How do I measure populations of bacteria?

1) Dilution and spread plating to give CFU/ml


2) Measure OD in a spectrophotometer


3) Observation and counting in something like a hemocytometer








#1 gives you an accurate count, but in theory if you have sticky bacteria (stick to each other) it will introduce error as 5 bacteria stuck together will give 1 colony





#1 also takes a day or two (or more) depending on the bacteria, some grow slower than others.





#2 doesn't discriminate between bacteria and other junk that blocks the flow of light such as dead bacteria.





#2 also doesn't give you an actual count, but a number you can use for comparitive purposes. in order to get cfu from a spec reading, that bacteria in that medium needs to be grown, diluted to a range of ODs, plated, counted and those numbers used to make a standard curve that future ODs can be compared to to extrapolate CFU. (hassle).





#3 sucks to do, but it gives you an accurate count, assuming you are able to discriminate between live and dead bacteria in a microscope (some you can, some you can't).

How do I measure populations of bacteria?
That rules me out then.......
Reply:You can dilute the population with a known quantity of diluting agent say by weight. Say 10:1 ratio. Then put a sample of the solution of a known weight on a graduated slide and count the numbers of bacteria per square under the microscope. Then back calculate how many would be in the whole solution. .