An Internal Reaction Control for Routine Detection of Clavibacter michiganensis subsp. sepedonicus Using a Real-Time TaqMan PCR-Based Assay.

An internal reaction control was integrated into a TaqMan polymerase chain reaction (PCR) assay for the detection of Clavibacter michiganensis subsp. sepedonicus, the causal organism of bacterial ring rot of potato. The reaction control, cloned into plasmid pCmsC4, consisted of a sequence unrelated to C. michiganensis subsp. sepedonicus flanked by the primer sequences used in the TaqMan PCR, thus eliminating the need for multiplexing. Inclusion of the reaction control plasmid in the TaqMan assay had no effect on either the limit of detection or the specificity of the method. Addition of SYBR Green permitted melt analysis of PCR products. The 242-bp reaction control amplicon, with a melt temperature of approximately 94.5°C, could easily be distinguished from the 152-bp primary diagnostic target amplicon, which had a melt temperature of about 85.5°C. Electrophoretic analysis showed that appearance of either melt peak correlated well with the presence of the appropriate amplicon. Two different substances, guanidine-HCl and humic acid, inhibited the amplification of the reaction control at concentrations lower than those that inhibited the primary diagnostic target, demonstrating the reaction control's effectiveness in detecting inhibition or reaction failure. Using the reaction control plasmid, a quantitative threshold for inhibitor detection was established. This permitted the validation of negative results, and thus facilitated the use of TaqMan real-time PCR in the routine testing of diagnostic samples for C. michiganensis subsp. sepedonicus.