Lauryl tryptose broth

Culture Media for Food Microbiology, J.E.L. Corry et al. (Eds.)

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9 1995 Elsevier Science B.V. All rights reserved

Lauryl tryptose broth Description and history A selective medium for the detection of coli-aerogenes bacteria in water, dairy products and other foods (Mallman and Darby, 1941). The surface active agent lauryl sulphate acts as the selective agent in restricting the growth of bacteria other than the coli-aerogenes group. The broth is specifically designed to allow rapid multiplication and copious gas production from a small inoculum of target organisms. Unlike other media it is possible to test directly for indole production.

Composition (grams)

Tryptose Lactose Sodium chloride Di-potassium hydrogen orthophosphate Potassium di-hydrogen orthophosphate Sodium lauryl sulphate Distilled or deionized water

20.0 5.0 5.0 2.75 2.75 0.1 1000.0

Preparation Add the ingredients to the water and mix until completely dissolved. Dispense in 10 ml amounts into suitable plugged test tubes or screw cap bottles and add a Durham tube to each tube or bottle. Sterilize by autoclaving at 121~ for 15 min. To avoid bubbles in the Durham tubes, allow the autoclave temperature to drop to 75~ before opening.

Physical properties Appearance pH

Light amber, clear to slightly opalescent. 6.8 + 0.2

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Shelf life R e a d y to use m e d i u m

1 m o n t h at 4 _+ 2~

in screw capped containers.

Inoculation method for samples Use M P N tests for presumptive e n u m e r a t i o n of coli-aerogenes bacteria and

Escherichia coli as described in the A m e r i c a n Public H e a l t h Association Standard M e t h o d s for water and waste water (1971), dairy products (1967) and foods (1966).

Incubation method To detect m e m b e r s of the coli-aerogenes group incubate at 35 or 37~ for 2 4 - 4 8 h. For dairy purposes and where it is desired to detect the full range of coliform bacteria, incubate at 30~ for 2 4 - 4 8 h. For Escherichia coli the t e m p e r a t u r e of 44 _+ 0.1~ for 18 h is specifically r e c o m m e n d e d . Psychrotrophic coliforms can be d e t e c t e d by incubation at 4~ for 10 days.

Reading of results and interpretation Turbidity provides presumptive evidence of the presence of coli-aerogenes bacteria, particularly when accompanied by gas formation. F u r t h e r confirmation of the presence of Escherichia coli may be obtained from the indole reaction and subculture to V R B agar (q.v.)

Quality assessment (i) Productivity Test strains

Inoculation m e t h o d (ii) Selectivity Test strain Inoculation m e t h o d

Escherichia coli50034 Hafnia aluei 50037 Dilution to extinction.

Enterococcus faecalis 50030 Dilution to extinction.

References American Public Health Association (1971) Standard Methods for the Examination of Water and Waste Water, 13th edn., pp. 651, 664-665. American Public Health Association (1967) Standard Methods for the Examination of Dairy Products, 12th edn., edited by W.G. Walter, New York NY. American Public Health Association (1966) Recommended Methods for the Microbiological Examination of Foods, 2nd edn., edited by J.M. Scharf, New York NY. Mailman, W.L. and Darby, C.W. (1941) Use of lauryl sulphate tryptose broth for the detection of coliform organisms. Am. J. Publ. Hlth, 31, 127-134.