Metchnikoff researched the effect of lactic acid produced by Lactobacilli on other non-desirable bacteria in the digestive tract. He thus expanded Döderlein's (No. 6279) concept of "normal flora" beyond the vagina.

Translated into English as Notes on soured milk, and other methods of administering selected lactic germs in intestinal bacterio-therapy. London: John Bale, Sons & Danielsson, 1909.

For what he called bacterio-therapy, Metchnikoff recommended eating yogurt. From p. 20: "The inexperienced reader may be surprised at our suggesting the ingestion of large quantities of microbes, the popular idea being that microbes are necessarily injurious. This view, however, is altogether erroneous; for there are many useful microbes, and among them the lactic organism occupies a foremost place."

(Thanks to Juan Weiss for the reference to the English translation and its interpretation.)