Role of the biosurfactant viscosin in broccoli head rot caused by a pectolytic strain of Pseudomonas fluorescens

The lipopeptidic biosurfactant viscosin was examined as a pathogenicity factor of a pectolytic strain of Pseudomonas fluorescens that causes broccoli head rot. The critical micellar concentration (CMC) of viscosin was 4 𝛍g/mL in a 2 mM phosphate buffer (pH 7.0), and the surface tension was reduced from 71 mN/m to 25 mN/m. When broccoli florets were immersed in viscosin solutions of increasing concentration, the tissues became wetted at 10 𝛍g/mL and electrolytes were induced to leak at a concentration between 10 and 25 𝛍g/mL. Erythrocytes were lysed at concentrations of 10 𝛍g/mL and above. Since membrane effects occurred above the CMC, it appears that viscosin does not act as a membrane toxin, but rather as a nonspecific detergent. A viscosin deficient mutant, induced by Tn5 mutagenesis, caused decay of wounded florets only, but the decay failed to spread to adjacent nonwounded florets as had occurred with a wild strain. When the mutant strain (1 x 107 cfu/mL) was incubated with viscosin (25 𝛍g/mL), it was...