within calculus are primarily bacterial, but also include at least one species of archaea (Methanobrevibacter oralis) and several species of yeast (e.g., Candida albicans). The organic extracellular matrix in calculus consists primarily of proteins and lipids (fatty acids, triglycerides, glycolipids, and phospholipids), as well as extracellular DNA. Trace amounts of host, dietary, and environmental microdebris are also found within calculus, including salivary proteins, plant DNA, milk proteins, starch granules, textile fibers, and smoke particles. Calculus formation The processes of calculus formation from dental plaque are not well understood. Supragingival calculus formation is most abundant on the buccal (cheek) surfaces of the maxillary (upper jaw) molars and on the lingual (tongue) surfaces of the mandibular (lower jaw) incisors. These areas experience high salivary flow because of their proximity to the parotid and sublingual salivary glands. Subgingival calculus forms below the gumline and is typically darkened in color by the presence of black-pigmented bacteria, whose cells are coated in a layer of iron obtained from heme during gingival bleeding. Dental calculus typically forms in incremental layers that are easily visible using both electron microscopy and light microscopy. These layers form during periodic calcification events of the dental plaque, but the timing and triggers of these events are poorly understood. The formation of calculus varies widely among individuals and at different locations within the mouth. Many variables have been identified that influence the formation of dental calculus, including age, gender, ethnic background, diet, location in the oral cavity, oral hygiene, bacterial plaque composition, host genetics, access to professional dental care, physical disabilities, systemic diseases, tobacco use, and drugs and medications. Clinical significance Plaque accumulation causes the gingiva to become irritated and inflamed, and this is referred to as gingivitis. When the gingiva become so irritated that there is a loss of the connective tissue fibers that attach the gums to the teeth and bone that surrounds the tooth, this is known as periodontitis. Dental plaque is not the sole cause of periodontitis; however it is many times referred to as a primary aetiology. Plaque that remains in the oral cavity long enough will eventually calcify and become calculus. Calculus is detrimental to gingival health because it serves as a trap for increased plaque formation and retention; thus, calculus, along with other factors that cause a localized build-up of plaque, is referred to as a secondary aetiology of periodontitis. When plaque is supragingival, the