The increased resistance of mice to experimental staphylococcal mastitis following inoculation of endotoxin.

Intramammary inoculation of endotoxin in mice three days before intramammary challenge with staphylococci induced resistance to the staphylococci. Endotoxin given by the intraperitoneal route also induced resistance when inoculated three days before challenge. Resistance was not induced, however, if the mice were allowed to continue lactating during the induction period. Histological examination of mammary glands following different combinations of lactation/involution and endotoxin treatment indicated that endotoxin given by either the intramammary or the intraperitoneal route caused an accelerated involution of the mammary gland and suggested that the involuted gland was inherently more resistant. This was confirmed by demonstrating the same resistance in mice after five days of natural involution when the mammary glands had reached the degree of involution produced by endotoxin in three days.