A comparative study of two food model systems to test the survival of Campylobacter jejuni at -18 degrees C.

The survival of Campylobacter jejuni NCTC 11168 was tested at freezing conditions (-18 degrees C) over a period of 32 days in two food models that simulated either (i) the chicken skin surface (skin model) or (ii) the chicken juice in and around a broiler carcass (liquid model). In the skin model, cells were suspended in chicken juice or brain heart infusion broth (BHIB) and added to 4-cm2 skin pieces, which were subsequently stored at -18 degrees C. In the liquid model, cells were suspended in chicken juice or BHIB and stored at -18 degrees C. The decrease in the number of viable C. jejuni NCTC 11168 cells was slower when suspended in chicken juice than in BHIB. After freezing for 32 days, the reductions in the cell counts were 1.5 log CFU/ml in chicken juice and 3.5 log CFU/ml in BHIB. After the same time of freezing but when inoculated onto chicken skin, C. jejuni NCTC 11168 was reduced by 2.2 log units when inoculated in chicken juice and 3.2 log units when inoculated into BHIB. For both models, the major decrease occurred within the first 24 h of freezing. The results obtained in the liquid model with chicken juice were comparable to the reductions of Campylobacter observed for commercially processed chickens. The survival at -18 degrees C in the liquid model was also tested for three poultry isolates and three human clinical isolates of the serotypes 1.44, 2, and 4 complex. As observed for C. jejuni NCTC 11168, all the strains survived significantly better in chicken juice than in BHIB and were not notably influenced by serotype or origin. The findings indicate that the composition of the medium around the bacteria, rather than the chicken skin surface, is the major determining factor for the survival of C. jejuni at freezing conditions. The liquid model with chicken juice was therefore the best model system to study the freezing tolerance in Campylobacter strains.