Strain-specific distribution and expression of pili-encoding genes. (A) Levels of spa gene transcripts in different C. diphtheriae strains. Total RNA was isolated from the indicated C. diphtheriae strains and hybridized with probes monitoring 16SrRNA for control as well as spa gene transcription. (B) PCR detection of spa genes. Chromosomal DNA of the indicated C. diphtheriae strains was used as template for PCR using specific oligonucleotide pairs for the spa genes indicated at the right side of the figure.