Tryptone Bile Salts Agar

Tryptone provides nitrogen, vitamins, minerals and amino acids essential for growth. Bile salts are inhibitors to Gram-positive organisms and suppress coliform bacteria. Bacteriological agar is the solidifying agent.

The fecal presence and level is an important factor in the evaluation of a water mass and the infection risk which it poses to human health. The analysis of water samples to detect E. coli, which usually is found in the intestinal tract of humans and other warm-blooded animals, provides an indication of this type of contamination.

This medium is recommended for the quick test method, which is based on the fact that 99% of Escherichia strains produce indole from tryptophan at 44 °C when grown by the membrane filtration technique.

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

Preparation

Suspend 36.5 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. Sterilize in autoclave at 121 °C for 15 minutes. Cool to 45-50 °C, mix well and dispense into plates.

Instructions for use

1. Incubate the filtration membrane at 36 ± 2 °C for 4-5 hours in Tryptone Soy Agar (LC-1138).
2. Transfer the filtration membrane to Tryptone Bile Salts Agar and incubate at 44 ± 0.5 °C for 19-20 hours.
3. If desired, the two agar media can be combined in a Petri dish in a double layer. In this case, it is convenient to place the membrane over a recently prepared double-layered plate of TSA (LC-1138) and TBA. Incubate at 36 ± 2 °C for 4-5 hours, and incubate again at 44 ± 0.5 °C for 19-20 hours.
4. Carry out the indole test: all indole-positive strains which give red colonies when stained with indole reagent are considered E. coli.

• International Standard ISO 9308-1 Water quality. Detection and enumeration of Escherichia coli and coliform bacteria. Part 1: Membrane filtration method.
• Sahidi S.A. and Ferguson A.R. (1971) Appl. Microbiol., 21 500-506.
• Harmon S.M., Kauttar D.A. and Peeler J.T. (1971) Appl. Microbiol. 21. 922-927.
• Hauschild A.H.W and Hilsheimar R. (1973) Appl. Microbiol. 27. 78-82.