Cation content and cation-exchange capacity of intact cells and cell envelopes of viomycin-sensitive and -resistant strains of Rhizobium meliloti

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Abstract
The envelope (wall plus plasma membrane) of a viomycin-resistant mutant of Rhizobium meliloti exhibited a higher calcium concentration than that of the viomycin-sensitive parent strain. However, the cation exchange capacity of the sensitive cells was almost twice that of the resistant cells, indicating that the excess calcium in the resistant cells is not available for exchange with external cations. Since externally supplied cations can block viomycin uptake by sensitive cells it is suggested that viomycin penetration in such cells involves an initial attraction of the basic antibiotic molecules to negatively charged sites in the envelope. Viomycin resistance in the mutant strain appears to be the result of (a) a decrease in the number of negatively charged sites and (b) the association with these sites, via calcium-bridging, of previously discovered envelope phospholipids capable of complexing with the drug.Chelators decreased viomycin resistance in the mutant strain, presumably by removal or destruction of the barrier in the cell envelope involved in decreased drug penetration. The decrease in viomycin resistance caused by cold shock was accompanied by a release of envelope phospholipid without a concurrent release of Ca2+.

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