Miles and Mishra Enumeration - Surface Drop, Introduction, Formula, Procedure, Disadvantage

Introduction to Miles and Mishra Enumeration

Principle of Miles and Mishra Enumeration

The Miles and Mishra enumeration or Surface Drop method is a Surface Viable Count (SVC) method that involves placing drops of (0.02ml) of test microorganisms on the surface of a solid agar plate medium followed by counting the number of colonies that grow on the inoculated agar after incubation.

This method is mostly followed if the suspected microorganisms are able to grow on the surface of the medium i.e. obligate aerobes or when an opaque medium is used.

The Miles and Mishra enumeration is also called the modified technique to pour plate technique for the total viable count.

Formula of Miles and Mishra Enumeration

To calculate the no. of organisms for Miles and Mishra Enumeration is by:

No. of organisms = (No. of colonies x Dilution Factor) / Inoculum size

OR

No. of organisms = (No. of colonies) / (Dilution Factor x Inoculum size)

Procedure of Miles and Mishra Enumeration

The procedure for Miles and Mishra Enumeration includes the following steps

1. Take sterile dilution blanks with each containing 9ml and mark it as 10^-1, 10^-2,......10^-n

2. Take 1ml or 1 gram from the sample with the help of a sterile pipette and mix it with the first dilution blank measured 10^-1, thus preparing an initial dilution

3. Shake gently to mix the content to obtain uniform distribution of cells

4. From the initial dilution i.e. 10^-1, take 1 ml of suspension by using a sterile pipette and mix it with 10^-2

5. Repeat the procedure until the desired dilution (10^-n) is obtained

6. From selected dilutions, 0.02 ml of the suspension is transferred onto the Nutrient agar plate from a 2.5 cm height

7. Incubate the inoculated agar plates at 35-37°C for 18-24 hours

Enumeration of Miles and Mishra Enumeration

Enumeration method for Miles and Mishra Enumeration is done by using the formula:

No. of organisms = (No. of colonies x Dilution Factor) / 0.02

Disadvantage of Miles and Mishra Enumeration

The disadvantage of Miles and Mishra Enumeration are:

• If the colonies are too crowded or mixed, enumeration may be difficult.

• The moisture on the surface of the media should be dried off before inoculation

• The sample should be dropped onto the media surface from a low height

• The inoculum on the media surface should be absorbed before incubation