# A Lab-Made E-Nose-MOS Device for Assessing the Bacterial Growth in a Solid Culture Medium
## 3 Results and Discussion
### Quantification of Bacteria CFUs Using the E-Nose-MOS Lab-Made Device
Thus, it was confirmed that the lab-made E-nose-MOS device combined with MLRMs could be implemented as a fast, green, and non-invasive tool to estimate the number of CFUs of each of the four bacteria studied inoculated separately in BHI agar medium, based on the sensors' response towards the VOCs emitted by each individual strain during a 24 h growing period in a solid synthetic medium. \begin{table}
\begin{tabular}{l l l l l l l} \hline \hline
**Microorganism** & \multicolumn{1}{c}{\(\mathbf{R^{2}}\)} & \multicolumn{1}{c}{**Slope**} & \multicolumn{1}{c}{**Slope CI**} & \multicolumn{1}{c}{\begin{tabular}{c} **Intercept** \\ **(log\({}_{10}\)(CFU))** \\ \end{tabular} } & \multicolumn{1}{c}{\begin{tabular}{c} **Intercept CI** \\ **(log\({}_{10}\)(CFU))** \\ \end{tabular} } \\ \hline Gram-negative & \begin{tabular}{l} _E. coli_ \\ _P. aeruginosa_ \\ \end{tabular} & 0.978 & 0.990 & \begin{tabular}{c} [0.857, 1.124] \\ 0.995 \\ \end{tabular} & \begin{tabular}{c} \(-0.016\) \\ 0.943 \\ \end{tabular} & \begin{tabular}{c} [-0.690, 0.658] \\ 0.943 \\ \end{tabular} \\ \hline Gram-positive & \begin{tabular}{l} _E. faecalis_ \\ _S. aureus_ \\ \end{tabular} & 0.943 & \begin{tabular}{c} 1.044 \\ 0.994 \\ \end{tabular} & \begin{tabular}{c} [0.834, 1.254] \\ 0.998 \\ \end{tabular} & \
\begin{tabular}{c} 0.167 \\ 0.893, 1.103] \\ \end{tabular} &
\begin{tabular}{c} [-1.060, 0.726] \\ 0.004 \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Parameters of the single linear regressions established between decimal logarithmic of the CFU predicted (LOO-CV) by the E-nose-MOS-MLM and the decimal logarithmic of the CFU experimentally determined by the conventional plate counting technique: coefficient of determination (\(R^{2}\)); slopes, intercept values and respective confidence intervals (CI) at 95%. \begin{table}
\begin{tabular}{l l l l l l l} \hline \hline
**Microorganism** & \multicolumn{1}{c}{\begin{tabular}{c} **Concentration Range** \\ **(log\({}_{10}\)(CFU))** \\ \end{tabular} } & \multicolumn{1}{c}{\begin{tabular}{c} **Selected Extracted Feature** \\ **Parameters** \\ \end{tabular} } & \multicolumn{1}{c}{\begin{tabular}{c} **Goodness of Fitting Parameters** \\ \end{tabular} } \\ \cline{3-6} Gram-negative & \begin{tabular}{l} _E. coli_ \\ _P. aeruginosa_ \\ \end{tabular} & \begin{tabular}{c} [0.342, 8.342] \\ [0.079, 6.079] \\ \end{tabular} & \begin{tabular}{c} **S4.LP;** **S6.SUM;** **S9.MEAN** \\ \end{tabular} & \begin{tabular}{c} 0.978 \\ 0.995 \\ \end{tabular} & \begin{tabular}{c} 0.436 \\ 0.174 \\ \end{tabular} \\ \hline Gram-positive & \begin{tabular}{l} _E. faecalis_ \\ _S. aureus_ \\ \end{tabular} & \begin{tabular}{c} [0.079, 8.079] \\ [0.484, 5.484] \\ \end{tabular} & \begin{tabular}{c} S9.INT; **S9.MIN;** **S8.MEAN**; **S5.SD** \\ \end{tabular} & \
\begin{tabular}{c} 0.943 \\ 0.94 \\ \end{tabular} &
\begin{tabular}{c} 0.602 \\ 0.158 \\ \end{tabular} \\ \hline \hline \end{tabular}  
* Experimental concentration range in log\({}_{10}\)(CFU): 9 independent levels for _E. coli_; 7 independent levels for _P. aeruginosa_; 10 independent levels for _E. faecalis_; and 6 independent levels for _S. aureus_. b Feature extracted parameters from the E-nose-MOS response, selected by the SA algorithm and included in each MLRM. \({}^{\circ}\)c \(R^{2}\): determination coefficient; RMSE: root mean square error.  
\end{table} 
Table 3: Predictive performance (LOO-CV) of the MLRM developed based on selected feature extraction parameters derived from the interaction of the E-nose-MOS sensors with the VOCs emitted by each one of the four bacteria studied: _E. coli, P. aeruginosa_, _E. faecalis_, and _S. aureus_ (number of each bacterium, i.e., contents in log\({}_{10}\)(CFU)).