Glucose inhibition of the characteristic melanoid pigment of Xanthomonas phaseoli var. fuscans

Abstract

The brown melanoid pigment of Xanthomonas phaseoli var. fuscans is a reliable cultural character to distinguish this pathogen, the cause of fuscous blight of beans (Phaseolus vulgaris), from X. phaseoli, the causal organism of common bean blight. The pigment appears to be exogenous because it developed mainly in nutrient agar and only slightly in water agar held side by side in the same petri plate inoculated by a single streak of X. phaseoli var. fuscans. Both pathogens produced similar tyrosinase-type diffusible enzymes in nutrient agar plates as detected by DL-dopa (3,4-dihydroxy-DL-phenylalanine) reaction. Presently unknown specific factors, besides the detectable enzymes, may be involved in pigment production.High glucose concentration in culture media caused inhibition of pigment development. One of the reasons for this inhibition seemed to be the repressive effect of glucose on the synthesis or secretion of tyrosinase-type enzymes as well as on the autooxidation of substrates, like dopa. Therefore, the amount of glucose in culture media should be carefully regulated when the characteristic brown pigment of X. phaseoli var. fuscans is used to distinguish this pathogen from X. phaseoli.