Attenuation by Divalent Cations of the Effect of the Phytoalexin Rishitin on Erwinia carotovora var. atroseptica

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Abstract
Rishitin at 300 .mu.g ml-1 rapidly decreased O2 uptake by E. carotovora var. atroseptica suspended in 0.1% (wt/vol) peptone water. Variation in the composition of the suspending medium affected sensitivity to rishitin, with Mg2+ and, to a lesser extent, Ca2+ decreasing bacterial sensitivity. Addition of 100 .mu.g rishitin ml-1 affected cell membrane permeability causing an increase in conductivity of the suspending medium. Inhibition of O2 uptake by the cationic surfactant hyamine 2389 or by sodium lauryl sulfate was also alleviated by the addition of Mg2+ suggesting that rishitin may act directly on cell membranes of bacteria, possibly in a manner similar to a cationic surfactant or a membrane-active antibiotic. The sensitivity of E. carotovora atroseptica to rishitin in potato tubers may be affected by variation in their Mg2+ or Ca2+ content. Phaseollin did not inhibit respiration of E. carotovora atroseptica.

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