Technical Abstract: Campylobacter jejuni is an important foodborne gastrointestinal pathogen. Research has shown that changes in culturability, cell morphology, and viability occur when C. jejuni cells are subjected to stresses such as low nutrient availability, entry into stationary phase, or low CO2/high O2 conditions. In this study, molecular and microscopic methods, including real-time PCR, ethidium monoazide (EMA) in combination with real-time PCR (EMA-PCR), BacLight bacterial viability staining, and agar plate counting were used to quantitatively analyze viable, stressed, and dead C. jejuni strain 81-176. With the real-time PCR assay, highly sensitive and specific quantification of total cell numbers of C. jejuni in different growth phases was achieved. In addition, our results revealed that real-time PCR can be used for direct quantification of Campylobacter genome release into the PBS as an indicator of cell death and is suitable for quantitative determination of dead cells in suspensions. By EMA-PCR, we obtained a dynamic range of greater than 3 log for differentiating viable versus dead cells. The viability and morphological characteristics of the stressed cells after one week incubation at 25 degrees C in air, and under nutrient-poor conditions were investigated. Our results indicated that, over 99 percent of the stressed cells were converted from the spiral to the coccoid form and became non-culturable within 1 week. However, more than 96 percent of the coccoid cells retained their viability based on membrane integrity as suggested by both the BacLight staining and EMA-PCR analyses. Thus the cell culturability obtained by plate counting is the least sensitive method in detecting viable cells, and should be used in conjunction with other molecular methods such as EMA-PCR for detection of food-borne pathogens that are particularly sensitive to stresses.