Leifson’s Deoxycholate Agar, Modified

Intended Use

Used for isolation and identification of Bacillus species and pathogenic Staphylococci.

Composition**

Final pH (at 25°C): 7.0±0.2

**Formula adjusted, standardized to suit performance parameters

# Equivalent to Beef extract

Directions

Suspend 48.52 grams in 1000 ml purified / distilled water. Heat to boiling to dissolve the medium completely. DO NOT AUTOCLAVE OR REMELT. Excessive heating is detrimental. Cool to 45-50°C. Mix well and pour into sterile Petri plates.

Principle And Interpretation

Leifson Deoxycholate Agar, was originally described by Leifson (1) and further modified by Hynes (2) for selective isolation and differentiation of Salmonella and Shigella species. This medium is the modification of Leifson Agar for the isolation and maximum recovery of intestinal pathogens. Leifson Deoxycholate Agar, Modified is a less selective medium and is used for direct sampling of faeces.

Peptone and HM peptone B provide nitrogeneous and carbonaceous compounds, long chain amino acids and other essential growth nutrients. Sodium citrate and sodium deoxycholate inhibit all gram-positive bacteria and coliforms but allow the gram-negative bacilli to grow. Lactose is added to the medium to allow differentiation of lactose fermenting bacteria such as, Escherichia coli from non-lactose fermenting species, such as Salmonella, Proteus and Shigella.

Lactose fermenting strains grow as red to pink colonies because of absorption of neutral red indicator. Nonfermenting species grow as colourless colonies. Ferric citrate and sodium thiosulphate help in H2S determination.

Type of specimen

Clinical samples - faeces; Food samples

Specimen Collection and Handling:

For clinical samples follow appropriate techniques for handling specimens as per established guidelines (3,4)

For food samples, follow appropriate techniques for sample collection and processing as per guidelines (5,6).

After use, contaminated materials must be sterilized by autoclaving before discarding.

Warning and Precautions :

In Vitro diagnostic Use. For professional use only. Read the label before opening the container. Wear protective gloves/protective clothing/eye protection/face protection. Follow good microbiological lab practices while handling specimens and culture. Standard precautions as per established guidelines should be followed while handling clinical specimens. Safety guidelines may be referred in individual safety data sheets.

Limitations :

1. Each lot of the medium has been tested for the organisms specified on the COA. It is recommended to users to validate the medium for any specific microorganism other than mentioned in the COA based on the user's unique requirement.

Performance and Evaluation

Performance of the medium is expected when used as per the direction on the label within the expiry period when stored at recommended temperature.

Quality Control

Appearance Light yellow to pink homogeneous free flowing powder

Gelling Firm, comparable with 1.5% Agar gel

Colour and Clarity of prepared medium Reddish orange coloured clear to slightly opalescent gel forms in Petri plates

Reaction Reaction of 4.85% w/v aqueous solution at 25°C. pH : 7.0±0.2

pH 6.80-7.20

Cultural Response Cultural characteristics observed after an incubation at 35-37°C for 28-48 hours.

Key: (*) Corresponding WDCM numbers.

Storage and Shelf Life

Store between 10-30°C in a tightly closed container and the prepared medium at 2-8°C. Use before expiry date on the label. On opening, product should be properly stored dry, after tightly capping the bottle in order to prevent lump formation due to the hygroscopic nature of the product. Improper storage of the product may lead to lump formation. Store in dry ventilated area protected from extremes of temperature and sources of ignition. Seal the container tightly after use. Product performance is best if used within stated expiry period.

Disposal

User must ensure safe disposal by autoclaving and/or incineration of used or unusable preparations of this product. Follow established laboratory procedures in disposing of infectious materials and material that comes into contact with clinical sample must be decontaminated and disposed of in accordance with current laboratory techniques (3,4).

Reference

1. Leifson E., 1935, J. Pathol. Bacteriol., 40:581.
2. Hynes M., 1942, J. Pathol. Bacteriol., 40:581.
3. Isenberg, H.D. Clinical Microbiology Procedures Handbook 2nd Edition.
4. Jorgensen, J.H., Pfaller, M.A., Carroll, K.C., Funke, G., Landry, M.L., Richter, S.S and Warnock., D.W. (2015)Manual of Clinical Microbiology, 11th Edition. Vol. 1.
5. American Public Health Association, Standard Methods for the Examination of Dairy Products, 1978, 14th Ed., Washington D.C.
6. Salfinger Y., and Tortorello M.L., 2015, Compendium of Methods for the Microbiological Examination of Foods, 5th Ed., American Public Health Association, Washington, D.C.