Phenol Red Broth
What is the initial pH range of phenol red broth, and what color does it appear at this pH?
Around pH 7.3; the broth appears red
What is the pH cutoff for positive MR test, and what color indicates a positive result?
pH greater than 4.4; red color
What reaction does catalase perform?
Breaks down H₂O₂ → H₂O + O₂.
What is the pH indicator and what is a positive color result?
Bromothymol blue; positive = blue.
In a decarboxylase test, what does a yellow result indicate about the bacteria being tested?
It indicates that the organism fermented glucose, which produced acidic end products and lowered the pH of the medium, but the organism did not produce the specific decarboxylase enzyme
At what pH does phenol red turn yellow, and what does this indicate?
Below pH 6.8; indicates acid production from carbohydrate fermentation
What metabolic pathway does VP detect?
Butanediol fermentation, producing acetoin.
Why use fresh cultures?
Older cultures may lose enzyme activity.
Enzyme for citrate use?
Citrate permease.
What were the three amino acids that were tested for? What chemicals did they convert to?
Arginine, Lysine - cadaverine, Ornithine - putrescine
What does a pink color in phenol red broth after incubation signify?
pH above 7.4; alkaline reversion due to peptone utilization
Which reagents are added in VP test?
Barritt’s A (α-naphthol) and Barritt’s B (KOH); positive = red color.
What does a negative catalase test indicate?
Organism likely anaerobic or microaerophilic.
Why does medium turn blue?
Alkaline products (ammonia) raise pH.
What pH indicator is used in Møller’s decarboxylase medium, and what color does it appear at a pH below 5.2?
The medium uses bromocresol purple, which appears yellow at a pH below 5.2
What is the purpose of the Durham tube, and what does a bubble indicate?
Detects gas production; bubble = CO2 or H2 produced during fermentation
Why must VP reagents be added in order and shaken?
Ensures oxygen exposure for acetoin oxidation.
What does the enzyme superoxide dismutase do?
Catalyzes the conversion of superoxide radicals, which are particularly damaging toxic oxygen intermediates, to the comparatively less damaging molecule, hydrogen peroxide
Role of ammonium phosphate?
Nitrogen source for growth.
Why must a layer of sterile mineral oil be added to the tubes after inoculation?
Mineral oil seals the medium from the air to prevent "artificial alkalization" from the atmosphere and to promote a low-oxygen environment that favors glucose fermentation
If the broth remain red with no bubble, what does this mean?
No fermentation and no gas production
If MR-positive and VP-negative, what does this suggest?
Organism performs mixed acid fermentation.
What percent hydrogen peroxide solution can we use to determine whether bacteria makes catalase or not?
3%
Growth without color change?
Still positive; growth = citrate use.
What are the two primary byproducts of the decarboxylation of an amino acid?
The reaction produces an amine and carbon dioxide (CO₂)