0 1995 The Pathological Society of Great Britain and Ireland DIAGNOSTIC MICROBIOLOGY An evaluation of pre-poured selective media for the

Fourteen commercial media supplied as pre-poured plates were compared with an ‘in-house ’ selective medium for their ability to support the growth of 105 gonococcal isolates (representing a wide variety of serovars encountered in natural infection), 25 meningococcal and 20 Neisseria lactamica isolates, and to inhibit the growth of 71 isolates of non-pathogenic neisseriae and miscellaneous organisms. Only two of the pre-poured plate media and the in-house selective medium yielded growth of duplicate cultures of all 105 gonococcal isolates after incubation for 24 h: one other medium provided growth of all the isolates after incubation for 48 h. The ability of the various media to suppress the growth of the 71 isolates of non-pathogenic neisseriae and miscellaneous organisms ranged from 97.2 to 71.8 % of isolates inhibited. Of the four media that enabled growth of all the gonococcal strains, inhibition was 94.4 % for the in-house medium, 85.9 % and 80.3 % for the two media on which all gonococci grew after 24 h and 71.8 % for the medium on which all of the gonococci grew after 48 h. Failure of growth of gonococci was associated with: serogroup IA isolates (p < 0.001), AHU auxotype (p < 0.001) and the presence of vancomycin rather than lincomycin in the selective medium (p < 0.02). The use of 10 % blood and a highly nutritious medium based on the original New York City (NYC) or modified New York City (MNYC) formulation were also important in supporting growth of gonococci. One of the main problems in lack of selectivity was a failure to inhibit the growth of yeasts. As effective inhibition of yeasts was obtained with other media containing the same concentration of amphotericin, failure may be due to batch variation of supplement, media preparation, or reduced shelf life of the media. None of the commercially available pre-poured media performed as well as the in-house medium despite the fact that some of the media were prepared to a very simila