ULTRASTRUCTURE AND CYTOCHEMISTRY OF IN VITRO INTERACTIONS OF THE ANTAGONISTIC BACTERIA BACILLUS CEREUS X16 AND B THURINGIENSIS 55T WITH FUSARIUM ROSEUM VAR SAMBUCINUM
M. Chérif, N. Sadfi, N. Benhamou, A. Boudabbous, A. Boubaker, M.R. Hajlaoui, Y. Tirilly
The present studies were undertaken to investigate the interaction of the bacterial antagonists Bacillus cereus X16 and B. thuringiensis 55T with Fusarium roseum var. sambucinum, the causal agent of potato dry rot. On wounded potato tubers, both bacilli effectively suppressed the development of Fusarium dry rot. Nevertheless, confrontation of the fungal pathogen with the antagonists on nutrient agar revealed that B. cereus X16, induced a strong visible inhibition zone but B. thuringiensis 55T did not. Light microscopy of the interaction regions after confrontation with B. cereus X16 on nutrient agar generally showed apparently intact fungal cells with densely stained protoplasm, indicating that the inhibition was due to stasis rather than toxicity. By contrast, in presence of B. thuringiensis, Fusarium cells appeared markedly damaged, with partial to complete cell wall disintegration and disorganization and generally complete loss of protoplasm. These observations suggest that, contrary to our expectations, antibiotic and chitinolytic activities of B. thuringiensis 55T may be highly significant in the parasitism of the pathogen. Confrontation in liquid medium revealed that both bacteria completely inhibited the germination of Fusarium macroconidia and resulted in their destruction. This suggests that, to produce their fungitoxic and hydrolytic effects, the two bacteria should make intimate contact with Fusarium cells.