# Bacterial Semichemicals and Transkingdom Interactions with Insets and Plants
## 3 BVOC Biosynthetic Pathways and Their Intersections with Primary and Secondary Metabolism
### BVOCs and Primary Metabolism  
A number of BOVAs may be produced as intermediates, byproducts, or endproducts of the bacterial primary metabolism through well-conserved pathways. Most of these pathways are fundamental for the energy metabolism, for the biosynthesis of cellular structure, and for bacterial growth and survival. Simple carboxylic acids, such as formic acid, acetic acid, lactic acid, and succinic acid, may be produced by mixed acid fermentation in facultative anaerobic enteric bacteria. When their intracellular concentration is high as a result of an intense metabolic activity, the molecules are released passively in the environment as BVOCs. Other enteric bacteria may vigorously produce acetoin (3-hydroxybutan-2-one) and its reduced form 2,3-butanediol through butanediol fermentation. Several Clostridia possess the acetone-butanol-ethanol fermentation that produces propanone and butanol. Clostridia are strictly anaerobic bacteria [99] that, thanks to this peculiar energy metabolism pathway, are used to produce solvent or fuels including hydrogen [100]. _Propionibacterium_- and propionate-fermenting microorganisms are other bacteria with a peculiar energy metabolism that contributes to propionate production from succinate. Many fermentative pathways lead to acetate production in addition to other metabolic processes such as those occurring in strictly anaerobic homoacetogenic bacteria. Amino acid metabolism by the Ehrlich pathway [101] through the catabolism of branched-chain amino acids (leucine, valine, and isoleucine), aromatic amino acids (phenylalanine, tyrosine, and trytophan), and the sulfur-containing amino acid (methionine) leads to the formation of fused acids and fused alcohols, and many of these BVOCs may act as semiochemicals. In particular, 2-phenylethanol and phenylacetic acid are produced by phenylalanine catabolism. Additionally, guaiacol and phenol are formed through tyrosine catabolism, while the indole is a direct precursor of tryptophan. Many BVOCs can be produced through the catabolism of branched-chain amino acids. It is interesting to note that research in this field intersects with biotechnology. A number of compounds, which act as chemical signals, are also studied for some biotechnological applications. An example is biofuel production using _Corynebacterium glutamic_ that was engineered for producing 2-methyl-1-butanol and 3-methyl-1-butanol via the Ehrlich pathway from 2-keto-3-methylvalerate and 2-ketoisocaproate, respectively [102]. Another example is provided by an _E. coli_ strain that was genetically engineeredto implement the production of 2-methyl-1-butanol, 3-methyl-1-butanol and 2-phenylethanol using 2-keto-isocaproate 2-keto-methyl-valerate 2-keto-isovalerate 2-phenyl-pyruvate as a substrate [103; 104]. Volatile sulfur compounds are produced through methionine catabolism. Methionine gamma-lyase is central in this catabolic pathway. This enzyme directly degrades sulfur-containing amino acids to \(\alpha\)-cheto acids, ammonia, and thiols [105]. Methionine is decomposed by various bacteria with production of methanethiol, dimethyl sulfide, and dimethyl disulfide [106; 107]. All these compounds, particularly dimethyl disulfide, have proven biological activities against insects, and their use in insect pest management has been proposed as repellent [108]. Other BVOC-producing pathways are linked to biological matter processing, including the catabolism of fatty acids and lipid degradation, aromatic compound degradation, and protein and carbohydrate degradation. Catabolism of diet-introduced macromolecules by gut bacteria have a central role in the biosynthesis of semiochemicals such as those that are produced by the aromatic compound degradation pathways, i.e., benzaldehyde, benzoic acid, benzyl alcohol, guaiacol (2-methoxyphenyl), phenol, phenylacetic acid, phenylethanol, phenylmethanol, and propionate. Many of these pathways have some intersections between them and, therefore, common intermediates.