Incidence of Thermoduric Bacteria and Spores on Selected Midwest Dairy Farms
Thermodurics can be present in milk even after pasteurization. The objectives of this study were to identify the origin and common species of heat-resistant bacteria occurring during summer and winter on Midwest dairy farms. Bulk tank milk samples were taken from ten dairy farms along the South Dakota section of Interstate 29 with herd sizes ranging from 650 to 3500 lactating dairy cows. Milk samples were profiled for the prevalence of thermoduric bacteria (TDB) and spore counts (SC). In addition, corn silage samples and swabs of the milking clusters were taken at nine of the ten dairies to profile the potential sources of TDB and SC. The samples were taken three times during winter (January–March) and summer (June-August), to track seasonal changes in the bacterial flora.
During winter the average TDB counts in bulk tank milk were 2.61 log compared to 2.76 log TDB counts in summer. The SC was 1.03 log in winter which is half the 2.06 log SC present in summer season (P<0.0001). Corn silage sampled in winter contained 7.57 log TDB compared to an increased 10.77 log TDB during summer sampling. Concentrations of SC in corn silage reached an average of 6.3 log in winter compared to 11.81 log for summer (P<0.001). The seasonal effect was evident with an increase in summer counts across the board for TDB and SC both, in the feed and bulk tank. Bacillus licheniformis was the predominant species identified in 62.4% of winter (85 total) and 49.4% of summer (83 total) samples. Bacillus subtilis made up 9.4% of the remaining winter isolates followed by Bacillus sonorensis at 8.2%, conversely, B. sonorensis made up 12% of summer isolates followed by Bacillus pumilus at 10.8%. Bacillus licheniformis is a ubiquitous microbe and was isolated from both TDB and sporeformer categories in all three sample types. There were larger increases in SC than TDB indicating summer conditions potentially increase the ability of sporeforming bacteria to proliferate over TDB. In conclusion, samples from bulk tank milk, milking cluster swabs, and corn silage at each of the 10 sites indicated B. licheniformis was the major contaminant regardless of season. In this experiment corn silage was determined as the major source of both TDB and SC over the milking clusters and relative to the levels in bulk tank milk showing significant higher concentrations in summer than winter.
Keywords: spores, thermoduric bacteria, corn silage, Bacillus