Fish Microbiology and Quality Assurance

2. Methyl Red (MR) Test

This test detects the organisms that do not convert glucose into neutral products, but ferment glucose to acidic end products which can be demonstrated by lower of pH in growth medium. Because of this low pH, the methyl red indicator will turn to red colour indicating positive reaction. Negative reaction is indicated by development of yellow color.

Procedure:
Inoculate the organisms into the Glucose Phosphate Peptone (GPP) broth and incubate at 37 ⁰C for 48 to 96 h. After the incubation, add a few drops (about 5 drops for 2.5 ml of culture) of alcoholic solution of methyl red and read the results immediately. Positive reaction is indicated by the culture broth turning to red colour and yellow colour is a sign of negetaive reaction.

Glucose Phosphate Peptone (GPP) broth

Peptone : 7.0 g
Glucose : 5.0 g
Sodium chloride : 5.0 g
Dipotassium hydrogen phosphate: 5.0 g
Distilled water : 1000.0 ml
pH : 7.1 ± 0.1

Heat to dissolve the ingredients and distribute in test tubes and sterilize at 110⁰C for 10 minutes.

Alcoholic Methyl Red

Methyl red : 0.1 g
Alcohol (95 %) : 300.0 ml
Distilled water : 200.0 ml

Dissolve methyl red in alcohol and add distilled water to the alcohol-indicator mixture. Store the reagent at 4⁰C.