## 2 Results and Discussion  
### Possible Cause and Origin of Detected Volatiles  
Preliminary experiments (item 1.4 of Supplementary Materials) suggested that incubated samples were superior to \"fresh\" ones because detected compounds displayed greater signal intensity in this case. It is supposed that the 1-day incubation process, at body temperature, may favor bacterial activity, then, selectively enhancing the content of bacterial volatiles. Incubated samples also allowed better discrimination between samples' group. The total number of detected VOCs in samples was 164 (Figure 1). The largest number of volatiles (130) was found for patients with submandibular abscess, followed by halitosis sufferers (85) and, finally, healthy volunteers (78). Evaluation of Figure 1 led to general observations. First, the most predominant classes of compounds for three investigated groups were alcohols and ketones, corresponding, respectively, to 23.2% and 23.8% of the total number of detected VOCs. This finding complied with previous publications regarding salivary volatile profiles [22; 23]. Esters corresponded to 9.8%, hydrocarbons to 9.1%, aldehydes to 8.5%, acids and volatile nitrogen compounds (VSCs) to 6.7%, volatile sulfur compounds (VSCs) to 6.1%, ethers to 1.8%, and \"others\" to 4.3% of the volatiles found in saliva samples. Few compounds were ubiquitous for all three groups of patients, such as 2-methyl-1-propanol,  
Figure 1: Functional group distribution of VOCs (volatile organic compounds) for healthy patients (HE), subjects with submandibular abscesses (AB), and halitosis (HA), where: VNCs—volatile nitrogen compounds, VSCs—volatile sulfur compounds. 1-pentanol, 1-dodecanol, 2-heptanone, 6-methyl-5-hepten-2-one, and 2-tetradcanone. These volatiles were commonly found by previous investigators [24; 25]. The \"abscess\" (AB) profiles were characterized by increased number of VNCs (10 of the detected volatiles), hydrocarbons (14 of the detected VOCs), and aldehydes (14 among the total VOCs). Five VNCs were present just in AB profiles: 2,6-dimethylpyrazine, 2,3-dimethylpyrazine, 3-ethenylpyridine, \(N\)-furfurylpyrrole, and benzyl nitrile. Pyrazines derivatives can origin from bacterial activity as well as they are important flavoring components [26]. 3-ethenylpyridine is a compound considered as an environmental tobacco smoke marker [27; 28]. \(N\)-furfurylpyrrole is found in various foods, such as coffee, chocolate, popcorn, and roasted chicken [29]. Benzyl nitrile (also known as phenylacetonitrile) can be biochemically synthesized by bacterial aldoxime-nitrile pathway, reported in _Escherichia coli_[30] species. Increased number of hydrocarbons can be due to microbial production since bacteria are the main cause of abscesses in mouth. Alkanes are addressed as products of metabolic and catabolic routes of bacterial fatty acids pathway [26]. \(Z\)-ocimene and myrcene are monoterenes that, despite being ingredients for the perfumery industry, can be generated by the mevalonate pathway in bacteria [31; 32]. Inflammation is often connected to the formation of skin abscesses as a reaction to the infectious process, i.e., the presence of bacteria or parasites. Volatiles representative for inflammatory processes are often compounds generated during oxidative processes and specific for this condition like nitric oxide (NO) and nitrate [33]. Thus, the increased number of aldehydes in AB samples can be considered as a manifestation of local inflammatory processes. Ten aldehydes were specific for the abscess group. Monofunctional C\({}_{3}\)-C\({}_{10}\) aldehydes are generated as break down products from unsaturated fatty acids after the attack of reactive oxygen species (ROS) onto membrane structures inducing peroxygenation. Hexanal and octanal are examples of such biomarkers [34]. Regarding other aldehydes, 2-methylbutanal and 3-methylbutanal are end-products of protein oxidation occurring in cell structure damage induced by reactive oxygen species [35].