MCB 2010C Final Lab Exam
What kind of media is EMB? - ANS-complex, selective, and differential
What is the EMB media composed of? - ANS-gelatin, lactose, and the dyes eosin Y and
methylene blue
What kind of microbe does EMB agar inhibit? - ANS-gram positive
If the bacterium changes to a purple color on EMB agar, what does that mean? -
ANS-The bacterium is gram negative, and ferments lactose.
What bacterial stocks turn purple on EMB agar, meaning that they ferment lactose? -
ANS-Enterobacter aerogenes, Proteus vulgaris, and Esherichia coli
What sheen does E. coli give off on EMB agar? - ANS-A metallic green sheen
What kind of media is MAC? - ANS-Selective and differential
What is the MAC media composed of? - ANS-Lactose, bile salts, neutral red, and
crystal violet.
What is the action of bile salts and crystal violet in MAC media? - ANS-Inhibits gram
positive bacteria growth
What is the pH indicator in MAC media? - ANS-Neutral red
If a bacterium does alter the color of MAC media, what does this mean? - ANS-The
bacterium is gram negative, lactose fermenting, lowers the pH of the media (acidic) and
changes the media from pink to red.
What bacterial stocks turn red on MAC agar, meaning that they ferment lactose and
lower pH? - ANS-Escherichia coli and Enterobacter aerogenes
What is the pH indicator in PR-G tubes? - ANS-Phenyl red
If the bacterium changes to a yellow color, what does this mean? pink? - ANS-If the
media turns yellow, this indicates glucose fermentation, and an acidic pH. If the media
turns pink, this indicates peptone from NH3 production, raising the pH alkaline.
, What does the durham tube in the PR-G tube indicate? - ANS-Gas production indicates
fermentation.
What bacterium changes yellow in PR-G media? - ANS-Enterobacter aerogenes,
Escherichia coli, and proteus vulgaris (both produced gas)
What kind of media is PR-G and PR-L tubes? - ANS-Differential
What bacterium changes to yellow on PR-L media, meaning it ferments lactose? -
ANS-Enterobacter aerogenes and Escherichia coli (both produced gas)
What kind of media is urea broth? - ANS-Differential
What does color change in urea broth indicate? - ANS-If an organism contains urease in
order break down urea
If an organism contains urease, what color does it change to in urea broth? What color if
it does not? - ANS-If it contains urease, broth changes to a magenta. If it does not
contain urease, broth changes orange/yellow indicating an acidic pH.
What bacterial stock contains urease and turns urea broth magenta? -
ANS-Enterobacter aerogenes and Proteus vulgaris
What does MR-VP media contain? - ANS-Peptone, glucose, and a phosphate buffer
What is the role of the peptone and glucose in MR-VP media? - ANS-Provides nitrogen
and protein and ferments carbohydrates.
What is the role of the phosphate buffer in MR-VP media? - ANS-Resists changes in
pH.
What does the MR test indicate? - ANS-Detects organisms capable of mixed acid
fermentation and the capability to overcome the phosphate buffer.
What reagent is used in the MR test? - ANS-Methyl red indicator dye.
What color does MR-VP media change when the MR test is performed? - ANS-The
media will change red if it can do mixed acid fermentation. The media will change yellow
if it cant.
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