Failure to detect capsule gene bexA in Haemophilus influenzae types e and f by real-time PCR due to sequence variation within probe binding sites.

Detection of the conserved capsule gene bexA is used to distinguish capsulate from non-capsulate Haemophilus influenzae. While developing a real-time PCR assay to detect bexA, it was found that bexA probes produced a detectable signal for H. influenzae types a to d, but failed to do so for H. influenzae types e and f. Sequencing revealed differences compared with H. influenzae types a to d within probe binding sites. To prevent misclassification of strains as non-capsulate, assays must detect all capsular types.