Gram-Negative Bacteria in Raw Milk

Abstract

A study was conducted to determine the relationship between the numbers of gram-negative bacteria in raw bulk tank milk and the results from other microbiological tests used to evaluate the sanitation of milk production. Samples taken from quarters of individual cows were either free of gram-negative organisms or counts were very low. However, when farm bulk tank milk was examined, this group of bacteria was frequently noticed in large numbers in spite of relatively low standard plate counts. The enumeration of gram-negative organisms in raw milk was found to provide more valuable information about dairy farm sanitation than either coliform tests or standard plate counts, but should be complemented with a thermoduric count for more accuracy. Results indicate that gram-negative bacteria counts of < 1,000/ml are easily obtainable and, depending on local factors, counts of 500/ml or less should be aimed for in raw market milk.