Detection and quantification of thermotolerant Campylobacter spp. on eggshells by cultivation and viability Real-time...

The implication of eggs as source for campylobacteriosis is not yet completely understood, since quantitative data on the contamination level are sparse. In this study 459 pooled eggshell samples derived from different production types and collected at German packaging stations and at retail were analysed by culture and culture-independent qPCR in parallel. Thermotolerant Campylobacter spp. were isolated from 1.5% of 457 samples (detection by culture). qPCR revealed presence of Campylobacter spp. in 79.9% of the samples (total DNA), with a median of 3.7 log₁₀ (SD, ± 0.8 log₁₀) and a maximum of 6.0 log₁₀ genomic copies of Campylobacter spp. per 10 eggs. Visible contamination grade and production type were significantly associated with the number of genomic copies detected. A subset of the samples (n = 90) was both analysed by culture and by live/dead differentiating viable (v-)qPCR, showing that 6.6% of these samples (6/90) were culturally positive and 11.0% (10/90) contained viable Campylobacter spp. according to v-qPCR. Culturability correlated with egg age. Based on v-qPCR, positive eggs contained a median of 3.7 log₁₀ and a maximum of 5.4 log₁₀ viable Campylobacter per 10 eggs. Artificial contamination with a mixture of C. jejuni and C. coli strains originally isolated from eggs confirmed rapid loss of culturability most likely due to dryness and 10.3% of maximal transfer of Campylobacter spp. from the eggshells by wet contact. In conclusion, although eggs were very frequently contaminated with Campylobacter spp. (around 80% positive samples), most of the bacteria (84% of positives of the analysed subset) were dead probably due to dryness. In a few cases, a fraction of the remaining viable bacteria may be transferred from the eggshell to raw egg dishes during wet contact (opening of the shell) at maximum levels of 3.4 log₁₀ per egg. However, the infectious potential of viable but not culturable bacteria is likely lower than that of culturable Campylobacter. This should be investigated further in order to improve risk assessment.