O157 Millichrome^™ plus Agar

O157 Millichrome^™ plus Agar is a selective media for the isolation and detection of E. coli O157. The distinguishing feature of these media is the inclusion of a chromogenic substrate mix that produces visible color changes upon enzymatic activity of E. coli O157. Millichrome^™ chromogenic media is a specialized type of culture medium used to detect and isolate specific groups of bacteria based on their ability to produce enzymes with chromogenic substrates. For optimal growth, peptone and yeast extract provide nitrogenous nutrients and essential growth factors. Sodium chloride is crucial for maintaining osmotic balance. The chromogenic mix contains substrates that change color when characteristic enzymes cleave them, allowing for easy differentiation.To create a more selective and specific medium, it is recommended to include a potassium tellurite supplement. If there is expected to be a high load of Proteus spp., cefixime can be added. Similarly, a high load of Pseudomonas and/or Aeromonas can be inhibited by including cefsulodin. Agar serves as the solidifying agent.

O157 Millichrome^™ plus Agar Base is a selective chromogenic culture medium intended for use in the qualitative direct detection, differentiation and presumptive identification of pathogenic biotypes of E. coli O157 from various samples.

Step 1 (Preparation)

• Disperse slowly 29.2 g of powder base in 1 L of purified water.
• Stir until agar is well thickened.
• Heat and bring to boil (100 °C) while swirling or stirring regularly.

DO NOT HEAT TO MORE THAN 100 °C. DO NOT AUTOCLAVE AT 121 °C.
Warning: If using an autoclave, do so without pressure.

Advice 1: For the 100 °C heating step, mixture may also be brought to a boil in a microwave oven: after initial boiling, remove from oven, stir gently, then return to oven for short repeated bursts of heating until complete fusion of the agar grains has taken place (large bubbles replacing foam).
Advice 2: if a more selective, and more specific, medium is needed, add a solution of Potassium Tellurite to obtain a final concentration of 2.5 mg/L at 45-50 °C.
Advice 3: in case of product samples containing a high load of Proteus, Cefixime can be added at 0.025 mg/L at 45-50 °C.
Advice 4 : in case of product samples containing a high load of Pseudomonas and/or Aeromonas, Cefsulodin can be added at 5 mg/L at 45-50 °C.

Step 2 (Pouring plates)

• Cool in a water bath at 45-50 °C, swirling or stirring gently.
• Pour into sterile Petri dishes.
• Let it solidify and dry.

Store prepared media below 8°C, protected from direct light (max. 1 month).Store dehydrated powder, in a dry place, in tightly sealed containers at 2-25°C.

The E. coli O157 is detected by a characteristic mauve color after only 24 h of incubation at an optimal growth temperature of 37°C, while most other E. coli form blue color colonies after incubation at the same temperature

MilliChrome is a trademark of Merck KGaA, Darmstadt, Germany