|MadSci Network: Microbiology|
Kim - This is a very good question, and is of importance when you need an idea of how many bacteria are actually present. As you may have already seen, temperature will have marked effects of bacterial growth. Microbiologists base their counting on the assumption that one single bacteria can give rise to one colony on solid media. Each colony is referred to as a Colony Forming Unit or CFU for short. If you are measuring bacteria in water, a known amount would be plated out (i.e. 1 ml of water/ agar plate). Once colonies are visible, you can count these (say you count 100 colonies) and then express your final value as 100 CFU/ml of water. Another word on counting. If you have so many bacteria that you begin to loose count, divide the plate into quarters, halves, etc. and count one section. To get the total number, multiply back by how many divisions the plate was split into (i.e. 100 CFU in one quarter of the plate x 4 = 400 CFU total). If this is not feasible for your experiments, you could just express them as CFU. You could count the different types of colonies based on their morphology (shape) and color and graph these CFU. If you have access to different agar media, you could also compare how your bacteria grow on these. Some bacteria require many special nutrients for growth (fastidious) while others can grow with limited nutrients (non-fastidious). For graphing purposes, CFU are usually expressed on a log scale. So 400 CFU would be equal to 2.60 CFU (on a log scale). The reason that we use a log scale is because bacteria multiply by fission and therefore undergo exponential growth. Numbers of bacteria can be as high as 10,000,000,000 CFU/ ml, which is easily expressed as 10 CFU/ml on a log scale. As far as your studies go, you may classify your bacteria into three classes based on their temperature requirement. The three general classes are psychrophiles (<7 C - 20 C), mesophiles (22 C - 45 C), and thermophiles (45 C - 90 C!). These ranges are pretty rough, but from these you may conclude that you may find psychrophilic bacteria in your refrigerator while you may find mesophilic bacteria in your mouth. Hope that this information has helped you! Good luck! Glynis Kolling Graduate Student Food Microbiology
Try the links in the MadSci Library for more information on Microbiology.