Azide Dextrose Broth, is a specialized dehydrated culture medium formulated for the selective detection and enumeration of fecal streptococci in water and food samples. This medium is instrumental in microbiological analysis, particularly in environmental monitoring and food safety, to ascertain the presence of these bacteria as indicators of fecal contamination.
Composition and Principle
The Azide Dextrose Broth comprises several components, with the primary ingredients being Tryptose, Beef Extract, Glucose, Sodium Chloride, and Azide. The inclusion of Azide, a critical component, inhibits the growth of Gram-negative bacteria, facilitating the selective cultivation of fecal streptococci. The broth’s formulation ensures an optimal environment for the growth of the target organism, providing reliable results in microbial detection.
Preparation and Usage
To prepare the medium, 34.7 g of the dehydrated powder (or 69.4 g for double strength medium) is suspended in 1 liter of distilled or deionized water. This mixture is then heated until completely dissolved and dispensed into final containers. The medium is sterilized in an autoclave at 121 °C for 15 minutes. The prepared medium is used for inoculating samples, followed by incubation at 36 ± 1 °C for 20-48 hours. Turbidity in the broth indicates the presence of fecal streptococci.
Interpretation of Results
The results are interpreted based on the turbidity of the broth. The presence of fecal streptococci is indicated by the broth’s turbidity, which is evaluated using the most probable number method (MPN). Azide Dextrose Broth is used for presumptive detection, and further culture tests must be done for confirmation.
For quality control, the dehydrated medium should appear free-flowing and homogeneous with a beige color. The prepared medium should be clear with a light to medium amber color. Microbiological control involves testing with specific organisms like Escherichia coli (ATCC 25922), which should show no growth, and Streptococcus faecalis (ATCC 19433 and 29212), which should exhibit good growth.
Storage and Precautions
The powder is very hygroscopic and should be stored at 10-30 °C in a dry environment in its original container, tightly closed. It should be used before the expiry date on the label or until signs of deterioration or contamination are evident. Prepared tubes should be stored at 2-8 °C. The broth contains sodium azide, which is harmful by inhalation and if swallowed, and irritating to the eyes, respiratory system, and skin. Therefore, it is essential to consult the safety data sheet for further details and take appropriate precautions.
Azide Dextrose Broth is an essential medium in environmental and food safety microbiology, offering a reliable and selective method for detecting fecal streptococci. Its ease of preparation and use, combined with stringent quality control measures, ensures its effectiveness in accurately indicating fecal contamination in various samples.