Periodontal disease increases the severity of bad breath. Periodontal pathogenic bacteria such as Porphyromonas densa, Porphyromonas gingivalis, Prevotella intermedia, Fusarium species and Clostridium nucleatum produce hydrogen sulphide and methyl mercaptan, irritating gases from cysteine and methionine in salivary proteins, which cause bad breath; periodontitis worsens bad breath through volatile sulphur-containing compounds (increased interdental and subgingival areas at the site of production), increased sulphur-containing substrates (shed epithelial cells and leucocytes) and increased methionine.