Lab 3 Flashcards
What is the first step to obtaining a pure culture
Streak the mixed sample onto Columbia CNA agar
What is the second step of obtaining a pure culture
Streak the mixed sample onto the macconey agar
What is the third step of obtaining a pure culture
Streak the mixed sample onto the TSA (Trypticase Soy Agar)
What is the fourth step of obtaining a pure culture
Obtain isolated colonies: After incubation, pick isolated colonies from each selective medium:
What is the fifth step of obtaining a pure culture
From Columbia CNA agar, select colonies of Gram-positive bacteria.
• From MacConkey agar, select colonies of Gram-negative bacteria.
What is the last step of obtaining a pure culture
Subculture each colony onto fresh plates to ensure purity:
• Streak the selected colonies from Columbia CNA on TSA to check the morphology and confirm the Gram-positive identity.
• Streak the selected colonies from MacConkey on TSA for Gram-negative bacteria.
What is a selective medium
A medium that favors the growth of a particular type of organism by inhibiting the growth of others.
Example: Columbia CNA agar for Gram-positive bacteria.
What is a differential medium
A medium that allows the growth of multiple organisms but differentiates between them based on biochemical properties.
Example:
MacConkey agar, which differentiates lactose fermenters from non-fermenters.
What is a enrichment medium
A medium that enhances the growth of a specific
microorganism by providing specific nutrients or growth factors.
Example: Blood agar for fastidious organisms.
Combination Selective-Differential Medium:
A medium that is both selective and differential, allowing for the selection of specific organisms and the differentiation of those organisms based on some biochemical property.
Example:
MacConkey agar, which is selective for Gram-negative bacteria and differentiates lactose fermenters from non-fermenters.
What is Columbia cna agar
It is selective gram positive bacteria due to the inhibitory effects of colistin and nalidixic acid on Gram-negative
bacteria. This medium is useful for isolating Gram-positive organisms from a mixed culture.
What is a macconkey agar
It is selective for Gram-negative bacteria due to the bile salts and crystal violet, which inhibit Gram-positive bacteria. It is also differential, allowing for the identification of lactose-fermenting organisms through color changes in the medium.
Appearance of Specific Bacteria on MacConkey
Agar: Escherichia coli
Colonies appear pink or red due causing a pH shift in the medium.
Appearance of Specific Bacteria on MacConkey
Agar: Klebsiella aerogenes
Colonies will appear pink or red for the same reason as E. coli, though they may produce more mucoid colonies
Appearance of Specific Bacteria on MacConkey
Agar: Pseudomonas aeruginosa
Colonies will appear colorless or translucent, as no acid is produced to shift the pH. The medium remains neutral or yellow.
What causes a color change in maconkey agar
Because of the ability to ferment lactose. Remember that macconkey agar selects for gram negative bacteria and the ph indicator turns pink/red. Colonies remain colorless, typically pale or yellow when the microbes are non fermenters. So the differential characteristic helps in identifying and differentiating gram pos/neg bacteria.
Columbia CNA agar
Selects for positive bacteria. Seeing large grey colonies may indicate a species of staphylococcus and seeing small grey colonies may indicate a species of staphylococcus aureus.
How would
a. Escherichia coli
Appear on macconkey agar
E.coli is a strong lactose indicator so it’s colonies will appear pink or red because the acid produced lowers the ph which changes the color
How would appear on macconkay agar
b. Klebsiella aerogenes
Is a lactose fermenter but may ferment less. Colonies will still be pink or red but will appear larger than e coli
How would Pseudomonas aeruginosa appear on macconkey agar
This microbe is a NON lactose fermenter and therefore colonies will be colorless/ pale because they do not produce acid to lower the ph and cause a color change.
