Lactose Broth
Practical information
Aplications Categories
Enrichment Salmonella
Detection Coliforms
Industry: Water / Food
Principles and uses
Lactose Broth is included in many Standard Methods for testing foods, dairy products and other materials for Enterobacteria and other Gram-negative microorganisms. It is also widely used for testing for coliforms in water and foods.
It is used as preenrichment for testing foods and dairy products for Salmonella, where conservation processes may have injured or reduced their numbers. Preenrichment in a non-selective medium allows for repair of cell damage, diluting toxic or inhibitory substances, and provides a nutritional base for Salmonella. A preenrichment medium provides a higher ratio of Salmonella to non-Salmonella organisms after incubation. Most non-Salmonella bacteria ferment lactose whilst Salmonella does not. As lactose is metabolized, the pH decreases creating a bacteriostatic effect on competing
organisms.
Large water samples may require a double-strength of Lactose Broth to maintain correct concentration of the medium.
Gelatin and Beef extract provide nitrogen, vitamins, minerals and amino acids essential for growth. Lactose is a complex carbohydrate energy source.
Formula in g/L
Beef extract 3 Gelatin pancreatic digest 5
Lactose monohydrate 5
Typical formula g/L * Adjusted and/or supplemented as required to meet performance criteria.
Preparation
Suspend 13 grams of the medium in one litre. of distilled water. Mix well and dissolve by heating with frequent agitation. Dispense into tubes with Durham gas collecting tubes for gas detection. Sterilize in autoclave at 121 ºC for 15 minutes. Cool as quickly as possible.
Instructions for use
- Check the sterilization of the medium by incubating the tubes at 35°C for 24 hours prior to inoculation.
- Ensure that the fermentation tubes are free from air bubbles before inoculation.
- Seed aliquots of 1, 10 or 100 ml of the sample liquid in adequate containers according to the quantity of the medium.
- Incubate at 35 ± 2°C for 18-24 hours and check for the presence of gas, which constitutes a presumptive coliform test.
- Subculture to VRBGL (Cat. 1144) for detection and quantification tests.
