Development of tolerance to the phytoalexin 6-methoxymellein in microorganisms.

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Abstract
6-Methoxymellein (6MM), a phytoalexin of carrot, exerted a static effect on the growth of fungi, yeast and bacteria at low concentrations. It retarded the spore germination of Chaetomium globosum and extended the lag phase in the growth of Bullera alba, Candida albicans and Staphylococcus aureus. When B. alba and C. albicans were first treated with 6MM and then a second time their growth was not inhibited by the 2nd dose, suggesting that they become tolerant to 6MM through some induced factor. By contrast, S. aureus appeared to tolerate 6MM in a constitutive manner, since the bacterial growth was inhibited by the 2nd dose of 6MM as well as the first. In S. aureus culture, however, the duration of growth inhibition by 6MM was dependent on the inoculum size, and the inhibitory effect was negligible when the bacterial concentration in the culture was sufficiently high. Chaetomium globosum and B. alba incorporated appreciable amounts of 6MM, but no metabolite of the phytoalexin appeared in the culture. The results suggest that 6MM tolerance occurred in these microorganisms independently of its degradation or transformation.