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MacConkey Agar with Sorbitol, SMAC
Used for selective isolation and differentiation of enterohemorrhagic (EHEC) E. coli 0157:H7-strains from foodstuffs and stool.
Catalog Number: AS-1284
A major pathogen linked to hemorrhagic colitis and hemolytic uremic syndrome, Escherichia coli O157:H7, is mainly isolated and differentiated using MacConkey Sorbitol Agar (MSA), a selective and differential culture medium.
The medium is selective because it contains crystal violet and bile salts, which inhibit Gram-positive organisms from growing. Essential nutrients for bacterial growth are provided by peptone, while osmotic equilibrium is preserved by sodium chloride. The main carbon source in MacConkey sorbitol agar is sorbitol, which takes the place of lactose. Neutral red, a pH indicator, is responsible for the distinct characteristics of MSA. Usually sorbitol-negative, E. Coli O157:H7 forms colorless colonies on MSA. On the other hand, most other strains of E. coli ferment sorbitol, producing acid and changing its color to pink as a result.
It is important to remember that although MSA is a useful tool for identifying E. Coli O157:H7 presumedly, more biochemical and serological testing is necessary for a conclusive diagnosis.
Storage
Keep the container at 15-30 °C and prepared medium at 2-8 °C.
| Composition | gr/L |
| Peptone | 20 |
| Sorbitol | 10 |
| Bile Salts Mixture | 1.5 |
| Sodium Chloride | 5 |
| Neutral Red | 0.03 |
| Crystal Violet | 0.001 |
| Agar | 15 |
| Final pH at 25°C | 7.1 ± 0.2 |
| Dehydrated Appearance | Pinkish beige, free-flowing, homogeneous. |
| Prepared Appearance | Reddish purple, slightly opalescent. |
| Reaction of 5.15% Solution at 25°C | pH 7.1 ± 0.2 |
| incubate at 35 ± 2 °C for 18 to 24 hours. | |||
| Organism (ATCC) | Recovery | Colony color | Bile PPT. |
| Escherichia coli (25922) | Good | Pink to red | + |
| Enterococcus faecalis (29212) | Marked to complete inhibition | – | – |
| Escherichia coli O157:H7 (35150) | Good | Colorless | – |
