Development of PMA real-time PCR method to quantify viable cells of Pantoea agglomerans CPA-2, an antagonist to control the major postharvest diseases on oranges

• Quantification of viable cells of a biocontrol agent by qPCR was assayed.
• The optimal conditions of PMA-qPCR protocol on an orange matrix were established.
• Significant difference of Cq was detected in dead cells among qPCR and PMA-qPCR counts.
• PMA-qPCR provided data on the VBNC fraction, not detected by culture methods.
• PMA-qPCR is a suitable tool to identify and quantify viable CPA-2 cells.

Dilution plating is the quantification method commonly used to estimate the population level of postharvest biocontrol agents, but this method does not permit a distinction among introduced and indigenous strains. Recently, molecular techniques based on DNA amplification such as quantitative real-time PCR (qPCR) have been successfully applied for their high strain-specific detection level. However, the ability of qPCR to distinguish viable and nonviable cells is limited. A promising strategy to avoid this issue relies on the use of nucleic acid intercalating dyes, such as propidium monoazide (PMA), as a sample pretreatment prior to the qPCR. The objective of this study was to optimize a protocol based on PMA pre-treatment samples combined with qPCR to distinguish and quantify viable cells of the biocontrol agent P. agglomerans CPA-2 applied as a postharvest treatment on orange. The efficiency of PMA-qPCR method under the established conditions (30 μM PMA for 20 min of incubation followed by 30 min of LED light exposure) was evaluated on an orange matrix. Results showed no difference in CFU or cells counts of viable cells between PMA-qPCR and dilution plating. Samples of orange matrix inoculated with a mixture of viable/dead cells showed 5.59 log10 CFU/ml by dilution plating, 8.25 log10 cells/ml by qPCR, and 5.93 log10 cells/ml by PMA-qPCR. Furthermore, samples inoculated with heat-killed cells were not detected by dilution plating and PMA-qPCR, while by qPCR was of 8.16 log10 cells/ml. The difference in quantification cycles (Cq) among qPCR and PMA-qPCR was approximately 16 cycles, which means a reduction of 65,536 fold of the dead cells detected. In conclusion, PMA-qPCR method is a suitable tool for quantify viable CPA-2 cells, which could be useful to estimate the ability of this antagonist to colonize the orange surface.