To provide a wholesome milk-supply: Streptococci and leukocytes.

Part 1

Early in the twentieth century the health quality of milk was of considerable importance, but little was known of how to evaluate it.

Streptococci in milk were supposed to be the cause of diseases in man; in England an outbreak of sour throat could be connected to a herd of the suspected dairy in which a cow was affected with acute mastitis. The problem was to establish suitable standards with regard to streptococci and “pus cells”, i.e. leukocytes, in milk supplies. Norman Harris, a bacteriologist in Chicago, gave an overview in 1907 of his own opinions and those of his colleagues about this problem¹.

In general it was believed that two groups of bacteria could be found in milk ducts of a cow: those that produce lactic acid fermentation and those that do nothing. After a long discussion of the literature Harris concludes that souring of milk by lactic acid is caused by Streptococcus lacticus. But then the question must be asked: ”in what light, then, are we to regard the presence of streptococci in milk as being an index of disease in the cows? It cannot be denied that cows suffer from an inflammation of the udder at one time or another during lactation, and that these lesions are largely caused by the ordinary pyogenic cocci … Of the cocci it would seem that a streptococcus is the more prominent factor, … particularly in that form of mastitis known as acute contagious mastitis.”

Thus, in Harris view, streptococci were responsible either for a harmless souring of milk or for an acute mastitis in cows. It would be very helpful if it were possible to distinguish the several “races” of streptococci, because that would give the possibility “to frame standards whereby we might be able to say that a given sample of milk was fit for consumption, whilst another was not, on the ground that each contained harmless and hurtful cocci respectively.”

Next Harris discusses various methods for making this distinction, such as possible differences in the fermentation of substrates, hemolytic properties, agglutination reactions, reactions to sera and pathogenicity to rabbits, mice and guine pigs, but all without good results.

This latter feature of pathogenicity was important in the light of the believe of Harris and colleagues the the pathogenic streptococci “have a virulence all [of] their own toward the human species … and that these cocci thus causing human infection are those giving rise to the acute contagious variety of mastitis or gelber Galt of the Germans.” Mastitis milk was a danger for humans and identification of harmful streptococci might help in recognizing dangerous milk. But unfortunately, no such test was found yet.

(to be continued)

1. N.M.Harris, ‘The relative importance of streptococci and leukocytes in milk’, The Journal of Infectious Diseases, 4 (1907) 50-62.