FIG. 6.

Intracellular survival of B. pertussis parental and mutant strains in human respiratory epithelial HTE cells. Each B. pertussis strain (7 × 10⁶ CFU) was added to a separate well of a 24-well tissue culture plate, each of which contained 7 × 10⁴ epithelial cells. The invasion of HTE cells by B. pertussis was assayed as described in Materials and Methods. A value of 100% corresponds to the absolute number of viable intracellular bacteria of each strain in a coincubation period of 5 h. For the 24- and 48-h points, the percentages of surviving intracellular bacteria of each strain compared to the 5-h point, independently of the parental strain, are shown. The persistence experiment was performed twice with the parental strain B. pertussis Tohama I and HTE cells and only once with the parental strain B. pertussis 18323 and the mutants. The values above the histograms for the 5-h time point correspond to the means ± SEM of the absolute number of intracellular bacteria of each strain per cell in all of the four to six experiments performed for this time point.