Cation exchange in cell walls of gram-positive bacteria

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Abstract
The relative affinities of various cations for anionic sites in isolated, bacterial cell walls were assessed by means of a technique involving displacement of one cation by another. The affinity series determined was H+ .mchgt. La3+ .mchgt. Cd2+ > Sr2+ > Ca2+ > Mg2+ > K+ > Na+ > Li+. High affinity was correlated with low mobility of the bound ions in an electric field. The net cation-exchange capacities of walls isolated from a variety of bacteria were estimated by preparing the Mg forms of the walls, washing them well with deionized water to remove supernumerary ions and then completely displacing the Mg with Na+ or H+. Total amounts of Mg displaced varied from 73 .mu.mol/g dry weight, for walls of the teichoic acid-deficient 52A5 strain of Staphylococcus aureus to about 520 .mu.mol/g for Bacillus megaterium KM walls. The amount of displacable Mg was inversely related to the physical compactness of the walls, except for walls of Streptococcus mutans GS-5. Mg or Ca ions can each neutralize, or pair with, 2 anionic groups in walls suspended in ion-deficient media. Previous work indicated that these ions may pair with only 1 anionic group at high ionic strength. There is apparently a great deal of flexibility in the arrangement of charged groups in the wall. For cells growing in commonly used laboratory media, which generaly contain large excesses of monovalent versus divalent cations, there is generally a mix of small, cationic counterions in the wall and monovalent cations may predominate even though the wall has higher affinity for divalent ions.

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