Science Buddies: "Ask an Expert"

I am doing my experiment on which is more effective, UV light water purification or iodine water purification tablets on stream water.... I treated 3-500ml samples of stream water with each purification method then took one drop (22drops/ml) of the treated water and spread it on a petri dish. The dishes have been sitting for three days now and have growth; how do I count the growth? I dont know what (?) bacteria it is but will be taking note (smooth, yellow shiny dots...). I have looked at some other posts and found some useful information in the Interpreting Plates guide. I am not a strong math person so I am wondering if I perform a "grid" count, then what do I multiply it by to get the # of bacteria in my 500ml sample? Thank you for your help! =D

Bacteria are way too small to count individually under a microscope and then multiply (that's the grid method, which I've used before for animal cells). You might be able to count them by measuring the area of the different colonies and adding up the areas. If you do this, and when you work with bacteria anytime, please wear protective gloves, goggles, and a lab coat/apron. Bacteria are dangerous and can harm you, so make sure you not only protect yourself, but clean up after you are done working with the bacteria (preferably use something like alcohol, which will kill left over bacteria).

Probably, you can also get a much more accurate measurement if you contact a local university or hospital and ask them to use their equipment to count the bacteria.

let me know if you have any more questions

good luck,
scienceexpert123

I was hoping to count Colony Forming Units rather than indv. bacteria....since I am performing all my stuff at home and dont have a microscope and "eye-balling" it.

Counting colony usually does not require one to have any special lab equipment, here is a procedure in quantitative urine culture method which can be apply to other type of fluid.

Quantitative Cultures
Quantitative cultures should be performed on all urine specimens so that the number of bacteria per milliliter of urine can be determined and expressed as CFU/ml. The most common method uses a calibrated platinum loop that delivers 0.001 ml or 0.01 ml of urine. Each plate is inoculated with one loopful of urine, and the colony count is determined by multiplying the number of colonies by the dilution factor. For example, if a 0.001 ml loop is used and 10 colonies are observed, the colony count would be 10 x 10000 or 10,000 CFU/ml (104 CFU/ml). If a 0.01 ml loop is used and 10 colonies are observed, the colony count would be 10 x 100 or 1000 CFU/ml (103 CFU/ml). Use the following procedure to perform a quantitative culture of urine sing a calibrated loop
1. Mix the urine well
2. Vertically insert a flamed and cooled calibrated loop into the specimen and immerse it just below the surface of the specimen. Only move the loop straight up and down
3. Remove a loopful of urine and inoculate each plate by making a straight line down the center and then a series of close perpendicular streaks throught the first line. Inoculate each plate with one loopful of urine

Michael

Hi there,

The other experts have given you some good advice. I just wanted to add to what they have said.

If you can see individual colonies on your plates, then you can just count the colonies. Each colony usually started from a single bacterium, which then multiplied. It is always good to take notes on the appearance of the colonies (e.g., smooth, yellow, shiny), because different types of bacteria look different when they grow.

If there are too many colonies to count, you can draw lines with a marker to divide the plate into four, count the number of colonies in one section, and multiply that by four.

If you cannot distinguish the individual colonies, you can describe how much of the plate is covered by bacteria.

Let us know if you have more questions.

Heather