e. coli 0157 is a pathogen that cannot ferment sorbitol, a sugar alcohol, to acid whereas the E. coli in our GI tract, a member of the normal flora, can ferment sorbitol to acid. What media would you use to differentiate between these two E. coli strains?
To differentiate between E. coli 0157, a pathogenic strain that cannot ferment sorbitol, and the E. coli in our GI tract, which ferments sorbitol, you would use a specific type of culture medium called MacConkey sorbitol agar or SMAC (Sorbitol MacConkey Agar).
Here's how you can use this medium to differentiate between the two strains:
1. Prepare the MacConkey sorbitol agar according to the manufacturer's instructions or using a standard recipe. This medium contains sorbitol as the sole source of carbon, along with other components that inhibit the growth of certain organisms and allow for the differentiation of lactose fermenters.
2. Swab or streak your sample containing both strains of E. coli onto the surface of the agar plate. Ensure to streak the culture in a way that differentiates individual colonies.
3. Incubate the plate at the appropriate temperature (usually 37 degrees Celsius) for a specific period, usually 18-24 hours.
4. After the incubation period, observe the colonies on the agar plate. E. coli strains that can ferment sorbitol will appear as colonies with a pink or red color, indicating a positive result.
5. The E. coli 0157 strain, which is unable to ferment sorbitol, will appear as colorless or pale colonies, indicating a negative result.
By differentiating between the pink/red and colorless/pale colonies on the MacConkey sorbitol agar, you can distinguish between the pathogenic E. coli 0157 strain and the normal flora E. coli strain found in the GI tract.