By adding an acid directly to the Enterobacteriaceae or Coliform vial, what will the resulting colour be?

Yellow. The Soleris vial contains an enrichment broth medium with glucose as a carbon source and Bromocresol purple (BCP) as a pH dye indicator. The reading frame contains a soft agar plug which reduces product interference with the detection of microbial growth. As microorganisms grow and ferment sugar into acid, the pH of the enrichment broth medium changes, causing a colour change in the indicator dye. The colour change diffuses down into the soft agar plug and is read by optical sensors in the Soleris instrument. Light from the L.E.D passes through the agar plug where the photo–diode reads the colour change in ‘real time’. The system measures when the change is greater than the background giving a “Detection Time”.