ZWITTERMICIN A-PRODUCING STRAINS OF BACILLUS-CEREUS FROM DIVERSE SOILS

Bacillus cereus UW85 produces a novel aminopolyol antibiotic, zwitterm icin A, that contributes to the ability of UW85 to suppress damping-of f of alfalfa caused by Phytophthora medicaginis. UW85 produces a secon d antibiotic, provisionally designated antibiotic B, which also contri butes to suppression of damping-off but has not been structurally defi ned yet and is less potent than zwittermicin A. The purpose of this st udy was to isolate genetically diverse strains of B. cereus that produ ce zwittermicin A and suppress disease. We found that most isolates of B. cereus that were sensitive to phage P7 or inhibited the growth of Erwinia herbicola produced zwittermicin A; therefore, phage typing and E. herbicola inhibition provided indirect, but rapid screening tests for identification of zwittermicin A-producing isolates. We used these tests to screen a collection of 4,307 B. cereus and Bacillus thuringi ensis isolates obtained from bacterial stock collections and from dive rse soils collected in Honduras, Panama, Australia, The Netherlands, a nd the United States. A subset of the isolates screened by the P7 sens itivity and E. herbicola inhibition tests mere assayed directly for pr oduction of zwittermicin A, leading to the identification of 57 isolat es that produced zwittermicin A; 41 of these isolates also produced an tibiotic B. Eight isolates produced antibiotic B but not zwittermicin A. The assay for phage P7 sensitivity was particularly useful because of its simplicity and rapidity and because 22 of the 23 P7-sensitive i solates tested produced zwittermicin A. However, not all zwittermicin A-producing isolates were sensitive to P7, and the more labor-intensiv e E. herbicola inhibition assay identified a larger proportion of the zwittermicin A producers. Preliminary phenotypic characterization of t he zwittermicin A-producing isolates obtained from soil and plant root s revealed a minimum of 31 distinct strains. We tested UW85 and 98 iso lates from this world-nide collection for the ability to suppress damp ing-off disease of alfalfa caused by P. medicaginis. The isolates that produced zwittermicin A and/or antibiotic B suppressed the disease mo re effectively than the isolates that produced neither antibiotic. Our results showed that B. cereus strains that produce zwittermicin A and antibiotic B are found in diverse soils and that these strains tend t o suppress damping off disease more effectively than B. cereus isolate s that do not produce these antibiotics. The rapid microbiological ass ays that we describe provide a way to exploit the genetic diversity of antibiotic-producing B. cereus strains for biological control.