Campylobacter spp. Strain Choice and Food Matrix Strongly Affect LOD50 Results

Introduction: Campylobacteriosis is the most commonly reported zoonosis in the EU and the occurrence of Campylobacter in broiler meat remains high. The detection in food may be hampered due to abundant growth of extended-spectrum β-lactamase (ESBL)-producing Enterobacteriaceae during enrichment, resulting in false-negative samples. Therefore, the ISO protocol (ISO-DIS 10272-1; 2015) was revised to include, next to Bolton Broth (BB), Preston Broth (PB) as a prescribed enrichment broth to inhibit competitive flora in samples with suspected high levels of ESBLs. An Inter-Laboratory Study (ILS) was performed to validate this protocol, using four food matrices and chicken caecal material.

Purpose:  The ILS validation included one different strain per food matrix; therefore, in the current study, enrichment procedures were carried out with all strains used in the ILS in each food matrix.

Methods: Enrichment procedures according to the ISO protocol were conducted using spinach, minced meat, raw milk, and chicken skin. Each matrix was inoculated with a different strain of Campylobacter jejuni (3 strains) or Campylobacter coli (2 stains). Results were expressed as LOD₅₀ (Level of Detection), which is the concentration at which the probability of detection is 50%.

Results: The LOD₅₀ for all strains tested in spinach was approximately 0.7 CFU/sample, which complies with the ILS results. Results for the other food products, however, showed a large variation in the LOD50, with statistically significant differences between food products and between strains in raw milk and minced meat.

Significance: When a laboratory is validating the ISO method, care should be taken to extrapolate the ILS results to other Campylobacter spp. strains. One of the strains used in the ILS (C. jejuni WDCM 00156) is not the best choice to use as the reference strain.