RECENT PROGRESS IN UNDERSTANDING THE MOLECULAR-GENETICS AND BIOCHEMISTRY OF CALCIUM-PHOSPHATE SOLUBILIZATION BY GRAM-NEGATIVE BACTERIA

For most of this century agricultural microbiologists and microbial ecologists have been interested in the ability of some bacteria to efficiently dissolve ''insoluble'' calcium phosphates. This phenotype has been termed mineral phosphate solubilizing (Mps). This minireview discusses recent work suggesting that the direct oxidation pathway for glucose dissimilation forms the metabolic basis for the strongly Mps+ phenotype observed in many species of gram negative bacteria. In the past, the physiological significance of the direct oxidation pathway for a given species was often considered obscure, giving rise to the term ''dissimilatory bypass'' to describe the apparent inefficient utilization of reducing power. The Mps function may provide a reason for the expression of this pathway in many bacteria whose bioenergetic needs are satisfied by the Embden-Meyerhof, Entner-Doudoroff or pentose phosphate pathways. The direct oxidation of glucose results in the production of gluconic acid and (often) 2-ketogluconic acid in the periplasmic space which, in turn, results in acidification of the region adjacent to the cell or colony. Calcium phosphates in the medium (soil) are dissolved by these acidic protons. It is proposed that extracellular acidification via the direct oxidation pathway may play an ecological role in some environments including soils where calcium phosphates provide a significant pool of unavailable mineral P. Specific strategies for testing this hypothesis including the use of DNA and antibody probes are discussed.