Mechanisms of Cation Exchange by Pseudomonas aeruginosa PAO1 and PAO1 wbpL , a Strain with a Truncated Lipopolysaccharide

Abstract

The ability of bacterial cells to sequester cations is well recognized, despite the fact that the specific binding sites and mechanistic details of the process are not well understood. To address these questions, the cationexchange behavior of Pseudomonas aeruginosa PAO1 cells with a truncated lipopolysaccharide (LPS) (PAO1 wbpL) and cells further modified by growth in a magnesium-deficient medium (PAO1 wbpL ؊ Mg 2؉ ) were compared with that of wild-type P. aeruginosa PAO1 cells. P. aeruginosa PAO1 cells had a negative surface charge (zeta potential) between pH 11 and 2.2, due to carboxylate groups present in the B-band LPS. The net charge on PAO1 wbpL cells was increasingly positive below pH 3.5, due to the influence of NH 3 ؉ groups in the core LPS. The zeta potentials of these cells were also measured in Na ؉ , Ca 2؉ , and La 3؉ electrolytes. Cells in the La 3؉ electrolyte had a positive zeta potential at all pH values tested. Growing P. aeruginosa PAO1 wbpL in magnesium-deficient medium (PAO1 wbpL ؊ Mg 2؉ ) resulted in an increase in its zeta potential in the pH range from 3.0 to 6.5. In cation-exchange experiments carried out at neutral pH with either P. aeruginosa PAO1 or PAO1 wbpL, the concentration of bound Ca 2؉ was found to decrease as the pH was reduced from 7.0 to 3.5. At pH 3.5, the bound Mg 2؉ concentration decreased sharply, revealing the activity of surface sites for cation exchange and their pH dependence. Infrared spectroscopy of attached biofilms suggested that carboxylate and phosphomonoester functional groups within the core LPS are involved in cation exchange.The ability of microbial cells to sequester metal ions from aqueous environments is important for microbial growth and biogeochemical processes, such as mineral formation/dissolution, metal transport, and the bioremediation of metal-contaminated sites (1). The external surfaces of gram-negative bacteria are comprised of phospholipids, lipoproteins, lipopolysaccharides (LPS), and proteins. These polymers contain carboxylic acids and phosphate esters, which are primarily responsible for the cells having a net negative charge (at neutral pH), and associated cations, which are retained even after thorough washing with water due to the electroneutrality condition (6, 12, 31). These cations may undergo cation exchange with the introduction of electrolyte solutions containing different cations. While many studies have focused on the amount of metal bound/adsorbed by cells and the development of bulk partitioning models (5), questions such as the mechanism of adsorption (20), the specific binding sites (9), and the impacts of different environmental conditions on adsorption (31) have received less attention. Such information is crucial for the development of models to predict the behavior of metals under a range of conditions and to determine the potential uses of these organisms for environmental remediation (11,15).Pseudomonas aeruginosa is a ubiquitous, well-characterized microorganism used in many metal-binding investigations (10). Suggest...

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