Silicon Nitride Bioceramics Induce Chemically Driven Lysis in Porphyromonas gingivalis

Organisms of Gram-negative phylum bacteroidetes, Porphyromonas gingivalis, underwent lysis on polished surfaces of silicon nitride (Si3N4) bioceramics. The antibacterial activity of Si3N4 was mainly the result of chemically driven principles. The lytic activity, although not osmotic in nature, was related to the peculiar pH-dependent surface chemistry of Si3N4. A buffering effect via the formation of ammonium ions (NH4(+)) (and their modifications) was experimentally observed by pH microscopy. Lysis was confirmed by conventional fluorescence spectroscopy, and the bacteria's metabolism was traced with the aid of in situ Raman microprobe spectroscopy. This latter technique revealed the formation of peroxynitrite within the bacterium itself. Degradation of the bacteria's nucleic acid, drastic reduction in phenilalanine, and reduction of lipid concentration were observed due to short-term exposure (6 days) to Si3N4. Altering the surface chemistry of Si3N4 by either chemical etching or thermal oxidation influenced peroxynitrite formation and affected bacteria metabolism in different ways. Exploiting the peculiar surface chemistry of Si3N4 bioceramics could be helpful in counteracting Porphyromonas gingivalis in an alkaline pH environment.