT) medium (selective medium without hemoglobin and containing 5 /Ag/ml of trimethoprim). The VCNT medium allowed colony typing of the primary isolate (17) ; it also suppressed an occasional Proteus which overgrew the surface of the standard selective medium (18).…”

Quantitative bacteriology of the vaginal flora during the menstrual cycle

“…For anaerobic incubation, samples (1OOpl) were spread on to the surface of the following media which had been poured in the chamber: Brucella blood agar, neomycin (1 00 pg/ml)-blood agar and kanamycin (50 pg/ml)-vancomycin (5pg/ml)-lysed blood agar, prepared according to the methods of Sutter et al (1980) except that horse blood lO%v/v was substituted for sheep blood 5% v/v; Rogosa Agar (Difco) with a finalpH of 5.4, as selective medium for lactobacilli. The dilution tubes were then removed from the chamber and 100-pl samples were spread on plates of the following media for aerobic cultivation : blood agar (Tissue Culture Services, 10 Henry Road, Slough, Berks), MacConkey Agar (Oxoid) and modified Thayer Martin Medium (Riddell and Buck, 1970). Anaerobic culture plates were incubated in the chamber for up to 7 days at 37°C and the aerobic plates for 2-3 days in an atmosphere of air + Different colony types were enumerated and subcultured on to fresh media for identification.…”