Microbiological deterioration of dental materials - A consideration of biochemical mechanisms behind bacterial metabolic activities -

Since the oral cavity is open to the external environment, a tremendous number of microorganisms adhere to its surface and form a biofilm on it. The microorganisms in the oral biofilm cohabit with the host as the indigenous microbiota and utilize nutrients, such as carbohydrates, proteins, and amino acids, which are supplied from bodily fluid (saliva, etc.) and dietary foods. However, when the balance between the host and the bacteria is disrupted by changes in the host's eating habits, prophylactic behavior, immunological activity, etc., oral diseases can occur. The excessive and frequent intake of carbohydrates might increase bacterial acid production from carbohydrates, initiating dental caries, while poor oral hygiene might cause bacteria to accumulate and promote the bacterial production of cytotoxic metabolites from proteins and amino acids, initiating periodontal diseases. When dental biomaterials are placed in the oral cavity, their surfaces also become covered by the oral biofilm and are influenced by bacterial metabolism. Our recent studies suggested that bacteria can invade polymethyl methacrylate and reduce its hardness and alter its components, and bacterial oxygen consumption can create oxygen concentration cells on titanium surfaces and cause then to corrode. Possible mechanisms that might underpin these microbiological deteriorations were discussed.