A microbiological study of dental calculus. IV. An electron microscopic study of in vitro calcified microorganisms.

Abstract

Cultures of microorganisms isolated from human calculus have been calcified using a simple in vitro system. All strains had previously produced hydroxyapatite (HAp) under similar experimental conditions.Ultra‐thin sections of specimens embedded in Epon 812 were examined, with a Philips 200 electron microscope, unstained or double‐stained with uranyl acetate and lead citrate.Intracellular crystals of HAp were seen in all strains examined. The majority of Grampositive rods produced only intracellular calcification, usually mesosome associated and extracellular vesicles were rarely seen. In contrast Gram‐negative rods produced extracellular material—vesicles and debris from disrupted cells—which was heavily mineralized. The association of initial intracellular calcification with mesosomes was not always observed.Extracellular calcification was also a feature of some Gram‐positive and all Gram‐negative cocci with cell‐associated deposits apparently originating from the cell wall. A feature of some species, particularly the Gram‐negative cocci, was the presence of randomly arranged spherules of amorphous calcium phosphate within the cytoplasm some of which appeared to be partially converted to HAp. In the majority of microorganisms examined early mineralization was apparently crystalline.It is suggested that extracellular mineralization of Gram‐negative microorganisms may be a significant factor in early matrix calcification in natural calculus.