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Media Both Selective & Differential
Some specialized media are both selective and differential, others are either or. The MacConkey’s and Mannitol Salt described above are both selective and differential bacterial growth media.
In addition to MacConkey’s agar only growing Gram-negative bacteria (the selective aspect of the medium), MAC has special additives that cause lactose fermenting bacteria (microbes that can
metabolize the sugar lactose) to grow in pink colonies, whereas Gram-negative non-lactose fermenting bacteria will grow in colorless colonies.
Mannitol Salt is also both selective and differential. This medium only grows salt-loving bacteria (so it is selective). In addition, bacteria that grow on MSA that can ferment mannitol, a sugar alcohol, will turn the medium from its original pink color to a bright, neon yellow.
Sources and Resources
Bauman, R. (2014) Microbiology With Diseases By Taxonomy, 4th ed., Pearson Benjamin Cummings.
Schauer, Cynthia (2009) Applied Microbiology HCR120 Laboratory Manual, Kalamazoo Valley Community College.
1. Sterile plate of MacConkey's Agar (MAC); 2, LAC- Salmonella on left, LAC+ E. coli on right, both plates are MAC; 3. Happy LAC+ microbe on MAC; 4. Lac+, Gram- microbe growing on MAC, 5. Colony directly under word "MacConkey's" os a colorless non-lactose fermenter. Click here for more MacConkey's Agar Images.
1. Sterile plate of Mannitol Salt Agar (MSA), 2. Staphylococcus epidermidis growing on MSA, 3 Mannitol + bacteria on left plate, Mannitol - bacteria on right plate, both plates are MSA; 4. Mannitol + bacteria on left, Mannitol - bacteria on right, both plated together on one MSA plate; 5. Same plate from bottom. Click here for more Mannitol Salt Agar Images.
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This color change is clinically significant in that Staphylococcus aureus, a disease-causing bacterium, is a mannitol fermenter, while S. epidermidis, a beneficial bacteria that normally grows on the surface of the skin and mucous membranes, is not a mannitol fermenter.