Seasonal occurrence and molecular diversity of clostridia species spores along cheesemaking streams of 5 commercial dairy plants.

Five commercial dairy plants were monitored over a 17-mo period to determine the seasonal occurrence of Clostridium spores in streams from the cheesemaking process. Every 2 mo, samples of raw milk (RM), separated cream (SC), pasteurized and standardized vat milk (PSVM), PSVM + lysozyme (PSVM+L), and manufactured cheese aged for 60 to 90 d were processed for analysis. Molecular diversity of the main species identified was determined using repetitive element palindromic PCR. The mean anaerobic spore counts (µ ± SE) were 3.16±0.054, 3.00±0.054, 2.89±0.059, and 2.03±0.054 log10 most probable number/L for RM, PSVM, PSVM+L, and SC, respectively. Although spore counts did not differ between dairy plants, seasonal variation was observed; spore counts of RM, PSVM, and PSVM+L were higher during winter (June to August) and summer (December to February) months, but no seasonal variation was seen in SC counts. The most frequently isolated species was Clostridium tyrobutyricum, ranging from 50 to 58.3% of isolates from milk and cream samples. Clostridium sporogenes was the second most common species identified (16.7-21.1%); Clostridium beijerinckii and Clostridium butyricum were also found, although at lower prevalence (7.9-13.2%). Analysis of the C. tyrobutyricum and C. sporogenes population structure through repetitive element palindromic PCR indicated a high diversity, with unique isolates found in each positive sample. The occurrence of Clostridia spores in incoming streams to cheesemaking was most prominent in the winter and summer seasons, with higher prevalence of C. tyrobutyricum in the months of June and August.

RAPD-based screening for spore-forming bacterial populations in Uruguayan commercial powdered milk.

The occurrence of spore-forming bacteria in powdered milk is of concern to the dairy industry due to potential deleterious effects including those resulting from proteolytic and lipolytic activities. Twenty-two powdered milk samples representative of spring and summer production obtained from Uruguayan retail stores were analyzed for type and number of thermophilic and spore-forming bacterial species. Bacillus licheniformis isolates were found to be the most prominent milk powder contaminant followed by Anoxybacillus flavithermus representing 71.5 to 84% of the total microflora. Geobacillus stearothermophilus, however, was not found. B. licheniformis strains F and G were both found in this study but strain F was the prevalent isolate representing 98.9% of the total isolates of this species. A. flavithermus isolates corresponded to strain C in accordance with 16S rRNA gene sequence analysis, however, in contrast with other reports, the RAPD profiles showed three characteristic bands at approximately 650, 1000 and 1650 bp, but lacking a band at 1250 bp. A third group of isolates was identified corresponding to members of a Bacillus subtilis group and Bacillus megaterium. Isolates designated B. licheniformis, A. flavithermus, B. megaterium and the B. subtilis group represented 89.1 to 93.6% of those analyzed, and depended on previous heat treatment and incubation temperatures of the plates. The remaining isolates were Bacillus pumilus and unidentified spore-formers.