To calculate the cfu discrete colonies are required to get that gives the information that how many microbial cells we are going to inoculate into the plate. but for study of plates suppose for antibacterial study, bacterial growth should be homogenous over the media. so, how both of these things can be corelated?
These are two different measurements.
CFU are measured to study killing kinetics and minimal bactericidal concentration (MBC). The sample of bacteria in liquid culture is treated with the antibacterial substance for various amounts of time, then 10-fold serially diluted and plated on nutrient agar. The serial dilution assures that one of the dilutions will result in a suitable number of colonies on the plate that can be counted.
For measuring the antibacterial activity of a substance, one common method is the measure the diameter of the cleared area around a disc containing the substance, placed on a lawn of bacteria on a nutrient agar plate. In this measurement, the number of bacteria plated must be high enough that a single continuous growth of bacteria is on the plate, rather than a small number of colonies.
The bacterial suspension must undergo an appropriate dilution and spreading must be uniform throughout the Petri dish to give distinct colonies
Mr. Adam B Shapiro Can you tell me that for measuring antibacterial activity of a substance by disc diffusion method how can i calculate that how many bacteria i am going to plate on media?
It will depend on several factors, such as the strain, their growth phase when plated, the medium, and the size of the plate. A simple experiment with your conditions, trying out a set of serial dilutions, will give you the answer.
Effectively, for a good disk diffusion susceptibility test you should have confluent growth on the plate, so you can't count colonies. The way to know the bacterial concentration you are plating is comparing the turbidity of the broth you are using with the McFarland scale,which correlates with specific bacterial concentrations
EXPECTED VALUES
Standard No. 0.5 1.0 2.0 3.0 4.0 5.0
Approximate Cell Density (x 108/ml) 1.5 3.0 6.0 9.0 12.0 15.0
can you give me the detail information of Mc farland scale and its calculation procedure?
There are McFarland scales available commercially, made with latex suspensions. If you prefer to prepare the scale by yourself, you only need 1% barium chloride and 1% sulfuric acid. I'm sending you the proprtions, and the equivalence of each point of the scale in Colony Forming Units/ml.
McFarland Scale CFU (x106/mL) 1% BaCl2/ 1% H2SO4 (mL)
0.5
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