Lab Exam 2: Ex. 4-4; 4-5 Flashcards
What kind of medium is MacConkey Agar?
Undefined, selective, and differential. Selects for Gram-Negative bacteria
What are the components of MacConkey Agar and what does each do?
Lactose - decreases the pH and differentiates organisms that ferment lactose from those that don’t.
Bile salts and crystal violet - inhibit growth of Gram-positive bacteria.
Neutral red dye - a pH indicator that is colorless above a pH of 6.8 and red at a pH less than 6.8
What ingredient makes MacConkey Agar selective?
Bile salts and crystal violet select for Gram-negative bacteria
What ingredient makes MacConkey Agar differential?
Lactose differentiates organisms that ferment lactose from those that don’t
What is MacConkey agar typically used for?
to isolate and differentiate members of the Enterobacteriaceae based on the ability to ferment lactose. (Gram-negative gut bacteria)
What would poor growth or no growth of a bacteria on a plate of MacConkey agar mean?
The organism is inhibited by crystal violet and/or bile and is Gram positive
What would good growth of a bacteria on a plate of MacConkey agar mean?
The organism is not inhibited by crystal violet or bile and is Gram negative
What would pink to red growth with or without bile precipitate of a bacteria on a plate of MacConkey agar mean?
The organism produces acid from lactose fermentation and is most likely a Gram negative coliform
What would colorless growth (not red or pink) of a bacteria on a plate of MacConkey agar mean?
The organism does not ferment lactose and is noncoliform
What organisms did we test on the MacConkey plate in lab and what were the results?
Escherichia coli - red growth
Staphylococcus epidermidis - no growth
Providencia stuartii - no growth
What kind of medium is Eosin Methylene Blue Agar?
undefined, selective, and differential. Selects for Gram-negative bacteria.
What are the components of Eosin Methylene Blue Agar and what does each do?
Digests of gelatin (peptone, sucrose, and lactose) - provides nitrogen and organic carbon; provide fermentable carbohydrates for fecal coliforms.
Eosin Y and Methylene blue dyes - inhibit the growth of most Gram-positive organisms and they react with vigorous lactose fermenters whose acid end products turn the growth dark purple or black.
What is Eosin Methylene Blue Agar typically used for?
It is used for the isolation of fecal coliforms - it encourages the growth of Gram-negative Enterobacteria (“gut” bacteria)
What component makes Eosin Methylene Blue Agar selective?
Eosin Y and Methylene blue dyes
What component makes Eosin Methylene Blue Agar differential?
lactose
