## 2 Materials and Methods
### MVOCs Collection and Identification  
Two different headspace volatile collection methods, using different absorbent materials, were performed to maximize the chance of capturing a maximum of MVOCs. Dynamic headspace collection was performed using an air pump to actively push/pull air through a sorbent (i.e., Tenax(r)-GR), which was solvent eluted to produce an extract that can be stored and later used for chemical analysis and bioassay. In contrast, passive headspace collection using solid phase micro-extraction (SPME) consisted of a fiber that passively adsorbs molecules and requires thermal desorption promptly after collection. SPME avoids solvent use, enables the detection of smaller volatiles that are generally hidden by the solvent peak, and generally allows a shorter collection time (minutes compared to hours). SPME was performed for 1 h to collect and compare MVOCs emitted from bacterial cultures grown on different media (artificial and fruits), and at different pH concentrations. In contrast, Tenax(r) was used to collect bacterial headspace for 24 h, and the solvent-extracts were used later for electrophysiology and behavioral assays. All headspace collections were conducted in Mason jars with bacterial cultures growing either on TSB or fruits. For each bacterial culture, three replicates and a media control were collected via each aforementioned method. All collections were performed under BSL2 laboratory conditions for human risk pathogens at The New Zealand Institute for Plant and Food Research Limited (PFR) in Lincoln, New Zealand.  
Tenax(r) filters used for dynamic headspace collection were adsorbent filters made from 60 mg of Tenax(r)-GR 35/60 (Grace Davison Discovery Sciences, Epping, Australia) packed in a glass tube between two plugs of silane treated glass wool (Grace Davison Discovery Science, Chicago, IL, USA). Air was drawn in from the environment, passed through a charcoal filter, distributed over the culture, and adsorbed onto the Tenax(r) filter at a rate of 500 mL/min. In between each collection, the charcoal filters were heated at 150 \({}^{\circ}\)C overnight while Tenax(r) filters were conditioned at 250 \({}^{\circ}\)C for 3 h under a nitrogen flow. After 24 h of volatile collection, the Tenax(r) filters were eluted with 1 mL of hexane (Sigma-Aldrich, Saint Louis, MO, USA, 95% purity). Two hundred microliters of each extract were concentrated down to circa 80 \(\upmu\)L under a gentle flow of argon and used for chemical analysis. Pooled Tenax(r) extracts of each bacterium were made by mixing 100 \(\upmu\)Lof each of the three replicates. A pooled control was made by mixing 100 uL of each fourth replicate from each bacterial control containing the media only (e.g., TSB). The pooled bacterial Tenax(r) extracts were used for electroantennogram (EAG) and behavioral choice assays with QFF. All the bacterial Tenax(r) extracts were from cultures grown at pH 7 (\(\pm\)0.5). Differences in the proportions of MVOCs emitted from kiwifruit, apples, and oranges, inoculated with either CF, EC, KO, or PA, were assessed at 24 h post-inoculation. Over time variations of chemical profiles, measured at 24 h, 72 h, and 5 days post-inoculation, were assessed for inoculated kiwifruit.  
The SPME fiber selected in our experiments was a grey Stableflex/SS 2 cm SPME (50/30 um DVB/CAR/PDMS, SUPELCO, Sigma-Aldrich, Saint Louis, MO, USA). The SPME fiber was directly inserted inside the Mason jar and exposed to the headspace of the culture a few centimeters above it for 1 h. Before each use, the SPME fiber was conditioned as recommended by the manufacturer (i.e., 270 \({}^{\circ}\)C for 30 min). MVOCs were collected from each bacterium at pH 3, 5, 7, 9, and 12, as well as mixtures of all bacteria adjusted to each pH, using SPME. Differences in chemical profiles of inoculated fruits with bacteria were also assessed with SPME at 24 h post-inoculation and compared with their Tenax(r) counterparts.