Mechanisms of action of Polymyxin B on Chlorella and Scenedesmus.

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Abstract
Chlorella pyrenoidosa is inhibited by 5 ppm of polymyxin B, whereas C. vulgaris and Scenedesmus obliquus are completely resistant. Previous work showed that certain susceptible bacteria are protected from inhibition by galactose. It was found that the enzyme, phosphoglucose isomerase is inhibited competitively by polymyxin B, and that galactose-6-phosphate is synthesized by those algae which are resistant to the inhibitor. These facts suggest a metabolic pathway for galactose as follows: galactose-6-phosphate arises by either mutation of galactose-1-phosphate or phos-phorylation of galactose. One of 3 subsequent transformations must occur: (1) tagatose-6-phosphate could be converted to fructose-6-phos-phate by means of a phosphotagatose epimerase; (2) the diphosphate ester of tagatose could be converted to fructose-1, 6-diphosphate under the influence of a diphosphotagatoisomerase; or, (3) tagatose-1, 6-diphosphate itself could be split by means of an aldolase to yield the 2 triose esters. An additional effect of polymyxin B is a disruption of the membrane resulting in a loss of 80% of the cellular phosphorous by the non-resistant C. pyrenoidosa.