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Physiology and Biochemistry

Protection of Pythium Species Against Antibacterial Antibiotics by Cholesterol. Carroll D. Rawn, Department of Biology, Seton Hall University, South Orange, NJ 07079; Michael Schwarz, Department of Biology, Seton Hall University, South Orange, NJ 07079. Phytopathology 77:319-323. Accepted for publication 27 May 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-319.

Cholesterol (10 ?g/ml) decreased the growth inhibitory action of tetracycline, erythromycin, and chloramphenicol against Pythium ultimum and P. debaryanum on a defined glucose-amino acid agar medium. For example, at 10 μg/ml of tetracycline inhibited radial growth almost completely in the absence of cholesterol but only 30–35% in the presence of cholesterol. Similarly, 50 μg/ml of erythromycin and 100 μg/ml of chloramphenicol reduced growth 70–75% in the absence of the sterol but only 20–35% in the presence of cholesterol. Cholesterol also significantly reduced the growth inhibitory effects of the antibiotics in stationary liquid culture, with growth measured as dry weight increase. About four to five times as much antibiotic was needed to give the same degree of growth inhibition in the presence of cholesterol. When tested against tetracycline, which impaired growth of both species even at 1 μg/ml in the absence of cholesterol, the effect of cholesterol in reversing inhibition was detectable at 0.03 ?g/ml (about 10^–7 M) and above but not at 0.01 μg/ml. The very small stimulation of growth by cholesterol in mycelia not treated with antibiotics was too small to account for the protective effect of cholesterol.