Spontaneous phase variation in Bordetella pertussis is a multistep non-random process.

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Abstract
Pathogenic strains of Bordetella pertussis undergo spontaneous phase variation and become non‐pathogenic upon culturing in vitro. Spontaneous variants of the Tohama and #165 pathogenic strains of B. pertussis were selected by their ability to grow on synthetic and semi‐synthetic solid media. The frequency of these variants was between 10(‐6) and 10(‐7). About 250 variant strains were screened for the presence of virulence‐associated traits, such as production of hemolysin, pertussis toxin and filamentous hemagglutinin (FHA). Only four different combinations of the traits were found: 7‐11% of the variants displayed all traits, 17% of the variants carried the toxin and FHA, 5‐11% carried FHA only and 66% were devoid of all virulence traits. The strains which had at least one virulence trait also demonstrated some adenylate cyclase activity. The disappearance of hemolysin quantitatively affected the other traits. These results suggest that phase variation in B. pertussis is a non‐random process, involving multistep disappearance of virulence factors in the following order: hemolysin, pertussis toxin and FHA. In contrast, all 300 variants of strain #18323 of B. pertussis, which were able to grow on the selective solid media, carried all the virulence traits. This is in accordance with the strain's unique intracerebral growth capability.