Cultures and culture filtrates of Bacillus cereus UW85 suppress damping-off of alfalfa caused by Phytophthora medicaginis. We studied the role in disease suppression of two
antibiotics from culture filtrates of UW85 that reversibly inhibited growth of P. medicaginis. We purified the two
antibiotics by
cation-exchange chromatography and high-voltage paper electrophoresis and showed that one of them, designated
zwittermicin A, was an aminopolyol of 396 Da that was cationic at pH 7.0; the second, designated
antibiotic B, appeared to be an
aminoglycoside containing a
disaccharide. Both
antibiotics prevented disease of alfalfa seedlings caused by P. medicaginis. Purified
zwittermicin A reversibly reduced elongation of germ tubes derived from
cysts of P. medicaginis, and
antibiotic B caused swelling of the germ tubes. Mutants generated with Tn917 or
mitomycin C treatment were screened either for
antibiotic accumulation in an
agar plate diffusion assay or for the ability to suppress damping-off disease of alfalfa. Of 2,682 mutants screened for
antibiotic accumulation, 5 mutants were substantially reduced in
antibiotic accumulation and disease-suppressive activity. Of the 1,700 mutants screened for disease-suppressive activity, 3 mutants had reduced activity and they accumulated less of both
antibiotics than did the parent strain. The amount of
antibiotic accumulated by the mutants was significantly correlated with the level of disease suppression. Addition of either
zwittermicin A or
antibiotic B to alfalfa plants inoculated with a culture of a nonsuppressive mutant resulted in disease suppression. These results demonstrate that B. cereus UW85 produces two fungistatic
antibiotics that contribute to suppression of damping-off disease of alfalfa.