The oral cavity supports a rich bacterial microflora, much of which thrives in plaque on tooth surfaces. Bacterial plaque on the crown surface of a tooth constantly presents antigen to the marginal gingiva, stimulating an inflammatory response and resulting in gingivitis. The bacteria in plaque are predominantly nonmotile, gram-positive aerobes, including Staphylococcus spp and Streptococcus spp, but many others are also present. Although this microbiota does stimulate an immune response, the bacteria in an otherwise healthy mouth exist in relative commensal harmony with the host. They may even be beneficial by helping to limit the numbers of periodontopathogenic bacteria. If the plaque becomes very thick because of poor oral hygiene and oxygen within the plaque is depleted, the bacterial population can become more pathogenic, with a higher percentage of nonmotile, gram-negative anaerobic rods. The bacteria found in the presence of teeth with periodontal disease include Bacteroides fragilis, Peptostreptococcus, Porphyromonas gulae, Porphyromonas salivosa, Porphyromonas denticanis, Prevotella intermedia, Treponema spp, Bacteroides splanchnicus, and many others. Interestingly, some of the common human periodontopathogens such as Haemophilus (formerly Actinobacillus) actinomycetemcomitans are notably absent in animals. Subgingival plaque (plaque on the tooth surface below the gingival margin) is also commonly inhabited by these more periodontopathogenic species of bacteria. Periodontitis is caused by the host's response to subgingival plaque. Inflammatory mediators produced by the host directly result in bone and tissue damage around the root. The bacteria themselves and their metabolic products also contribute to the bone damage. Development of periodontitis is also affected by other intrinsic (eg, genetics, tooth crowding, thin alveolar bone, age) and extrinsic (eg, diet, stress, concurrent disease, oral hygiene) factors.