## 2 Materials and Methods
### Chemical Analysis  
Chemical analyses of bacterial Tenax(r) extracts and SPME samples were performed on a gas chromatograph (GC, 7890B, Agilent Technologies, Santa Clara, CA, USA) coupled to a mass-spectrometer (MS, 5977A, Agilent Technologies, Santa Clara, CA, USA). The general GC operating conditions were split-less injections for 0.6 min using helium carrier gas at 1.6 mL/min. The GC column was non-polar (DB-5 ms, Agilent Technologies, Santa Clara, CA, USA) with the following dimensions: 30 m \(\times\) 0.25 mm i.d. \(\times\) 0.25 um film thickness. The temperature program started at 40 \({}^{\circ}\)C (2 min hold) and was increased by 4 \({}^{\circ}\)C/min to 280 \({}^{\circ}\)C followed by a 10 min hold. The injector temperature for the column was 250 \({}^{\circ}\)C. The transfer line was maintained at 250 \({}^{\circ}\)C. The MS system was operated in electron impact mode at 70 eV. Automatic integration of each peak was performed with Mashhunter software (Agilent Technologies, Santa Clara, CA, USA), which provided a list of compounds (i.e., peak) for each sample. Identification of compounds was then performed by matching the spectra of each peak with the NIST Mass Spectral Database library (Ver 2.2, 2014). In addition, Kovats retention indices (KI or RI) for each compound were calculated by running an external alkanes series using the aforementioned GC/MS method. Finally, identification of peaks-of-interest was confirmed with synthetic standards purchased from Sigma-Aldrich and marked as bold in Table 1.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline  & \multicolumn{2}{c}{**Mean RT \({}^{**}\)**} & \multicolumn{2}{c}{**Mean KI \({}^{*}\)**} & \multicolumn{2}{c}{**CF**} & \multicolumn{2}{c}{**EC**} & \multicolumn{2}{c}{**KO**} & \multicolumn{2}{c}{**PA**} & \multicolumn{2}{c}{**TSB**} \\  & \multicolumn{2}{c}{**(min.)**} & & & & & & & & & & & \\ \hline
**Compounds** & **Texas** & **SPME** & **Texas** & **SPME** & **Texas** & **SPME** & **Texas** & **SPME** & **Texas** & **SPME** & **Texas** & **SPME** \\ \hline
2-Butanone & & 3.35 & & 599 & & 0.04 & & 0.03 & & 0.03 & & \\ \hline Isobutyl alcohol & & 5.86 & & 6.35 & & & & & & & 0.01 & \\ \hline
3-Hydroxy- & & & & & & & & & & & & \\
2-butanone, (syn. & 4.84 & & 704 & & & & & & 0.06 & \\ acetoin) & & **5.48** & & **731** & **0.36** & & **0.3** & & **0.46** & **0.45** & \\ \hline Methyl-pyrazine & & 7.85 & & 823 & & 0.01 & & 0.01 & & 0.01 & & 0.01 & \\ \hline
1,2-Dimethyl-benzene & 8.7 & & \(<\)900 & & & 0.01 & & 0.01 & & & & \\ \hline \end{tabular}
\end{table}
Table 1: Mean relative proportions of microbial volatile organic compounds (MVOCs) identified for each bacterium grown at pH 7 collected by 24 h headspace sample with either Tenax® filter or solid phase micro-extraction (SPME) fiber, followed by solvent extraction. The MVOCs in bold letters were identified to trigger SPME-Gas-chromatopn coupled with electroantennographic detection (GC/EAD) responses, and MVOCs highlighted in grey were tested in Y-tube olfactometer with 11 synthetic compounds. (CF: _Citrobacter freundii_; EC: _Enterobacter cloacae_; KO: _Klebsiella oxytocin_; PA: _Enterobacter_ (syn. _Pantoeus_) _agglomerans_ and tryptone soya broth (TSB) media). \({}^{*}\) KI: Kovats retention index; \({}^{**}\) RT: retention time (in min).