Growth-enhancing factor for human tubercle bacilli using minute inocula.

It has been difficult to cultivate human tubercle bacilli in agar medium unless bovine serum, or a fraction therefrom, was added aseptically, since the factor present in the serum is thermolabile (Dubos and Middlebrook, 1947). On the other hand, egg yolk or whole egg media may be used, but the difficulties in preparing the media render the methods uncertain and cumbersome. Hence, a thermostable agar medium which can be prepared by autoclave sterilization has been needed. For three years the author studied the metabolism of "tween-80" by the tubercle bacilli (Minami et al., 1954; Minami and Yamane, 1955) and devised a medium in which tween-80 was the sole source of carbon (Yamane et al., 1954), but this medium did not permit growth from small inocula. The latter was accomplished by adding whole egg yolk to the tween-agar medium (Yamane et al., 1955). This growth factor could be extracted from the yolk by fat solvents and has been obtained in a crystalline form. It is heat stable, so that it can be added to tween-agar medium which can be sterilized by autoclaving, and its addition enables the bacilli to grow from small inocula. The present paper deals with the nature of this crystalline substance and the synthetic agar medium containing it.