A1 MEDIUM Dehydrated culture Media for Microbiology CAT Nº: AG- 1252;
For detection of coliforms in water and foods
FORMULA IN g/l
Final pH 6.9 ± 0.2 at 25ºC
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 for gas detection and sterilize in autoclave at 121°C for 15 minutes. The prepared medium should be stored at 2-8ºC. The color is amber, slightly opalescent.Note: for 10 ml water samples, prepare a double-strength medium to ensure the ingredient concentrations are not reduced below those of the standard medium.
The dehydrated medium should be homogeneous, free-flowing and beige in color. If there are any physical changes, discard the medium.
A1 MEDIUM is also known as A1 Broth. It is used for purity analysis (source water, marine water, trated wastewater) and for foods, particularly seafood, using the detection of Coliforms and in particular Escherichia coli. This medium was created to reduce the timing of recovery and also to be used without a need of a preenrichment step. Furthermore, the use of this medium improves reliability as it reduces the presence of false positives.
Tryptone provides the nitrogen, vitamins, minerals and aminoacids for growth; Lactose is the fermentable carbohydrate providing carbon and, together with Salicin, provides energy for growth; Triton X-100 is a surfactant and Sodium chloride supplies essential electrolytes for transport and osmotic balance.
Inoculate sample into tubes and incubate at 35ºC for three hours. Transfer tubes to a 44.5ºC water bath and incubate for an additional 21±2 hours.
Gas production is a positive reaction indicating the presence of Coliforms. Gas may be produced in the inverted vial or may appear as dissolved gas that forms gas bubbles when slighty agitated. Calculate densities using MPN standard methods.
The following results were obtained in the performance of the medium from type cultures after incubation at a temperature of 35 ± 2°C for three hours, and at 44.5ºC for 21 ± 2 hours.
Enterobacter aerogenes ATCC 13048
Escherichia coli ATCC 25922
Bacillus cereus ATCC 6633
Enterococcus faecalis ATCC 19433