Production of low molecular weight carboxylic acids by Xanthomonas campestris pv. campestris in relation to the amino acid composition of the medium and their possible involvement in pathogenesis

A major component of cultures of Xanthomonas campestris pv. campestris grown on Watanabe broth was crystallized as colourless needles and identified as trans-3-methylthioacrylic acid. A second major component of the same culture filtrates was purified by high performance liquid chromatography and identified as 3-methylthiopropionic acid. Radiolabelling experiments with [¹⁴C] and [³⁵S] methionine gave approximately equally high levels of incorporation of radioactivity into the two above acids. When the amount of methionine suppleid to cultures was increased or decreased above or below that of Watanabe broth, a corresponding change in levels of production of these acids resulted. Neither 3-methylthiopropionic acid nor trans-3-methylthioacrylic acid was detected in cultures when methionine was omitted from the medium. When, for example, the amino acids leucine and phenylalanine were substituted for methionine, other carboxylic acids including isovaleric and phynylacetic acids were detected in cultures of the bacteria. The various carboxylic acid biotransformation products of X. campestris pv. campestris were bioassayed against protoplasts and seedlings of cabbage. All compounds expressed only weak phytotoxicity when prepared in unbuffered media and phytotoxicity was effectively eliminated by supplying the compounds under test in buffered solution at pH 6·5. Benzoic acid and 3-methylthiopropionic acid, which are structurally dissimilar but which possess similar pK_as, expressed similar degrees of activity. Contrarily, both within and between the various carboxylic acids tested there was good correlation between level of activity expressed and pH of the treatment solution.