SS Selective Agar, Improved, (Twin Pack) (Salmonella Shigella Selective Agar, Improved (Twin Pack))

Intended Use

Recommended for the selective detection and isolation of Salmonella & Shigella species.

Ingredients

Final pH (at 25°C): 7.4±0.2

**Formula adjusted, standardized to suit performance parameters

Directions

Suspend 81.93 grams in 1000 ml purified / distilled water. Add 4.6 ml of Part B. Boil with frequent agitation to dissolve the medium completely. DO NOT AUTOCLAVE OR OVERHEAT. Overheating may destroy selectivity of the medium. Cool to 45-50°C. Mix and pour into sterile Petri plates.

Principle And Interpretation

SS Selective Agar, Improved is recommended as selective medium for the isolation of Salmonella as well a Shigella species from clinical specimens (1,2,3). It provides significantly greater sensitivity and specificity in the detection of both the organisms. The other selective medias like HE, SS and XLD largely fail to suppress the growth of Salmonella interfering organism like Citrobacter and Proteus which resemble the presence of Salmonella (4).

It is a highly nutritious medium which provide essential growth nutrients like nitrogen and vitamins. Though the sugars xylose, lactose and sucrose provide sources of fermentable carbohydrates, xylose is mainly incorporated into the medium since it is not fermented by Shigella but practically by all enterics. This helps in the differentiation of Shigella species. Degradation of xylose, lactose and sucrose to acid causes phenol red indicator to change its colour to yellow. Addition of Cellobiose controls the growth of false positive Salmonella suspect (5). Lysine is included to differentiate the Salmonella group from the non-pathogens. Salmonella rapidly ferment xylose and exhaust the supply. Subsequently lysine is decarboxylated by the enzyme lysine decarboxylase to form amines with reversion to an alkaline pH that mimics the Shigella reaction.

Bacteria that decarboxylate lysine to cadaverine can be recognized by the appearance of a red colouration around the colonies due to an increase in pH. Sodium thiosulphate is reduced by certain species of enteric organisms to sulphite and H₂S gas and this reductive enzyme process is attributed by thiosulphate reductase. Production of H₂S gas is detected as an insoluble black precipitate of ferrous sulphide, formed upon reaction of H₂S with ferric ions or ferric citrate, indicated in the centre of the colonies. Part B addition to the medium helps in improving the selectivity of the medium. It selectively inhibits the growth of gram positive organisms. The growth of Proteus species is also reduced.

Type of specimen

Clinical samples - Feces ; Food samples.

Specimen Collection and Handling

For clinical samples follow appropriate techniques for handling specimens as per established guidelines (6,7). For food samples, follow appropriate techniques for sample collection and processing as per guidelines (8). After use, contaminated materials must be sterilized by autoclaving before discarding.

Warning and Precautions

In Vitro diagnostic Use only. 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. The medium is highly selective and may be toxic to certain Salmonella or Shigella species. Hence it is recommended to use to inoculate plates of less inhibitory media parallel to SS Agar, such as Hektoen Enteric Agar (M467) or Deoxycholate Citrate Agar (M065) for easier isolation of Shigella species (8).

Quality Control

Appearance Part A: Light yellow to pink homogeneous free flowing powder. Part B: Colourless to pale yellow liquid

Gelling Firm, comparable with 1.35% Agar gel

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

Reaction Reaction of 8.65% w/v aqueous solution of Part A and 0.46 ml of Part B at 25°C. pH : 7.4±0.2

pH 7.20-7.60

Cultural Response Cultural characteristics observed after incubation at 35-37°C for 18-48 hours. Recovery rate is considered as 100%for bacteria growth on Soyabean Casein Digest Agar.

Key : (*) Corresponding WDCM numbers. (#) Formerly known as Enterobacter aerogenes

Storage and Shelf Life

Store between 10-30°C in a tightly closed container and the prepared medium at 20-30°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 (6,7).

Reference

1. Mallinson, E.T.et al.2000.Improved plating media for the detection of Salmonella species with typical and atypical hydrogen sulfide production.J.Vet.Diagn.Invest.12:83-87.
2. Mallinson, E.T.1991.Novelsalmonelladetectionsystemdeveloped;combines increased reliability,practicality.Feedstuffs 63:40-44.
3. Miller,R.G.et al.1991.Xylose-Lysine-tergitol 4:an improved selective agar medium for the isolation of Salmonella Poult.Sci.70:2429-2432.Erratum,Poult.Sci.71:398,1992.
4. Pollock, H.M. and B.J.Dahlgren.1974,Clinical evaluation of enteric media in the primary isolation of Salmonella and Shigella. Appl.Microbiol.27:197-201.
5. Miki,K.et al.1996.Re-speciation of the original strains of serovars in the Citrobacter freundii (Bethesda-Ballerup group) antigenic scheme of West and Edwards.Microbiol.Immunol.40:915-921.
6. Isenberg, H.D. Clinical Microbiology Procedures Handbook 2nd Edition.
7. 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.
8. Salfinger Y., and Tortorello M.L. Fifth (Ed.), 2015, Compendium of Methods for the Microbiological Examination of Foods, 5th Ed., American Public Health Association, Washington, D.C.