Hey, does any one know how many strains are needed to test before a selective/differential media can be confirmed as selective/differential for a particular microorganism?
The simple answer is to test all of the most common strains of the species. Any strain that occurs commonly in environmental or medical samples should be checked for reaction in both selective and differential reactions.
If i get the question rightly, I think, the parameter of strain number is not important, rather, it is better to critically evaluate the metabolic potential of the bacteria and then one can select a particular biochemical reaction to design a differential/selective medium for the same bacteria.
If you are talking about developing a new formulation for selective/differential medium. Then i think you must check the particular biochemical reaction response (evident from such media) with well established bacterial/yeast/fungal cells. in most of the cases, these are type strains which are permanent example of a species. You can purchase these type strains from microbial culture collection centres (example- DSMZ in Germany, www.dsmz.de; MTCC in India, www.imtech.mtcc.res.in etc.). Once you get authentic results you can establish the formulation.
If the the situation is first case then, it is not in the number of strain but It is always best to have positive and negative controls when biochemical reactions are monitored based on growth of bacteria on selective/differential media. Lets take a very simple example, say, MacConkey agar medium. If you see its composition it comes in many formulations. The most straightforward reaction that we monitor is to differentiate lactose fermenters from lactose non-fermenters. Usually lactose fermenting bacteria produce pink color due to acid production from lactose which turns the color to pink due to pH indicator. To monitor the reaction in best way one must have a positive control (E. coli type strain, which is lactose fermenting) and a negative control, (Proteus vulgaris which is lactose non-fermenting). The medium does not supports growth of Gram positive bacteria (due to presence of bile salts and crystal violet in some formulations). So, it may be considered selective for Gram negative group. But I have seen some Bacillus are exception and they can still grow on MAC agar even in presence of bile salt & CV. This is just an example.
However, if the situation is second case, then one must test the formulation with as many well established strains (normally must be procured from an authentic source) as possible. For such case, number of different representative strains giving positive/negative results may be important.
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