Bacteriology 102: What Is a Colony-Forming Unit? |
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When we are putting bacterial cells in contact with the medium in a petri plate – such as when we inoculate a certain amount of a natural sample into a plate or use a loop to streak a plate from a sample or culture (just to give two of many possible examples) – we expect the microscopic cells to multiply and ultimately form masses of cells visible to the naked eye. These macroscopic masses are called colonies, having been formed by cells which are able to utilize the nutrients in the medium under the conditions in which the plates are incubated. Besides nutrients, we have to consider temperature, oxygen availability, and other factors that are conducive for metabolism. 
As stated in the introduction to Experiment 1 in the lab manual: "When we observe colonies, we cannot assume each arose from just one cell originally planted on the medium, however. A pair, chain or cluster of cells which 'land' on the medium in close proximity to each other can multiply and produce a single colony. Thus, we use the term colony-forming unit when we consider the common origin for the cells of any colony." This term is usually abbreviated CFU. In the diagram at right (wherein we exaggerate the size of the cells as well as the amount of liquid in the tube), we have – as an example – a test tube containing one ml of a water sample which contains some bacterial cells, and we pour that one ml into a plate in order to determine something about the number of viable bacteria in that one ml sample of water. (Normally we would not know how many cells we are dealing with, nor how they are distributed on the plate that we inoculate, as we use this method to detect viable cells only by the visible colonies they ultimately produce.) Notice that in this scenario, three of the cells (#2, 3 and 4) are in a chain, and they land together; also two cells (#5 and 6) land close to each other. We also have a viable cell (#7) that is unable to metabolize at all under these conditions; perhaps it needs something that is not present in the medium, or the incubation temperature is too hot or cold, or there is some other reason. The cells able to metabolize and reproduce (usually by binary fission where 1 cell forms 2, the 2 form 4, the 4 form 8, etc.) ultimately form colonies during the incubation period. However, as you see, the common origin for the cells of any colony can be more than one cell, such as that chain of cells (#2, 3 and 4) or the two cells which "start out" very close together (#5 and 6). By finding three colonies arising from having inoculated that one ml of water, we cannot therefore say that there were three cells per ml of that water sample. We can only say that there were three colony-forming units per ml. Whether or not colonies are counted, they still arise from CFUs nonetheless. So "colony-forming unit" is not a term that is restricted only to quantitation. On occasion, cells of different species can constitute a CFU, and the resulting colony would then be considered a mixed culture. Hence, thorough mixing of the sample and good isolation methods are essential. Re-streaking isolated colonies on a non-selective medium can enhance the detection of different organisms. |
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Page last modified on 6/17/01 at 6:00 PM, CDT. |
