Microbial hydroxyapatite formation as a model of proteolipid-dependent membrane-mediated calcification.

The study of calcium hydroxyapatite (HA) formation in bacteria can be used as a model for membrane-associated normal and pathologic calcification in vertebrates. While the ability to deposit HA is common to a wide variety of microorganisms, the characteristic is not universal, even within closely related species (e.g., oral streptococci). Clues as to why some microorganisms can support HA formation while others cannot have been derived from analysis of calcification of Bacterionema matruchotii. These bacteria form intracellular HA which is crystallographically indistinguishable from bone mineral. Calcification is proteolipid (Pr)-dependent and involves Ca-phospholipid-Pi (CPLX) formation. Crystal deposition follows a defined series of events including changes in Ca:Mg:Pi ratios, membrane lipids, Pr concentration, protein and phospholipid composition, and mineral phase. Comparison with non-calcifiable Actinomyces naeslundii indicates that under appropriate conditions, most Pr-containing membranes can function as nucleating sites. Enrichment of Pr and CPLX in pathologic calcifications and matrix vesicle membranes suggests that similar mechanisms may operate in vertebrate HA formation.