What do colonies of thermotolerant coliforms look like when grown on MLSB?
Thermotolerant coliforms produce yellow colonies when grown on Membrane Lauryl Sulphate Broth (MLSB). The yellow colour is the result of a pH change in the medium that is caused by the synthesis of acids during the growth of the bacteria. It is important to note that the yellow colour will fade and change on cooling and standing; thus, colonies should be counted WITHIN 15 MINUTES of being removed from the incubator. The picture below shows the appearance of thermotolerant coliforms on MLSB. The recommendations for counting given in the text books and standard methods are that you should count all yellow colonies, however faint, and irrespective of size. In practice, most thermotolerant coliform colonies will be between 2 and 3 millimetres in diameter. Do not count colonies of any other colour; however, you should make a note on your results sheet if the petri dish is covered with a large number (greater than 100) of pink colonies. The growth of these non-coliform bacteria can obscure t
