Lauryl Sulphate Broth (Lauryl Tryptose Broth)

Lauryl Sulphate Broth is a selective liquid medium for the enumeration and detection of coliform bacteria in water, wastewater, dairy products, and other food samples. APHA recommends it for the detection of coliforms and it is also recommended by the ISO Committee. The medium is suitable for presumptive enumeration of coliforms in water, effluent or sewage using the Most Probable Number (MPN) test. The medium was introduced by Mallmann and Darby and later work showed selective action of sodium lauryl sulphate on coliform bacteria. Sodium lauryl sulphate also suppresses other organisms, which is why the medium can be used for detection of these in samples harbouring mixed populations of microbes.

Each component of the formulation has a role in the growth and detection process. Tryptose provides nitrogen, carbon compounds, vitamins, and amino acids necessary for bacterial growth. Lactose is a fermentable carbohydrate. Gas is produced as a fermentation product when lactose-positive bacteria metabolize lactose. An inverted Durham tube is an easy method to view this gas production within the broth. Sodium chloride maintains the osmotic pressure of the medium. Dipotassium phosphate and potassium dihydrogen phosphate act as pH regulators during fermentation of lactose. These ingredients, combined, create the ideal environment to identify lactose fermenting coliforms, while the selective agent inhibits the growth of other organisms.

The concentration of the medium used for the MPN method is chosen on the basis of the inoculum volume. Single-strength medium is used for inoculum volumes of 1 ml or less and double-strength or proportionate medium is used for inoculum volumes of 10 ml or more. After inoculation, tubes are incubated at 37°C for 24 to 48 hours. Tubes showing fermentation are assumed to have undergone primary fermentation.

For additional differentiation, a tube of material from a tube of primary fermentation is inoculated into two tubes of Lauryl Tryptose Broth. These are incubated at 37°C and 44°C respectively. An indole test is carried out with Kovacs reagent if fermentation is detected at 44°C from 8 to 24 hours. Gas production at 44°C together with a positive indole reaction indicates the presence of Escherichia coli. If fermentation is absent at 37°C after 24 hours, the primary fermentation is considered attributable to organisms other than coliforms.