## 3 VOCs Used as Indicators of Bacterial Growth  
Many bacteria are pathogens to humans but not to agricultural products. The reasons why bacteria produce VOCs are unclear; however, some reports indicate that they favor communication with the host plant and among species and play a critical role in the completion of bacterial pathogenesis by affecting bacterial virulence, promoting stress resistance, acting as signaling agents to warn the host plant encouraging it to modulate its response, have antifungal activity because bacteria often grow slower than fungi, and take part in biofilm formation [36]. According to [PERSON] and [PERSON] [37], bacteria are established in the host cell by surface proteins (adhesins), assembled into pilus-like structures, or anchored in the outer membrane. These organism opportunists can invade the horticultural products if cells are disrupted. They produce diverse VOCs to avoid fungal growth and destabilize cellular structures when it occurs. Some of these VOCs are the nonan-2-one, undecan-2-one, butanone, hexan-2-one, octan-2-one, butan-1-ol, propan-1-ol, heptan-1-ol, nonan-1-ol, pentadecan-1-ol, 2-phenyl ethanol, methyl mercaptoethanol, pyrazines, trimethylamine, 2-methyl propylamine, 2-methyl butylamine, 3-methyl butylamine, cyclohexylamine, 2-phenylethylamine, geosmin, and 2-methyl isoborneol, among others [38]. In addition, it has been reported that hexadecane is particularly abundant in cyanobacteria, and long-chain aliphatic alcohols such as 1-octanol, 1-decanol, and 1-dodecanol are produced through the \(\beta\)- and \(\alpha\)-oxidation of fatty acid and commonly associated with Enterobacteriaceaeae. _Proteobacteria_ and _Frimicutes_ produce short-chain alcohols (i.e., 2,3-butanediol) under low-oxygen conditions [36]. Factors such as the density of bacterial growth in the substrate, the bacteria genre, and the substrate type affect the volatilome of agricultural products. With regard to the density of bacterial growth, [PERSON] et al. [31] observed that the concentration of VOCs is mainly unchanged until the number of bacteria reaches values above the infectious dose (\(10^{6}\)-\(10^{8}\) CFU mL\({}^{-1}\)). On the other hand, according to [PERSON] et al. [39], the growth time and genre of bacteria could be correlated with the VOCs released. The researchers evaluated the VOCs of five types of bacteria that grew for 16 h in trypticase soy broth at 37 \({}^{\circ}\)C with agitation at 150 rpm min\({}^{-1}\). They found correlations c.a. 1 between bacteria and a group of VOCs. For example, 1-octanol, 1-decanol, 1-dodecanol, and indole correlated with _E. coli_ O157:H7, 2-undecanone, 1-octanol, 1-decanol, and 1-dodecanol with _S. enteritidis_ and _S. flexneri_, and 3-hydroxy-2-butanone with _L. monocytogenes_. Similarly, [PERSON] et al. [40] found differences between _Salmonella_ Typhimurium, _Escherichia coli_ O157:H7, _Pseudomonas fluorescens_, _Pseudomonas aeruginosa_, and _Staphylococcus aureus_ using electronic nose (e-nose) sensors based on thin films of an ionogel composite doped with different concentrations of iron oxide. The interaction between VOCs and sensors increased ionogel conductivity. Variations related to each bacterium's volatile metabolism were noted, leading to bacteria differentiation based on the relative response of each sample. Additionally, the VOC profile of bacteria is affected by substrate type and growth conditions. [PERSON] et al. [41] showed a clear difference in the volatilome of _Pseudomonas