Modeling the survival of Campylobacter jejuni in raw milk considering the viable but non-culturable cells (VBNC)

Campylobacter spp. cannot grow in raw milk, but it is able to transform into a viable but non-culturable (VBNC) state enabling the survival in such harsh conditions. In this study, Campylobacter jejuni survival in raw milk was investigated taken into consideration colony-forming units (CFUs) and VBNC cells. CFU from two different strains of C. jejuni (DSM 4688 and BfR-CA-18043) were enumerated at three temperatures (5°C, 8°C, and 12°C). In parallel, a viability real-time PCR was conducted to quantify intact and putatively infectious units (IPIUs) (comprising CFU and VBNC bacteria). The data generated were used to model the viability of C. jejuni during raw milk storage. Here, a one-step fitting approach was performed using parameter estimates from an intermediate two-step fit as starting values to generate tertiary models. Different primary model equations (Trilinear and Weibull) were required to fit the CFU and the IPIU data. Strain-specific linear secondary models were generated to analyze the effect of storage temperature on the maximum specific inactivation rate of the CFU data. The time of the first decimal reduction parameter of the IPIU models could be modeled by a strain-independent linear secondary model. The developed tertiary models for CFU and IPIU differ significantly in their predictions, for example, for the time required for a one log₁₀ reduction. Taken into consideration that VBNC could revert to a culturable state during the raw milk storage, our results underline the importance of considering IPIU and not only CFU to avoid underestimation of the survival of C. jejuni in raw milk.