Behavior of Pythium torulosum zoospores during their interaction with tobacco roots and Bacillus cereus

Bacillus cereus UW85 suppresses seedling damping-off diseases caused by Oom ycetes and produces antibiotics that inhibit development of Oomycetes in cu lture. The goal of this study was to determine how UW85 and its antibiotics affected the behavior of an Oomycete, Pythium torulosum, in its interactio n with plant roots. We studied tobacco seedlings inoculated with zoospores of P. torulosum and UW85 culture, culture filtrate, washed cells, antibioti cs (zwittermicin A or kanosamine), purified from cultures of UW85, and UW03 0, a mutant of UW85 that does not suppress disease and does not produce the antibiotics. Microscopic observation revealed that all of the treatments i nhibited zoospore activity around roots and encystment on roots. Treatment with UW85 culture, culture filtrate, zwittermicin A, or kanosamine delayed cyst germination and the elongation rate of germ tubes, whereas treatment w ith UW030 or washed UW85 cells did not. In an in vitro seedling bioassay of disease suppression, the antibiotics, zwittermicin A and kanosamine, suppr essed disease singly or together, although UW85 culture suppressed disease more effectively than did the antibiotics. The results show that B. cereus cultures affect zoospore behavior in the presence of roots, and B. cereus-p roduced antibiotics, zwittermicin A and kanosamine, contribute to disease s uppression and inhibition of germ tube elongation in the presence of the pl ant root.