A1 Medium

The enumeration of coliforms, specifically Escherichia coli, has been used to determine water purity by the most-probable-number method. This medium was created to hasten the recovery time of E. coli and to reduce the number of false positive cultures. A-1 Medium can be used in a single-step procedure, also in foods, not requiring a previous enrichment step.

Tryptone provides nitrogen, vitamins, minerals and amino acids. Lactose is the carbon source and, in combination with salicin, provides energy for organism growth. Ecosurf is a surfactant and sodium chloride supplies essential electrolytes for transport and osmotic balance.

Gas production is a positive reaction indicating the presence of coliforms. Gas may be produced inside the Durham tubes or may appear as dissolved gas that forms gas bubbles when slightly agitated.

Typical formula in g/L, adjusted and/or supplemented as required to meet performance criteria.

Preparation

Suspend 31.5 grams of the medium in one liter of distilled water. Mix well and dissolve by heating with frequent agitation. Boil for one minute until complete dissolution. Dispense into tubes with Durham gas collecting tubes and sterilize in autoclave at 121 °C for 15 minutes.

Note: for 10 mL water samples, prepare a double-strength medium.

Instructions for use

1. Inoculate the sample into tubes and incubate at 35 °C for three hours.
2. Then, incubate at a temperature of 44.5 °C for an additional 21 ± 2 hours.
3. Calculate densities using MPN standard methods.

• Standard Methods for the Examination of Water and Wastewater 15th Ed. American Public Health Association, Inc, Washington, D.C. 1980.
• Andrew, W.H., C.D. Diggs, and C.R. Wilson, 1975.