Selective and Differential Tests Flashcards
A media that encourages the growth of some organisms but suppresses the growth of others
Selective Media
This media causes an observable change (color or change in pH) in the medium when a particular biochemical reactions occurs. So most bacteria will grow, but they will grow different colors based on their interaction with the media
Differential Media
This media contains special nutrients that allow growth of a particular organism that might not otherwise be present in sufficient numbers.
Enrichment Media
Used for the isolation and differentiation of Staphylococcus Aureus
Mannitol Salt Agar (MSA)
What makes the MSA agar selective
Sodium Chloride (Salt)
Staphylococcus bacteria thrive in high salt concentrations. But what happens to other bacteria such a E.coli
They Dehydrate (shrivel) and die
What makes the MSA agar differential
The Mannitol
If Staphylococcus bacteria ferment the Mannitol what color will the growth be
Yellow
If Staphylococcus bacteria do not ferment the Mannitol what color will the growth be
Pink
When Mannitol is fermented what happens to the pH in the media
Because acids are released the pH goes down
Used to isolate and differentiate members of enterobacteriaceae based on the ability to ferment lactose
MacConkey Agar (MAC)
What makes the MAC agar selective
Bile salts and crystal violet
In MAC agar what type of bacteria are inhibited from growing
Gram positive bacteria
In MAC agar what makes the media differenital
The lactose
When bacteria such as E.coli ferment lactose what color is the growth on a MAC agar plate
Pink
Used for the isolation of fecal coliforms
Eosin Methylene Blue Agar (EMB)
What sugars are present in an EMB agar plate
Sucrose and Lactose
What type of bacteria are inhibited from growing on an EMB plate
Gram positive bacteria
What components of the EMB plate make it selective
Eosin Y Dye and Methylene Blue
What type of bacteria typically grow with a metallic green sheen on an EMB plate?
E.coli
What color will the bacteria grow on an EMB plate if they ferment sucrose?
They retain the color of the medium.
What color will the bacteria grow on an EMB plate is they ferment lactose?
Depending on the bacteria it could be dark purple, black or metallic green.
The citrate test is used to determine the ability of an organism to use citrate as its sole source of _____
Carbon
The enzyme _______ will transport citrate to the cell and perform citrate fermentation
Citrate Permease
Will the pH of the Citrate media increase or decrease after citrate has been utilized
Increase
A citrate positive test will turn which color (assuming you add enough bacteria)
Blue
During a urea test you are testing bacteria to see if they have what enzyme?
The enzyme urease
How long do you have to read the results of a urea test?
24 hours
Will the pH of the media increase or decrease when urea is broken down?
Increase
What color is a positive urea test?
Pink
Allows for differentiation of bacteria based on their ability to hemolyze RBCs
Blood Agar
The complete destruction of RBCs and hemoglobin, results in a clear growth around the colonies
Beta Hemolysis
The partial destruction of RBCs and produces a greenish discoloration of agar around the colonies.
Alpha Hemolysis
Non-hemolysis and appears as simple growth with no change to the medium.
Gamma Hemolysis
