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 fac tor of a pectolytic strain of Pseudomonas fluorescens that causes broccoli head rot. The critical micellar concentration (CMC) of viscosin was 4 mu g/ 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 s olutions of increasing concentration, the tissues became wetted at 10 mu g/ mL and electrolytes were induced to leak at a concentration between 10 and 25 mu g/mL. Erythrocytes were lysed at concentrations of 10 mu g/mL and abo ve. 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 wou nded florets only, but the decay failed to spread to adjacent nonwounded fl orets as had occurred with a wild strain. When the mutant strain (1 x 10(7) cfu/ml) was incubated with viscosin (25 mu g/mL), it was able to induce el ectrolyte leakage and decay of broccoli florets. Leakage and decay failed t o occur when florets were immersed in an extract of pectolytic enzymes alon e, but rapid leakage and decay occurred when viscosin was added (25 mu g/mL ). The mutant strain was unable to colonize the surface of intact broccoli florets, as shown by scanning electron microscope studies, bur in the prese nce of viscosin (20 mu g/mL), it was able to colonize surfaces and penetrat e stomata. Epicuticular wax was substantially altered on florets immersed i n a viscosin solution of 20 mu g/mL and appeared to dissolve with 200 mu g/ mL viscosin. Taken together, these results show that viscosin, as a powerfu l surfactant, is an important pathogenicity factor of this pectolytic strai n of P. fluorescens that causes decay of the difficult-to-wet waxy surfaces of broccoli heads.