SUMMARY: Gonococci from subcutaneously implanted chambers in guinea pigs produced, on agar, more than 95% small colonies showing a ‘double highlight’ (DH) effect in oblique reflected light combined with transmitted light. Laboratory strains of gonococci produced some DH colonies, but others showed a single highlight (SH) or no highlight (NH). Selection of DH colonies and comparison of their organisms with gonococci grown and with those from SH colonies, showed that the DH character was associated with high infectivity for guinea-pig chambers, resistance to killing by human phagocytes and heavy pilation. Furthermore, DH colonies were found in the first culture of three fresh samples of urethral pus. Thus, the DH colony characteristic may be a more reliable criterion of pathogenicity of gonococcal isolates than systems used previously. There were, however, some differences between the gonococci grown and the DH colony types. The gonococci grown and cultured once on solid medium possessed one or two antigens which differed from those of DH (or SH) colonies. They also formed smooth suspensions (which separated slowly) in saline, compared with the rough suspensions (which separated quickly) formed by gonococci from DH (or SH) colonies. Finally, the organisms grown were resistant to killing by human serum whereas the DH (and SH) colony types were susceptible; the resistance of the organisms grown was lost during one subculture on agar suggesting that the property is a phenotypic characteristic. Hence, in addition to selecting DH colony types the conditions produce organisms which differ, probably phenotypically, from cultured organisms.


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