Biochemical and Microbial Aspects of Oral Malodor Production

Fig. 3.1

Confocal laser scanning microscopy image showing VSC production (red) in the deep ­layers of the biofilm

Biochemical Aspects

Metabolism

Oral malodor is considered to derive primarily from the anaerobic degradation of proteinaceous materials by oral microorganisms. Proteins are readily available from saliva, exfoliated epithelial cells, food debris, blood and crevicular fluids, and possibly postnasal drip. The constituent proteins are hydrolyzed by proteolytic enzymes of bacterial origin, yielding free amino acids, which can then be further broken down. Some of the molecular by-products of this process are particularly foul-smelling.
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Deglycosylation: Salivary mucins are large glycoproteins comprised of a long protein core surrounded by carbohydrate side chains in a bottle brush-like structure. Unlike ordinary polypeptides, their proteolytic degradation requires the prior removal of their carbohydrate side chains, or deglycosylation
Mucin  →  Protein  →  Amino acids  →  Putrefactive end products
One of the key enzymes in the deglycosylation process is β-galactosidase. Our research has shown that β-galactosidase activity in saliva highly correlated with malodor ratings of 64 subjects (Sterer et al. 2002). Furthermore, addition of β-galactosidase or β-galactosidase producing bacteria (i.e., Streptococcus salivarius) to a mucin incubation mixture promoted mucin putrefaction by Porphyromonas gingivalis (Sterer and Rosenberg 2006).
The proteolytic process described above results in the breakdown of available oral proteins (or glycoproteins) into free amino acids. These free amino acids serve as nutrients for oral microorganisms that do not grow on carbohydrates (asaccharolytic), and their metabolites are important pH modulators and precursors for various molecules such as iron-scavenging siderophores. Amino acids are degraded via different metabolic pathways such as deamination, decarboxylation, and various oxidation–reduction processes, yielding different by-products, as described below.
Volatile fatty acids: The enzymatic cleavage of the amino group from various amino acids occurs at a pH range of 6–7 and results in the production of volatile fatty acids, and ammonia (NH3) (Table 3.1).

Table 3.1

Deamination products of anaerobic bacteria
Amino acid
VFA produced
Alanine, glycine, serine
Acetate
Threonine
Propionate
Glutamate, aspartate
Acetate, propionate, butyrate
Valine
Isobutyrate
Leucine
Isovalerate
Isoleucine
2-Methylbutyrate
Phenylalanine
Phenylacetate
Tyrosine
p-Hydroxyphenylacetate
Tryptophan
Indoleacetate  →  3-methylindole
Tyrosine
Phenylacetate, phenylpropionate
Source: Mackie et al. (1998)
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Amines: Decarboxylation of nitrogen-containing amino acids occurs in the mouth mainly at pH 6.5 (Gochman et al. 1959) and serves also as a means for the microorganisms to regulate pH conditions. The decarboxylation of these amino acids results in the production of various amine compounds (Table 3.2).

Table 3.2

Decarboxylation reactions by anaerobic bacteria
Amino acid
Amine produced
Glycine
Methylamine
Alanine
Ethylamine
a-Aminobutyrate
Propylamine
Ornithine
Putrescine  →  pyrrolidineb
Argininea
Putrescine  →  pyrrolidineb
Norvaline
Butylamine
Lysine
Cadaverine  →  piperidineb
Arginine
Agmatine
Histidine
Histamine
Cysteic acid
Taurine
Tyrosine
Tyramine
Tryptophan
Tryptamine
Phenylalanine
Phenylethylamine
Source: Mackie et al. (1998)
aDecarboxylation and hydrolysis
bRing closure reaction
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Indoles and Phenols: Production of indoles and phenols occurs as a result of the metabolism of various aromatic amino acids (Table 3.3).

Table 3.3

Indoles and phenols
Amino acid
Products
Tyrosine
Phenol, p-Cresol
Tryptophan
Indole, 3-Methyl Indole (Skatole)
Phenylalanine
Phenyl acetate, Phenyl propionate
Volatile sulfide compounds: Production of sulfur containing compounds results from sulfate reduction or the metabolism of sulfur containing amino acids:
Cysteine  →  Hydrogen sulfide
Methionine  →  Methyl mercaptan

Malodorous Volatile Organic Compounds (VOCs) in the Oral Cavity

Many of the above compounds, as well as various other VOCs, have been detected in saliva samples, saliva headspace, tongue-coating samples, and breath samples. These samples were analyzed using different techniques varying from classic colorimetric techniques to modern liquid and gas chromatography. These reported compounds are listed in Table 3.4, highlighting those that have been implicated in oral malodor production and stating their odor characteristics and thresholds:

Table 3.4

Oral VOCs detected in saliva, tongue coating samples, saliva headspace, and mouth air
Compound
Odor description
Odor threshold (ppm v/v)
References
Acids
     
Acetic acid
Butyric acid
Methylpropionic acid
   
(2, 6)
(2, 6)
(2)
Alcohols
     
Ethanol
Propanol
Decanol
Dodecanol
Tetradecanol
Hexadecanol
Phenylethanol
2-Ethylhexanol
Benzylalcohol
   
(1)
(1, 4, 5, 6)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
Aldehydes
     
2-heptanal
2-octanal
Nonanal
Acetaldehyde
Benzaldehyde
   
(2)
(2)
(5)
(6)
(1, 5)
Aromatics
     
C2-C4 Alkyl benzenes
Ethylbenzene
Styrene
Dimethylbenzene
Benzene
Naphtalene
Toluene
   
(1)
(2)
(1, 5)
(2)
(1)
(2)
(1)
Ethers
     
Dimethylfuran
   
(1)
Fixed gases
     
Carbon disulfide
Dimethyl selenide
Thiopropanal-s-oxide
   
(4)
(4)
(6)
Hydrocarbons
     
2-methyl-propane
dimethoxy-methane
trichloro-ethane
1(1 ethoxyethoxy)-propane
2-methyl 1-nitropropane
   
(2)
(2)
(2)
(2)
(2)
2,4 –pentadienenitryl
Isoprene
Caryophyllene
β-Pinene
Isobutene
Tridecane
Dodecane
Undecane
Pentadecane
Decane
Limonene
C8-C12 Alkanes
C17 Alkane
   
(2)
(5)
(5)
(5)
(5)
(5)
(5)
(5)
(5)
(5)
(2, 5)
(1)
(1)
Ketones
     
2-Butanone
2-Heptanone
2-Hexanone
2-Pentanone
6-Methyl-2-heptanone
Acetone
   
(4, 5)
(2)
(2)
(4)
(2)
(1, 4, 5, 6)
Nitrogen containing compounds
     
Diphenylamine
Pyrrolidine
Putrescine
   
(1)
(2)
(3, 7)
Cadaverine
Methenamine
Ammonia
Indole
Putrid
Pungent
Fecal
1.5
0.00030
(3, 7)
(5)
(6, 8)
(1, 2, 3, 4, 9)
2-Methyl pyridine (Picoline)
Sweat
0.031
(1, 2)
3-Methyl pyridine
4-Methyl pyridine
Ethyl-pyridine
   
(1)
(1)
(2)
3-Methyl-indole (Skatole)
Fecal
0.0000056
(1, 2, 3 ,4)
Pyridine
Rancid
0.063
(1)
Phenols
p-Cresol
Dimethylphenol
Phenol
   
(1, 2)
(2)
(1, 2, 5)
Sulfur-containing compounds
     
2-Methyl-thio-propane
Mercapto-acetic acid (2)
Methylsulphide (2)
Propane-thiol (2)
Methylthiopropane (2)
Thiocyanic acid (2)
Ethanethioic acid-S-ME (2)
Methylbenzotiophene (2)
Benzenecarbothiotic acid (2)
Methanesulfonylazide (2)
   
(2)
(2)
(2)
(2)
(2)
(2)
(2)
(2)
(2)
(2)
Dimethyl sulfide
Cabbage
0.0030
(4, 5)
Allyl methyl sulfide
Diallyl disulphide
   
(4)
(6)
Hydrogen sulfide
Methyl mercaptan
Dimethyldisulfide
Dimethyltrisulfide
Rotten egg
Sewer
Sulfur
0.00041
0.000070
0.0022
(4)
(2, 4)
(1, 2, 4, 5)
(1, 2, 4, 5)
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Nov 30, 2015 | Posted by in General Dentistry | Comments Off on Biochemical and Microbial Aspects of Oral Malodor Production
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