Abstract
Campylobacter jejuni and Campylobacter coli are the most common bacteria isolated in animals and humans suffering from diarrhoea. These organisms can also be found in animals which are clinically healthy. The most common sources of Campylobacter infection include undercooked meat, especially chicken, contaminated water, unpasteurized milk and raw milk. Initially, the effects of storage temperature on the survival of C. jejuni were examined in pasteurized and UHT skimmed milk. It was found that, C. jejuni NCTC 11168 survived for more than 5 days at 4 C in both UHT and pasteurized milk but that at 25 C it survived for 3 days in UHT milk and was not detectable after 6 h in pasteurized milk suggesting the presence of an antimicrobial system in the latter milk. C. jejuni was found to die at the same rate in pasteurized milk that had been filter sterilized to remove the residual microflora. This result indicates that the antimicrobial effect was not due to the natural flora but to a natural component in the milk.The inhibitory action of pasteurized milk was prevented by boiling and by addition of 1 mM of sodium meta-bisulphite (a specific inhibitor of lactoperoxidase) suggesting that lactoperoxidase in the pasteurized milk was responsible. This was confirmed by restoration of the inhibitory effect in UHT milk by addition of components of the lactoperoxidase system. When tested in pasteurized milk, and with the lactoperoxidase system, other foodborne pathogens such as Salmonella, Listeria monocytogenes and E. coli 0157 survived without significant reductions in viability indicating the unique sensitivity of Campylobacter to the lactoperoxidase system. This finding was exploited and this system was found to be effective at eliminating Campylobacters from artificial surfaces but not from chicken meat surfaces. A gene encoding the catalase from C. jejuni was cloned, an antibiotic marker inserted and allelic exchange used to generate a catalase-deficient mutant (katA). This was assessed for its sensitivity to the LPO system. The mutant C. jejuni was shown to be less sensitive to killing by LPO system compared to the parental strains. Other mutants deficient in Cgb (nitrosative stress resistance), and SodB (superoxide dismutase) were also compared to the wild type strains and shown to be less sensitive to LPO system. This unexpected result is unexplained but could be investigated more in future work. This study also addressed the effect of xanthine oxidase (XO) as this is another antimicrobial enzyme that exists in milk. The addition of this enzyme had very little effect on the survival of C. jejuni in UHT milk and the addition of an XO inhibitor (allopurinol) did not give rise to any enhancement in the survival of the organism suggesting that XO plays no role in influencing Campylobacter survival. The similar properties of Helicobacter and Campylobacter made it worthwhile to compare the effect of LPO system in reconstituted UHT milk on the survival of H. pylori in comparison to C. jejuni. H. pylori was, however, found not to be sensitive to the LPO system and there were less than a 1 log reduction in bacterial viable counts after 9 h. Camel’s milk is the most valuable of food resources for pastoral people in arid and semi-arid areas such as United Arab Emirates (UAE), Saudi Arabia, Mauritania, Kazakhstan and some other countries. Given the country of origin of the author of this study was UAE, this gave rise to unique opportunities to assess this milk for the presence of Campylobacter and for its lactoperoxidase activity. Of the 171 samples examined none tested positive for Campylobacter. Finally, the LPO system present in this type of milk did not have a pronounced effect on the survival of C. jejuni. In conclusion, the results of this study suggest that Campylobacters are very sensitive to the LPO system, and that the pasteurization process does not fully destroy the presence of LP in milk. This inherent sensitivity means that pasteurized milk and the LPO system could be used to eliminate the presence of C. jejuni in various environments, particularly artificial surfaces.