Electron microscopic observations of polysaccharide components in polymyxin B treated outer membranes from Serratia marcescens.

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Abstract
Treatment of outer membranes [OM] of S. marcescens with polymyxin B results in the formation of blebs. This effect may be due to the action of the antibiotic on the lipopolysaccharides [LPS] proteins, phospholipids or a combination thereof. This effect may be dissociative, degradative or due to an inhibition of the assembly of OM components. LPS and polymyxin B form complexes, but direct visualization of the in situ action of polymyxin B was not accomplished. Isolated OM normally exhibit a periodicity of the polysaccharide molecules when stained by the thiosemicarbazide-silver technique. Polymyxin B-treated OM display a change in their basic morphology. This effect is very drastic in the sensitive strain as demonstrated by the large gaps in the deposition of the granules in the modified OM structure. The polysaccharide molecules (probably the LPS) alone or in association with protein or phospholipids are apparently the primary targets of the antibiotic.