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Immune serum resistance of wild-type and ΔbrkA mutant strains of B. pertussis at various concentrations of serum. In vitro-grown bacteria were diluted to a concentration of 500 CFU/50 μl and treated with different concentrations of human immune serum for 1 h at 37°C. Serum resistance is expressed as the percentage of bacteria that survived ± the standard error. All samples were run in triplicate, and each strain was tested at least twice.
