Effect of O-side-chain-lipopolysaccharide chemistry on metal binding

Pseudomonas aeruginosa PAO1 produces two chemically distinct types of lipop olysaccharides (LPSs), termed A-band LPS and B-band LPS, The A-band O-side chain is electroneutral at physiological pH, while the B-band O-side chain contains numerous negatively charged sites due to the presence of uronic ac id residues in the repeat unit structure. Strain PAO1 (A(+) B+) and three i sogenic LPS mutants (A(+) R-, A(-) B+, and A(-) B-) were studied to determi ne the contribution of the O-side-chain portion of LPS to metal binding by the surfaces of gram-negative cells. Transmission electron microscopy with energy-dispersive X-ray spectroscopy was used to locate and analyze sites o f metal deposition, while atomic absorption spectrophotometry and inductive ly coupled plasma-mass spectrometry were used to perform bulk quantitation of bound metal. The results indicated that cells of all of the strains caus ed the precipitation of gold as intracellular, elemental crystals with a d- spacing of 2.43 Angstrom. This type of precipitation has not been reported previously for gram-negative cells and suggests that in the organisms studi ed gold binding is not a surface-mediated event. All four strains bound sim ilar amounts of copper (0.213 to 0.222 mu mol/mg [dry weight] of cells) at the cell surface, suggesting that the major surface metal-binding sites res ide in portions of the LPS which are common to all strains (perhaps the pho sphoryl groups in the core-lipid A region). However, significant difference s were observed in the abilities of strains dps89 (A(-) B+) and AK1401 (A() B-) to bind iron and lanthanum, respectively. Strain dps89 caused the pre cipitation of iron (1.623 mu moI/mg [dry weight] of cells) as an amorphous mineral phase (possibly iron hydroxide) on the cell surface, while strain A K1401 nucleated precipitation of lanthanum (0.229 mu mol.mg [dry weight] of cells) as apiculate, surface-associated crystals. Neither iron nor lanthan um precipitates were observed on the cells of other strains, which suggests that the combination of A-band LPS and B-band LPS produced by a cell may r esult in a cell surface which promotes the formation of metal-rich precipit ates. We therefore propose that the negatively charged sites located in the O-side chains are not directly responsible for the binding of metallic ion s; however, the B-band LPS molecule as a whole may contribute to overall ce ll surface properties which favor the precipitation of distinct metal-rich mineral phases.