Identification and characterization of elevated microbial counts in bulk tank raw milk

The bacterial composition of bulk tank milk from 13 farms was examined over a 2-wk period to characterize sudden elevations in the total bacterial count referred to as spikes. Bulk tank milk samples collected at each pick-up were analyzed for standard plate count, Petrifilm aerobic count, somatic cell count, Gram-negative organisms, and streptococci. Twenty standard plate count spikes were observed: 12 associated with streptococci, 4 associated with Gram-negative organisms, 2 associated with streptococci and Gram-negative organisms, and 2 that were not definitively characterized. Spikes ranged from 14,000 to 600,000 cfu/ml. Streptococcus uberis was isolated as the predominant organism from 11 spikes, and Escherichia coli was isolated from 4 spikes. Statistical analysis of total bacterial counts indicated a high correlation (r = 0.94) between standard plate counts and Petrifilm aerobic count. Regression analysis of standard plate counts and Petrifilm aerobic counts yielded the equation log(10) (standard plate count) = 0.73 + 0.85log(10) (Petrifilm aerobic count), indicating that the correlation, although strong, is not one to one. In a related pilot study, triplicate bulk tank milk samples were collected and analyzed for total bacterial count and presumptive streptococcus, Gram-negative, and staphylococcus counts. Two-way ANOVA of these triplicate data indicated a lack of significant variation among the triplicate samples, suggesting that one sample can reliably gauge the microbial status of the entire bulk tank.