LAB 7
1. Could an organism be both MR and VP positive? Explain. (6 pts)
Answer below:
An organism typically cannot be both MR and VP positive because the end-products involved in each test are
opposing. Organisms that are MR positive metabolizes pyruvic acid to other acidic end products by way of the
mixed acid pathway which lowers the pH of the medium, while VP positive organisms are those that use the
butylene glycol pathway to metabolize pyruvic acid to neutral end products which reacts with KOH and alpha-
napthol in Barritt’s B reagent producing a red color. An organism cannot both be positive at the same time; if the
organism in question possesses the ability to emote the conditions required for a positive MR as well as those
required for a positive VP test, it causes an either/or situation, but it can’t be both simultaneously. Serratia
marcescens is one of the very rare bacteria that possess the ability to effect a positive result in both the MR and
VP tests under certain conditions.
A B C
Microbiology A Laboratory Microbiology A Laboratory Laboratory Experiments in
Manual. 6th Edition. Manual. 6th Edition. Microbiology. 7th Edition.
2. What is the name of each of the three tests (A, B, and C) seen above? What is the result of each test
(positive or negative)? Name one organism that can have all of these results (i.e. taken as a combination of
the results shown). (6 pts)
Answer below:
A – Methyl Red test – positive
B – Carbohydrate Fermentation test – Positive
C – Phenylalanine Deaminase test – Negative
One organism that can have all of these results is Escherichia coli.
3. What is the name of each of the three tests (A, B, and C) seen above? What is the result of each test
(positive or negative)? Name one organism that can have all of these results (i.e. taken as a combination of
the results shown). (6 pts)
Answer below:
A – Methyl Red Test – Positive
B – Carbohydrate Fermentation Test – Negative
C – Phenylalanine Deaminase Test – Positive
One organism that can have all of these results is Proteus vulgaris.
4. What types of bacteria can the carbohydrate utilization test and MacConkey agar be used to differentiate?
What would a positive result look like for each test? What would a negative result look like for each test? (6
pts)
Answer below:
The carbohydrate utilization test and the MacConkey Agar (MAC) Test are used to differentiate certain bacteria based on
their ability to ferment various carbohydrates to acid and their capacity to further convert this acidic end product into
gas. Carbohydrate utilization tests employ various fermentable carbohydrates such as glucose, lactose and sucrose and
work by using a pH indicator, such as phenol red or bromcresol purple (which both change to yellow in the presence of
acid or remains unchanged-red/purple in its absence), to detect changes in medium pH due to acid end products of the
fermentation process and a Durham tube (small, inverted glass tube in the medium) to capture gaseous end products.
These tests help distinguish, for example, Gram-negative lactose-fermenting enteric bacteria such as Escheria coli,
Enterobacter aerogenes, and Klebsiella pneumoniae from non-lactose fermenting enterics such as Proteus vulgaris,
Shigella flexneri and Salmonella typhi. Also, with the Durham tube, carbohydrate utilization tests can further differentiate
between “positive” bacteria, based on their ability to produce gas. For example, E. coli and Serratia marcescens are both
glucose fermenters, and would therefore produce acid and a positive (yellow) result, but only E. coli produces gas.
MacConkey Agar test uses organisms’ (Gram-negative bacteria as most Gram-positive bacteria do not grow in this
medium due to its selective functions) lactose-fermenting capabilities to differentiate between them. The acid end-
product of lactose fermentation turn the pH indicator (neutral red) red or pink; depending on the quantity of acid
produced this coloration could just be isolated to the bacterial growth surface or it could precipitate to color the medium
surrounding the growth as well. Those bacteria that aren’t lactose fermenters will not produce an acid end product and
therefore will not lower the pH of the media so they affect no change in coloration; appearing normally-colored or
colorless. Coliform bacilli of the Enterobacteriaceae family such as Escherichia coli, Klebsiella pneumoniae, and
Enterobacter aerogenes are some examples of lactose-fermenting bacterium while non-coliform Enterobacteriaceae
such as Pseudomonas aeruginosa, Proteus, Salmonella, and Shigella are examples of non-lactose fermenting bacteria.
5. Why does hydrogen peroxide bubble when you put it on an open sore? (6 pts)
Answer below:
Hydrogen peroxide bubbles when it is put on an open sore because blood cells/live tissue cells contain the enzyme
catalase which converts (inactivating) the hydrogen peroxide into water and oxygen gas. The presence of the oxygen
gas causes the bubbles to form (oxidizing). It indicates the presence of catalase.
