![]() ![]() If a bacterium is able to grow on MSA (has visible colonies) but the medium stays pinkish-red, then it is not capable of fermenting mannitol since the medium doesn't change color. aureus streak in the left half of the plate shows growth of the bacterium and the yellow color indicating mannitol fermentation, while on the right half of the plate, Streptococcus durans is not capable of growing on this media (because it cannot tolerate high salt concentration) and thus there are no colonies and the media remains the same color, a pinkish red. Bacteria that can grow at high salt concentration and ferment mannitol to acid can be identified on MSA by color change, such as shown on this image 3- S. aureus are differentiated from colonies of bacteria that do not ferment mannitol. Thus, the mannitol-fermenting colonies of S. The medium also contains the pH indicator, phenol red, when the pH drops, such as when mannitol in the medium is fermented to acids, it will change color from red to yellow. If you wanted to isolate the common bacterium, Staphylococcus aureus and know that it has a tolerance for high concentrations of sodium chloride and can ferment the carbohydrate mannitol to form acids, you can use a selective-differential media called “Mannitol Salt Agar or MSA” which contains 7.5% sodium chloride, and will discourage the growth of competing microbes and thus select for (or favor the growth of) S. In some cases, selective and differential characteristics are combined into a single medium. Image by Rebecca Buxton, University of Utah, Salt Lake City, UT. The presence of beta-hemolytic colonies (clear zones around small colonies) indicates the possibility of Streptococcus pyogenes infection. Image 2: Normal Upper respiratory flora mixed with Streptococcus species. These species, such as Streptococcus pyogenes, that causes strep throat, will show a clear ring around their colonies where they have lysed the surrounding blood cells. Blood agar (which contains red blood cells/RBCs) is a medium often used to identify bacterial species that destroy RBCs. typhi).ĭifferential media makes it easier to distinguish colonies of your desired microorganism from other colonies growing on the same plate. Bismuth sulfite inhibits Gram positive bacteria and most Gram negative intestinal bacteria (other than S. For example, bismuth sulfite agar can be used to isolate Salmonella typhi from feces. Selective media suppresses the growth of unwanted bacteria and encourage the growth of desired microbes. Here’s a recap: In clinical microbiology, it is often important to detect the presence of specific microbes associated with disease or poor sanitation, for this task, selective and/or differential media are used. Back in Module 2, you were introduced to this idea. One of the common methods to identify bacteria is through the use of specialized media. \)īacterial Identification with Culture media ![]()
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