Periodontal disease increases the severity of halitosis. Periodontal pathogenic bacteria such as Porphyromonas gingivalis, Porphyromonas mesenteriae, Fusobacterium fusae and Clostridium nucleatum produce hydrogen sulfide and methyl mercaptan, irritating gases from cysteine and methionine in salivary proteins that cause dental malodor; periodontitis worsens halitosis through volatile sulfur-containing compounds (increased interdental and subgingival areas at the site of production), increased sulfur-containing substrates (shedding of epithelial cells and leukocytes) and an increase in methionine worsen bad breath.