The ability of Salmonella Enteritidis to survive in the presence of phage, SJ2, during manufacture, ripening, and storage of Cheddar cheese produced from raw and pasteurized milk was investigated. Raw milk and pasteurized milk were inoculated to contain 10(4) CFU/ml of a luminescent strain of Salmonella Enteritidis (lux) and 10(8) PFU/ml SJ2 phage. The milks were processed into Cheddar cheese following standard procedures. Cheese samples were examined for Salmonella Enteritidis (lux), lactic acid bacteria, molds and yeasts, coliforms, and total counts, while moisture, fat, salt, and pH values were also measured. Salmonella Enteritidis (lux) was enumerated in duplicate samples by surface plating on MacConkey novobiocin agar. Bioluminescent colonies of Salmonella Enteritidis were identified in the NightOwl molecular imager. Samples were taken over a period of 99 days. Counts of Salmonella Enteritidis (lux) decreased by 1 to 2 log cycles in raw and pasteurized milk cheeses made from milk containing phage. In cheeses made from milks to which phage was not added, there was an increase in Salmonella counts of about 1 log cycle. Lower counts of Salmonella Enteritidis (lux) were observed after 24 h in pasteurized milk cheese containing phage compared to Salmonella counts in raw milk cheese with phage. Salmonella Enteritidis (lux) survived in raw milk and pasteurized milk cheese without phage, reaching a final concentration of 10(3) CFU/g after 99 days of storage at 8 degrees C. Salmonella did not survive in pasteurized milk cheese after 89 days in the presence of phage. However, Salmonella counts of approximately 50 CFU/g were observed in raw milk cheese containing phage even after 99 days of storage. In conclusion, this study demonstrates that the addition of phage may be a useful adjunct to reduce the ability of Salmonella to survive in Cheddar cheese made from both raw and pasteurized milk.

Pilot-scale studies were carried out to determine the effect of different heat treatments on catalase activity during the manufacture and maturation of Cheddar cheese. Three trials were conducted to monitor catalase activity using disk flotation and polarographic methods. Cheese was manufactured from raw milk and from milk that had been treated at 60, 65 and 72 degrees C for 16 s using a high temperature, short time heat exchanger. Catalase activity was also determined in samples of commercial milk and in samples of mild, medium, sharp, and extra sharp Cheddar cheeses obtained from different manufacturers in order to verify that the enzyme could be used as an indicator of the type of heat treatment applied to cheese milk. Catalase activity was present in cheese made from raw milk but was only present at low concentrations in cheese manufactured from thermized milk. However, high catalase activity was observed in commercial samples of sharp and extra sharp Cheddar cheese that was apparently due to the growth of catalase-producing yeasts in the cheese during maturation.

Salmonella enterica subsp. enterica serotype Enteritidis is not readily subtyped beyond the level of phage type (PT). A recently developed method for ribotyping of this organism, which uses a mixture of PstI and SphI (PS) for restriction of DNA (PS ribotyping), has proved useful for further subtyping of a number of PTs of this organism, including PT 4. However, it has not been extensively tested with PT 8. In the present study the PS ribotyping method was used to investigate outbreaks of both S. enterica serotype Enteritidis PT 4 and PT 8 and provided subtyping data that were consistent with information obtained from epidemiologic investigations. The method proved to be more discriminatory than phage typing and pulsed-field gel electrophoresis (PFGE) combined and was useful for investigating a pseudo-outbreak involving isolates that had identical PTs and PFGE types but that could not be linked epidemiologically. Several PS ribotypes were found within the cluster of isolates indistinguishable by other subtyping methods, confirming the epidemiologic findings. Although the PS ribotyping method proved to have a superior discriminatory ability in resolving clusters, it did not have high enough throughput for use in outbreak investigations. This method has therefore been adapted for use in automated ribotyping with a RiboPrinter, and the results were compared with those obtained by manual ribotyping. Both methods produce equivalent results and are useful for obtaining epidemiologically relevant subtyping data for S. enterica serotype Enteritidis, including PT 8 strains not extensively tested previously.

No abstract