. The addition of Congo red, Trypan blue or haemin to the growth medium allowed the differentiation of phase-I and variant strains of . Phase-I strains produced red (CR), blue or dark brown colonies on a modified cyclodextrin solid medium containing Congo red, Trypan blue or haemin, respectively, whereas variant (Vir and phase IV) strains grew as pale (CR) colonies. Spontaneous CR variants were isolated and characterised and had a phenotype like that of Vir or phenotypically modulated, C-mode strains in that they did not produce the haemolysin, haemagglutinin(s), histamine-sensitising factor (pertussis toxin), heat-labile toxin and two major envelope polypeptides associated with phase-I strains. Two such variants had reduced virulence for mice. CR strains, when grown on a high nicotinic acid medium to induce modulation, gave CR colonies. Thus the CR phenotype is a characteristic of phase-I and its expression appears to be controlled in a manner similar to that of other phase I-related factors. CR variants of and were also deficient in these factors. Four isolates of were CR.


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