BROWN PIGMENTATION OF XANTHOMONAS-CAMPESTRIS PV PHASEOLI ASSOCIATED WITH HOMOGENTISIC ACID

The diffusible brown pigment produced by some strains of Xanthomonas campestris pv. phaseoli was found to be due to the secretion and subsequent oxidation of homogentisic acid (2,5-dihydroxyphenylacetic acid) rather than tyrosinase activity as previously reported. Homogentisic acid is an intermediate in tyrosine catabolism for a number of bacteria. Brown-pigmented strains appeared to be disrupted in tyrosine catabolism and could not use tyrosine as a nutrient. Nonpigmented strains, however, could utilize tyrosine as a nutrient, and although they secreted homogentisic acid, the levels were approximately 1/100th that of brown-pigmented strains. Production of brown pigment was stimulated by tyrosine and repressed by glucose. Growth in glucose resulted in a drop in the pH of the media, and the greatest pigment formation was associated with a shift to alkaline conditions in the culture media. By buffering the media near pH 7.0, brown pigment formation was repressed, even though unoxidized homogentisic acid accumulated in the culture media. The disruption of tyrosine catabolism corresponds to other reports describing differences between brown-pigmented and nonpigmented strains, and provides further support for the retention of a special taxonomic status for the brown-pigmented strains.