Selective and Differential Media and Pure Cultures Flashcards
What is a pure culture?
A pure culture is a culture made up of only a SINGLE species. Pure cultures are show in the presence of individual colonies
How do we obtain pure cultures to determine the needed bacteria?
We have to dilute the bacteria. To dilute bacteria we use the streak plate technique
Walk through the streak plate procedure
- incinerate loop
- Fill look with bacteria mixture
- streak loop on agar 4-5 times
- incinerate loop
- streak once or twice across zone one and then into zone 2
- Repeat with zones 2, 3, and 4
What is a contaminant in a pure culture?
A contaminant is a bacterium that was not in the original culture
Define selective and differential media
Selective Media: adds salt to the agar so that only certain TYPES of bacteria will grow. Only a certain gram type and shape will grow.
Differential Media: adds sugar to the agar so that we are able to tell the different SPECIES apart. Differentiating certain species of the same gram type and shape
Key words to remember: selective=type
differential=species
What are the four types of bacteria that can be selected for using selective media?
Gram + cocci
Gram - cocci
Gram + rods
Gram - rods
Mannitol Salt Agar (MSA)
What makes it selective, what bacteria is selected?
What makes it differential, what bacteria is differentiated?
A high concentration of salt, sodium chloride, makes MSA selective.
MSA is selective for Gram + cocci.
Mannitol, a type of sugar, makes MSA differential.
MSA is differential for Staph aureus.
MacConkey Agar (MaC) What makes it selective, what bacteria is selected? What makes it differential, what bacteria is differentiated?
Bile salts make MaC selective.
MaC is selective for Gram - rods.
Lactose, a type of sugar, make MaC differential
MaC is differential for E. coli.
What type of patient infection would MSA be best suited for?
A skin infection as Staph aureus is the major bacteria that is responsible for many skin infections. MSA mimics the skin as it is extremely salty. The salty environment is similar to the environment present on the patient’s skin.
What type of patient infection would MaC be best suited for?
Urinary tract infections are often caused by E. coli from a patient’s own body as it is found in the GI tract. MaC is differential for E. coli and is easily identifiable with MaC.
What is Tryptic Soy Agar (TSA)?
A very generic form of agar. It grows most bacteria but does not help with determining bacteria due to this nature
What does Staph aureus do to MSA
Ferments the mannitol in turn producing waste products. The waste products turn the part of the plate affected bright yellow.
What does E. coli do to MaC
E. coli can ferment lactose. When an indicator dye, CV, is added the color will change in the presence of of waste products. The E. coli will suck up the dye and then turn a bright pink after fermentation.
