The survival of serum resistant Escherichia coli in the bovine mammary gland following experimental infection.

Serum resistant strains of Escherichia coli were injected into one or two quarters of the udders of eight healthy dairy cows. Animals receiving infection into two quarters showed variation in their ability to eliminate the bacteria. This variation extended from elimination from both glands to complete failure to remove the organisms from either gland. In most cases, the organisms were removed from one gland before the clinical signs of infection were observed, but persisted in the other gland for three to four days. Following a single infection most animals eliminated the organisms before the appearance of clinical signs, but one retained the bacteria for four days. The retention of bacteria within the gland for a period longer than the initial inflammatory response resulted in their survival within neutrophils; and in some of these glands spasmodic clinical signs of mastitis reappeared for up to at least 40 days after the initial infection. These signs were associated with the reappearance of the same serological strain of E coli. Bovine serum albumin levels in the milk were found not to constitute the most effective marker of the serum components involved in the bactericidal activity.