Another common test in our laboratory is the detection of Escherichia coli. From Ying Ying’s first posting, we know that is LTB tubes are positive (gas production), we will have to continue by inoculating a loop of the positive LTB tube into a BGB and a EC tube.
If the food sample is positive for EC tube (gas production), we can conclude that the bacterium is a lactose fermenter. Further tests to confirm whether it is Escherichia coli needs to be carried out.
A loopful of the culture from the EC tube is being streaked onto an Eosin Methylene Blue agar.
Eosin Methylene Blue agar (EMB)
- Consists of Eosin Y and Methylene Blue dyes which inhibit gram-positive bacteria growth and differentiates lactose fermenting and non-lactose fermenting bacteria
- Lactose fermenting bacteria will appear blue-black
- Non-lactose fermenting bacteria will appear colourless and transparent.
- If positive for Escherichia coli, purple colour colonies with green metallic sheen will grow.
After incubating for 1 day at 35◦C, the EMB plate is taken out. If positive for Escherichia coli, purification of a single colony for the EMB is done and incubated.
After purification, biochemical tests are being carried out.
Indole test
- If positive for Escherichia coli, a red ring is being formed when Kovac’s reagent is added. This is because Escherichia coli produces tryptophanase which cleaves tryptophan to produce indole and other products. Kovac’s reagent will then react with indole to form a red ring. (Positive indole test)
Methyl Red test
- If positive for Escherichia coli, the methyl red indicator will turn red due to the acids produce by the degradation of glucose, lowering the pH. (Positive MR test)
Voges-Proskauer test
- If positive for Escherichia coli, there will be no change. This is because E.coli does not produce acetyl methyl carbinol which will react with alpha-napthol and potassium hydroxide to produce pink colour after standing for about 1 hour. (Negative VP test)
Simmon’s Citrate test
- If positive for Escherichia coli, there will be growth on the surface of the Simmon’s Citrate slant but no colour change from green to blue.
Gram-stain
- If positive for Escherichia coli: gram-negative bacillus. (pink, rod-shaped)
Gram stain of E.coli
Results of the biochemical tests are then recorded and compared with the results of the control to check whether it is positive for Escherichia coli.