Cat. no. G50 SS Agar, 15x100mm Plate, 18ml 10 plates/bag
Cat. no. J139 HE Agar/SS Agar, 15x100mm Biplate, 10ml/10ml 10 plates/bag


Hardy Diagnostics SS Agar is recommended for use as a selective and differential medium for the isolation of Salmonella and some Shigella species from clinical and non-clinical specimens.


SS Agar was originally developed as a selective medium for the isolation of Salmonella and Shigella species. It was also developed to aid in the differentiation of lactose and non-lactose-fermenters from clinical specimens, suspected foods, and other such samples.

The basis for differentiation on SS Agar depends on the fermentation of lactose and the absorption of neutral red as the bile salts precipitate in the acidic condition. Neutral red turns red in the presence of an acidic pH, thus showing fermentation has occurred. The inclusion of bile salts, sodium citrate, and brilliant green serve to inhibit gram-positive and coliform organisms. Salmonella, Shigella, and other non-lactose-fermenting organisms appear as transparent or translucent colorless colonies on SS Agar. Sodium thiosulfate is added to the medium as a hydrogen sulfide source, and ferric citrate is added as an indicator for hydrogen sulfide production.


Ingredients per liter of deionized water:*

Lactose 10.0gm
Bile Salts No. 3 8.5gm
Sodium Citrate 8.5gm
Sodium Thiosulfate 8.5gm
Beef Extract 5.0gm
Proteose Peptone 5.0gm
Ferric Citrate 1.0gm
Brilliant Green 0.33mg
Neutral Red 0.025gm
Agar 13.5gm

Final pH 7.0 +/- 0.2 at 25ºC.

* Adjusted and/or supplemented as required to meet performance criteria.


Storage: Upon receipt store at 2-8ºC. away from direct light. Media should not be used if there are any signs of deterioration (shrinking, cracking, or discoloration), contamination, or if the expiration date has passed. Product is light and temperature sensitive; protect from light, excessive heat and freezing.



Specimen Collection: Consult listed references for information on specimen collection.(1-3,5,7) Infectious material should be submitted directly to the laboratory without delay and protected from excessive heat and cold. If there is to be a delay in processing, the specimen should be inoculated onto an appropriate transport media and refrigerated until inoculation.

Method of Use: Allow the plates to warm to room temperature and the agar surface to dry before inoculating. Heavily inoculate and streak the specimen as soon as possible after collection. If the specimen to be cultured is on a swab, roll the swab over a small area of the agar surface. Streak for isolation with a sterile loop. Incubate plates aerobically at 35-37 degrees C. for 18-24 hours. Examine colonial morphology.


If lactose fermentation occurs, the medium will turn red due to the acidic pH. Salmonella, Shigella,and other non-lactose fermenters appear as transparent or translucent colorless colonies on SS Agar. Colonies of Salmonella spp. may appear with or without black centers.

Consult listed references for the identification of colony morphology and further biochemical tests required for identification.(1-3,5,7)


The incorporation of brilliant green into this medium makes it highly selective, and has been shown to inhibit the growth of some Shigella species. Thus, a nonselective but differential medium such as MacConkey Agar or HE Agar (Cat. nos. G35 or G63, respectively) should also be streaked to increase the recovery of fastidious, or low numbers of, gram-negative organisms.

The bile salts may crystallize over time. They appear as small spider-like puff balls within the medium and do not affect the performance of the medium.

Some strains of Shigella, such as S. sonnei and S. dysenteriae serovar 1, may ferment lactose relatively slowly, and colonies change to lactose-fermenting after cultivation for 2 or more days.(8)


Standard microbiological supplies and equipment such as loops, swabs, applicator sticks, other culture media, incinerators, and incubators, etc., as well as serological and biochemical reagents, are not provided.


Test Organisms Inoculation Method* Incubation Results
Time Temperature Atmosphere
Salmonella enterica
ATCC® 14028
A 18-24hr 35°C Aerobic Growth; colorless colonies with or without black centers
Shigella flexneri
ATCC® 12022
A 18-24hr 35°C Aerobic Growth; colorless colonies
Escherichia coli
ATCC® 25922
B 18-24hr 35°C Aerobic Partial to complete inhibition; pink to rose red colonies with precipitate
Enterococcus faecalis
ATCC® 29212
B 18-24hr 35°C Aerobic Inhibited


Physical Appearance

SS Agar should appear clear, and red-orange in color.

S. typhimurium growing on SS Agar

Salmonella enterica (ATCC® 14028) growing on SS Agar (Cat. no. G50). Incubated aerobically for 24 hours at 35ºC.

S. flexneri growing on SS Agar

Shigella flexneri (ATCC® 12022) growing on SS Agar (Cat. no. G50). Incubated aerobically for 24 hours at 35ºC.

E. coli growing on SS Agar

Escherichia coli (ATCC® 25922) colonies growing on SS Agar (Cat. no. G50). Incubated aerobically for 24 hours at 35ºC.

SS Agar

Uninoculated plate of SS Agar (Cat. no. G50)


1. Anderson, N.L., et al. Cumitech 3B; Quality Systems in the Clinical Microbiology Laboratory, Coordinating ed., A.S. Weissfeld. American Society for Microbiology, Washington, D.C.

2. Tille, P., et al. Bailey and Scott's Diagnostic Microbiology, C.V. Mosby Company, St. Louis, MO.

3. Isenberg, H.D. Clinical Microbiology Procedures Handbook, Vol. I, II & III. American Society for Microbiology, Washington, D.C.

4. MacFaddin, J.F. 1985. Media for Isolation, Cultivation, Identification, Maintenance of Bacteria, Vol. I. Williams & Wilkins, Baltimore, MD.

5. Versalovic, J., et al. Manual of Clinical Microbiology. American Society for Microbiology, Washington, D.C.

6. Quality Assurance for Commercially Prepared Microbiological Culture Media, M22. Clinical and Laboratory Standards Institute (CLSI - formerly NCCLS), Wayne, PA.

7. APHA Technical Committee on Microbiological Methods for Foods. Compendium of Methods for the Microbiological Examination of Foods, APHA, Washington, D.C.

8. Ito et al. 1991. Possible Mechanisms Underlying the Slow Lactose Fermentation Phenotype in Shigella spp. Appl. and Environ. Microbio. 57(10): 2912-2917.

ATCC is a registered trademark of the American Type Culture Collection.