Regarding hte presence of sterol in Yeast Extract or in peptone: don't
know about peptone, probbably not. There definitely IS sterol, ergosterol
and others, in Difco Yeast extract. Refer to:  Adams, B.G., and L.W.
Parks 1967. Biochem. Biophys. Res. Commun. 28: 490-494. and  Adams, B.
G., and L.W. Parks 1968. J. Lipid Res. 9: 8-11.
Bruce G. Adams, Dept. Microbiology, Univ HI at Manoa, Honolulu, HI