Expression of phase 2 antigen and phase variation frequency.

aWT, wild type; A46T and R140L, amino acid substitutions.b+, selectable; -, not detected.cND, not done;*, significantly lower than parental strain (p = 0.04).aWT：野生型（wild type）；A46T与R140L为氨基酸替换突变。b+：可筛选；-：未检测到。cND：未完成；*：显著低于亲本菌株（p = 0.04）。