# A Lab-Made E-Nose-MOS Device for Assessing the Bacterial Growth in a Solid Culture Medium
## 4 Conclusions  
The study carried out highlighted the feasibility of applying a self-built electronic nose prototype, comprising metal oxide semiconductor sensors, as a qualitative-quantitative tool for the identification and monitoring of four different bacteria. The designed device allowed the discrimination of the odors emitted by Gram-negative (_E. coli_ and _P. aeruginosa_) and Gram-positive (_E. faecalis_ and _S. aureus_) bacteria, grown overnight in a solid culture medium. However, it should be highlighted that the satisfactory predictive classification performance (sensitivities ranging from 80 to 100% and specificities varying between 83 to 100%) was achieved for the leave-one-out cross-validation variant, which could be slightly overoptimistic, being required an external validation in the future. Moreover,  
Figure 2: E-nose-MOS-MLR-SA model’s predictions (leave-one-out cross-validation) of the decimal logarithmic of the colony forming units (CFU), i.e., number of microorganisms, for the studied Gram-positive and Gram-negative bacteria grown in BHI agar medium. (**A**) _E. faecalis_: model based on 4 feature extraction MOS signal parameters (S9_INT; S9_MIN; S8_MEAN and S5_SD); (**B**) _S. aureus_: model based on 3 feature extraction MOS signal parameters (S5_INT, S4_SUM and S4_MEAN); (**C**) _E. coli_: model based on 3 feature extraction MOS signal parameters (S4_LP, S6_SUM and S9_MEAN); and (**D**) _P. aeruginosa_: model based on 3 feature extraction MOS signal parameters (S8_MAX; S9_MAX and S9_SD).  
the olfactory-sensor device allowed assessing the number of colony-forming units, (i.e., the number of microorganisms) of each of the four bacteria studied, grown separately in BHI solid culture medium, with the respective root mean square errors as low as 1 to 4 colonies. Once again, this accuracy was obtained for the leave-one-out cross-validation variant and thus, the satisfactory quantitative results should be considered with caution. Nevertheless, the performed study can be seen as a proof of concept that the electronic nose is a fast, non-invasive and green alternative/complementary strategy to the conventional analytical approaches. Finally, the promising and reliable classification and quantitative performances achieved with the electronic nose strengthen the hypothesis of applying the proposed sensing-chemometric approach to the bacterial growth analysis in solid food samples, although the influence of environmental conditions and the impact arising from the complexity and diversity of real samples matrices must be considered.  
Conceptualization, T.D., K.Z., A.C.A.V. and A.M.P.; methodology, T.D. and A.M.P.; validation, V.S.S., T.Z. and N.F.; formal analysis, T.D., K.Z., A.C.A.V. and A.M.P.; investigation, V.S.S., T.Z., N.F. and A.I.R.; resources, T.D., A.C.A.V. and A.M.P.; data curation, V.S.S., T.Z., N.F. and A.I.R.; writing--original draft preparation, T.D. and A.M.P.; writing--review and editing, V.S.S., T.Z., N.F., A.I.R., T.D., K.Z., A.C.A.V. and A.M.P.; funding acquisition, T.D., A.C.A.V. and A.M.P. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Foundation for Science and Technology (FCT, Portugal), through national funds FCT/MCTES (PIDDAC), to CIMO (UIDB/00690/2020 and UIDP/00690/2020), to CEB (UIDB/04469/2020) and to the Associate Laboratory SusTEC (LA/P/0007/2020).  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
The authors declare no conflict of interest.