Schiemann first described Yersinia selective agar as an alternative to MacConkey agar and other media commonly used for isolating Yersinia Enterocolitica, a causative organism of gastroenteritis. Yersinia Enterocolitica, a major food- or waterborne enteric pathogen, has been reported to cause epizootic outbreaks in animals such as diarrhea, lymphadenopathy and pneumonia. Yersinia Selective Agar, a selective and differentiated medium that supports the growth of Y. Enterocolitica and other Yersinia spp.
Principle of Yersinia Selective Agar
The medium distinguishes between non-fermenting and mannitol fermentation bacteria. The medium is affected by microorganisms which ferment sugar mannitol acid. This causes a drop in pH. The colonies turn red when there is neutral red. A zone of precipitated liver may be found due to the pH drop. Mannitol-negative organisms form translucent and colorless colonies. The medium is selective because of the presence of crystal violet and sodium deoxycholate, which both inhibit gram positive and some gram negative bacteria.
Yersinia can be made more selective by adding antibiotic supplement. Y. enterocolitica colonies will often form dark red colonies resembling bullseye. These colonies are usually surrounded by a transparent boundary. Different serotypes may have different sizes, smoothnesses and ratios of the border to the center diameter. The medium’s osmotic equilibrium is maintained by Sodium Chloride. Magnesium Sulfate and Sodium Pyruvate are stimulators of organism growth. Agar acts as a solidifying agent.
Composition of Yersinia Selective Agar
Final pH (at 25°C): 7.4±0.2
Note: Yersinia Selective Agar is used with cefsulodin and novobiocin for the isolation of Yersinia enterocolitica in a laboratory setting.
Preparation and Method of Use of Yersinia Selective Agar
- Take 29.02 grams and 500 ml of distilled water.
- To dissolve the medium completely, heat to boiling
- For 15 minutes, sterilize by using an autoclave at 15 lbs pressure (121degC).
- Cool the mixture to 45-50°C. Aseptically add 1 vial of Yersinia Selective Supplement, or 10 mL sterilized aqueous solution containing 4.5 mg Cefsulodin & 2.5 mg Novobiocin.
- Before pouring into sterilized Petri dishes, mix well.
- After the specimen arrives in the laboratory, you should immediately begin to treat it. The streak plate is used to separate pure cultures from mixed flora specimens.
- Alternativly, culture the material directly with a swab. Roll the swab across a small area at the edge of the surface and then streak from that inoculated area.
- For 24-48 hours, incubate the sample at 22-32degC or suspend it (food, feces etc.). Incubate in sterile Phosphate Buffer Saline for up to 21 days at 4degC
- Subculture periodic samples onto Yersinia Agar Plate, and incubate according to the instructions.
Result Interpretation on Yersinia Selective Agar
Yersinia Enterocolitica colonies are translucent or translucent with dark pink center. The edges of colonies can be either straight or uneven. After 48 hours of incubation, colonies turn dark pink and have a translucent border. They may also be surrounded with precipitated bile. Non-Yersinia organisms are markedly inhibited from growing.
|Yersinia enterocolitica||Translucent with dark pink center & bile precipitate (bulls-eye colonies)|
|Yersinia pseudotuberculosis||Growth with dark pink center & bile precipitate. No transparent zone around the colonies.|
Uses of Yersinia Selective Agar
- It is highly recommended to isolate and count Yersinia Enterocolitica from food samples and clinical specimens.
- Yersinia Selective Agar can also be used to isolate Yersinia species not Y. enterocolitica.
Limitations of Yersinia Selective Agar
- Serratia liquefaciens and Citrobacter freundi may resemble Y. Enterocolitica, which can be further confirmed by biochemical testing.
- MacConkey Agar or another less selective medium should be used to inoculate the specimen. It should be incubated at 35 +/-2 degrees C for detection of any other pathogens.
- Sometimes, Yersinia Enterocolitica may need to be isolated from stool specimens or other materials using “cold enrichment”.
- For the complete identification of suspect isolates, biochemical and serological verification is required.
- Yersinia Selective Agar should not be used in diagnosing any disease or other condition in humans.
- Some strains may fail to grow or grow well on this medium due to nutritional variation. Additional tests are required to confirm the existence of Yersinia spp.
- Complete medium does not limit growth of Yersinia frederiksenii and Y. kristensenii as well as Y. intermedia. These organisms should be distinguished from Y. enterocolitica by having additional characteristics.