bacterial content contains a great proportion of aerobic bacteria and yeast, or those bacteria which utilize and can survive in an environment containing oxygen. Subgingival plaque contains a higher proportion of anaerobic bacteria, or those bacteria which cannot exist in an environment containing oxygen. Several anaerobic plaque bacteria, such as Porphyromonas gingivalis, secrete antigenic proteins that trigger a strong inflammatory response in the periodontium, the specialized tissues that surround and support the teeth. Prolonged inflammation of the periodontium leads to bone loss and weakening of the gingival fibers that attach the teeth to the gums, two major hallmarks of periodontitis. Supragingival calculus formation is nearly ubiquitous in humans, but to differing degrees. Almost all individuals with periodontitis exhibit considerable subgingival calculus deposits. Dental plaque bacteria have been linked to cardiovascular disease and mothers giving birth to pre-term low weight infants, but there is no conclusive evidence yet that periodontitis is a significant risk factor for either of these two conditions. Prevention Toothpaste with pyrophosphates or zinc citrate has been shown to produce a statistically significant reduction in plaque accumulation, but the effect of zinc citrate is so modest that its clinical importance is questionable. Some calculus may form even without plaque deposits, by direct mineralisation of the pellicle. Calculus in other animals Calculus formation in other animals is less well studied than in humans, but it is known to form in a wide range of species. Domestic pets, such as dogs and cats, frequently accumulate large calculus deposits. Animals with highly abrasive diets, such as ruminants and equids, rarely form thick deposits and instead tend to form thin calculus deposits that often have a metallic or opalescent sheen. In animals, calculus should not be confused with crown cementum, a layer of calcified dental tissue that encases the tooth root underneath the gingival margin and is gradually lost through periodontal disease. Archaeological significance Dental calculus has been shown to contain well preserved microparticles, DNA and protein in archaeological samples. The information these molecules contain can reveal information about the oral microbiome of the host and the presence of pathogens. It is also possible to identify dietary sources as well as study dietary shifts and occasionally evidence of craft activities. Sub-gingival calculus formation and chemical dissolution Sub-gingival calculus is composed almost entirely of two components: fossilized anaerobic bacteria whose biological composition has been replaced by calcium phosphate salts, and calcium phosphate salts