Campylobacteriosis is the most frequently reported foodborne infection in the EU, with a staggering 127,840 cases reported in 2021 alone. The symptoms of this infection include abdominal pain, diarrhea, and fever, with the most common culprit being C. jejuni. This bacterium typically enters the human body through the cross-contamination of raw meat or raw milk, posing a significant public health concern.
In 2020, the EU witnessed four outbreaks of C. jejuni that were attributed to raw milk vending machines. Despite the presence of cooling equipment, the milk obtained from these machines exceeded the recommended temperature of 4°C. In fact, a disturbingly high proportion of the milk samples had temperatures as high as 18.6°C, endangering consumers.
One noteworthy attribute of C. jejuni is its capacity to enter a state called ‘viable but non-culturable’ (VBNC). In this state, the bacteria can survive adverse conditions without being detected by cultural detection methods. Although it is unable to grow in raw milk under standard storage conditions, it remains in this latent state until better conditions arise for reactivation and reinfection.
A study published in the Journal of Food Safety emphasized that the presence of C. jejuni in VBNC form within raw milk is a genuine public health concern due to the challenges involved in detecting it. To explore this further, researchers conducted an experiment using a reference strain of C. jejuni obtained from a strain collection and a field strain from the feces of a dairy cow. Raw milk spiked with these strains was monitored at different temperatures ranging from 5°C to 12°C.
The experiment involved the use of culture-dependent and culture-independent methods to quantify the presence of C. jejuni. Results indicated that relying solely on colony-forming units (CFUs) severely underestimates the presence of this bacterium in raw milk. Furthermore, a higher level of C. jejuni was observed in the field strain compared to the reference strain. The use of a real-time v-qPCR method allowed for the detection of intact and potentially infectious units (IPIUs), which contain both CFUs and VBNC cells. Notably, the data from the IPIUs revealed significantly higher quantities of C. jejuni.
Although VBNC cells of C. jejuni are thought to be non-infectious, they possess the potential to transform into CFUs and gain infective capacity under suitable conditions. The researchers discovered that extremely low partial pressure of oxygen at 5°C can facilitate the transition of VBNC cells to culturable forms. This time-dependent recovery into infectious CFUs raises concerns and highlights the necessity of extending our scrutiny beyond the existing CFU-based assessments of raw milk.
While the conditions necessary for the VBNC form to convert back to CFU form are highly specific, it remains uncertain whether these conditions can be replicated within the human gut. Nonetheless, caution is advised to prevent any potential risks associated with the consumption of raw milk.
Source: Journal of Food Safety, ‘Modeling the survival of Campylobacter jejuni in raw milk considering the viable but non-culturable cells (VBNC)’ published on June 30, 2023, DOI: https://doi.org/10.1111/jfs.13077, Authors: A. Knipper, C. Plaza-Rodríguez, M. Filter, I. F. Wulsten, K. Stingl, T. Crease.