# A Lab-Made E-Nose-MOS Device for Assessing the Bacterial Growth in a Solid Culture Medium
## 3 Results and Discussion
### Bacterial Species Discrimination Using the E-Nose-MOS Lab-Made Device
As previously pointed out, the VOCs' profiles established in this study by HS-SPME-GC-MS clearly pointed out the existence of volatiles emitted by only one of the four bacteria, which would generate different signal responses during the E-nose analysis, justifying the discrimination power of this MOS sensor device. It should be highlighted that, although not confirmed in the present study, other VOCs have been reported in the literature, as bacterial species' biomarkers, namely isovaleric acid, or 2-methyl-butanal for _S. aureus_; 1-undecene, 2,4-dimethyl-1-heptane, 2-butanone, 4-methyl-quinazoline, hydrogen cyanide, or methyl thiocyanide for _P. aeruginosa_; and, methanol, pentanol, ethyl acetate, or indole for _E. coli_[6]. In which concerns the predictive performance, an overall correct classification of 90% was achieved for the LOO-CV procedure, with a global specificity of 91% (Table 2). As  
Figure 1: E-nose-MOS-LDA-SA model’s discrimination of the four studied Gram-positive and Gram-negative bacteria grown in BHI agar medium: (\(\blacktriangle\)) _E. faecalis_; (\(\blacklozenge\)) _S. aureus_; (\(\blacksquare\)) _E. coli_; and, (\(\bullet\)) _P. aeruginosa_. Classification model based on 25 feature extraction MOS signal parameters (S6_LP, S1_INT, S5_INT, S8_INT, S9_INT, S1_MAX, S2_MAX, S4_MAX, S6_MAX, S2_MIN, S4_MIN, S5_MIN, S6_MIN, S2_SUM, S3_SUM, S5_SUM, S6_SUM, S8_SUM, S9_SUM, S2_MEAN, S5_MEAN, S1_SD, S2_SD, S3_SD, and S6_SD). can be inferred, among the four bacteria, only _S. aureus_ was not misclassified (sensitivity of 100%) but had the lowest specificity of (83%). On the contrary, one of the 10 assays with _E. coli_ was misclassified as _S. aureus_ (sensitivity and specificity of 90%); one _E. faecalis_ was misclassified as _P. aeruginosa_ (sensitivity of 90% and specificity of 100%); and finally, one sample of _P. aeruginosa_ was incorrectly classified as _E. coli_ and another as _S. aureus_ (sensitivity of 80% and specificity of 89%). The misclassifications observed, although in a low number (4 in 40 independent bacteria samples) can be attributed to the fact that some VOCs are emitted by different bacterial species, which mitigate their use as unique chemical fingerprints. Indeed, the VOCs profiles established in this study showed that several volatiles were emitted by two or more of the four bacteria under study, although in different amounts (e.g., 1-butanol, 1-pentanol, phenylethyl alcohol, phenol, isocetane, 1-undecene, isovaleric acid, undecane, methyl valerate, 2,5-dimethylpyrazine, 2-ethyl-6-methyl-pyrazine, \(\beta\)-pinene, 2,4-thujadiene, or indole). Additionally, a systematic review performed by [PERSON] et al. [6] revealed that, for example, the four bacteria produce isopentanol, formaldehyde, methyl mercaptan, and trimethylamine, although not observed in the present study. Nevertheless, it should be remarked that the discrimination rates (training and LOO-CV) are similar to the E-nose classification performances previously reported in the literature, in which sensitivities varied from 84 to 100%, allowing differentiating/discriminating several bacterial species, including _E. coli_, _E. faecalis_, _P. aeruginosa_, or _S. aureus_, among other pathogenic bacteria [1; 2; 15; 16; 17; 18; 19; 20; 21; 22; 23].