Relationship Between Oral Malodor and Oral Microbiota

Oral malodor, also called halitosis or bad breath, is one of the major complaints made by patients visiting the dentist, ranking behind only dental caries and periodontal disease. It can originate from either systemic or oral conditions, but is usually related to an oral cause. Clinical causes of oral malodor include periodontitis, poor oral hygiene, tongue debris, deep caries, inadequately fitted restorations, endodontic lesions, and low salivary flow [1-5]. Under such conditions, it is thought that either bacterial cell numbers increase or oral bacterial communities shift towards a composition producing high levels of malodor. The major compounds that contribute to oral malodor are volatile sulfur compounds (VSCs), such as hydrogen sulfide (H2S), methyl mercaptan (CH3SH), and dimethyl sulfide (CH3SCH3) [6, 7]. In addition, methylamine, dimethylamine, propionic acid, butyric acid, indole, scatole, and cadaverine are reported to cause oral malodor. About 90% of the VSCs in mouth air are H2S and CH3SH [7], which are produced through bacterial metabolism of sulfur-containing amino acids, such as cysteine and methionine. Gram-negative anaerobes are important producers of VSCs. Persson et al. [8] reported that periodontal pathogens isolated from subgingival plaque, such as Porphyromonas gingivalis, Prevotella intermedia, Tannerella forsythensis and Treponema denticola, generated significant amounts of H2S and CH3SH. An examination of the microbiota composition of the tongue biofilm of individuals with no periodontitis, or only a slight degree of periodontitis, suggested that the major species of H2S-producing bacteria were Veillonella, Actinomyces, and Prevotella [9]. On the other hand, Gram-positive oral bacteria, primarily streptococci, may also promote VSC production by Gram-negative bacteria [10]. Recently, Takeshita et al. [11] determined the bacterial composition of saliva, based on terminal-restriction fragment length polymorphism (T-RFLP) profiles using hierarchical cluster analysis, and associated the global composition of indigenous bacterial populations with the severity of oral malodor. The human oral cavity contains more than 500 bacterial species that interact both with each other and host tissues, suggesting that various bacteria may be related to malodor production. Oral-derived malodor is classified into physiological and pathogenic odor. The microbial composition of the oral cavity varies according to clinical condition; therefore, the most appropriate management strategy may also differ. However, the common goal of regimen for the treatment of oral malodor is the acquisition of a healthy oral condition, including