Expert: Paras Anand - 1/12/2009

Question
QUESTION: How do you measure the growth of bacteria?
How do you measure growth of inhibition of bacteria?
How many days do you leave bacteria in incubator?


ANSWER: The most common method for measuring the growth of bacteria is taking the optical density (OD) of the culture at 600nm wavelength. Another method is to plate the bacteria on the agar plates and count the colony forming units (CFU).

The growth of inhibition of bacteria can be checked if there is less OD or fewer colonies in the treated samples than in untreated controls.

For normal E. Coli, the colonies can be seen after overnight in incubator (16-18h).

---------- FOLLOW-UP ----------

QUESTION: Thank you so very much for the help.
Is there any device available to count the colonies or do we use just a ruler?
Can you please tell us how many days do we leave Staphylococcus epidermidis and Pseudomonas fluorescens to incubate?
Thanks much


Answer
There are devices present but unfortuanately they are quite costly at the moment. The easiest method is to plate the bacteria at different dilutions (diluting 10 times every time) and plating them. And then you just count by eye and make calculations to reach at the number of bacteria/ml. Please see the attached image taken from web showing E.Coli colonies.

For Staphylococcus its overnight incubation. I think it may be the same for Pseudomonas.