Sorbitol MacConkey Agar

Sorbitol MacConkey agar is a differential medium for the isolation of Escherichia coli 0157:H7 based on the formulation by Rappaport and Henig. It differs from other MacConkey mediums in that lactose has been replaced by sorbitol.

As Escherichia coli 0157:H7 does not ferment sorbitol it produces pale translucent colonies whereas most other strains of Escherichia coli are sorbitol-positive and produce pink colonies.

However, it should be noted that colonies that are sorbitol-positive can revert and possibly be mistaken as sorbitol-negative. Tryptone and meat peptone provide the required carbon, nitrogen and vitamins.

Sorbitol is a fermentable carbohydrate and neutral red is a pH indicator. Bile Salts no.3 and crystal violet are selective agents and together inhibit Gram-positive cocci. Sodium chloride maintains the osmotic balance.

If required, the selectivity of the medium may be increased by the addition of cefixime (0.05 mg/L) and potassium tellurite (2.5 mg/L).

Differential medium for the isolation of Escherichia coli 0157:H7

Additional Information

Shelf Life (days):1095 days

Storage Temp (C): 10-30C

Dehydrated Medium Appearance: Fine Powder

Prepared Medium Appearance:  Firm Gel

Dehydrated Medium Colour: Pale Straw

Prepared Medium Colour: Pale Violet-Red

Dehydrated PH:7.1± 0.2

Prepared PH: 7.1 0.2

Recommended Incubation: Aerobically at 35-37C for 18-24 hours

Formula g/L

Tryptone: 17.0

Meat peptone: 3.0

Sorbitol: 10.0

Bile Salts No.3: 1.5

Sodium chloride: 5.0

Neutral red: 0.03

Crystal violet: 0.001

Agar: 12.0

Total: 48.5

*Adjusted/supplemented as required to meet performance requirements

 

	g/L
Tryptone	17.0
Meat peptone	3.0
Sorbitol	10.0
Bile Salts No.3	1.5
Sodium chloride	5.0
Neutral red	0.03
Crystal violet	0.001
Agar	12.0
Total	48.5
*Adjusted/supplemented as required to meet performance requirements