Question

Practice Questions

1. You were given a bacteria for unknown investigation. To begin with, you performed a Methyl Red test after inoculating and incubating them for 48 hours. You obtained no color change after addition of Methyl Red. You proceed forward and performed Voges Proskauer test. For this particular test, you obtained a red color change after addition of Barritt solution A and B. From the result, what kind of inference can you draw from the 2 results obtained about the fermentation pattern of bacteria?

2. You are analysing a food sample of beef. You make a slurry of it and serially dilute it. After serial dilution, you plate it out on a HEA plate. The plate is incubated at 37C for 48 hours. After incubation at 37C for 48 hours, you obtain the following set of results.

a. Green colonies with black center

b. Light green colonies

c. Yellow orange colonies with organish halo

Name bacteria A, B and C. What are the differentiating agents present in HEA?

3. You obtained 20 plaques on a plate where E.coli and T4 phage were inoculated at the same time, and incubated at 37C for 48 hours. The Plaque Forming Units were 2*10^5. What dilution of T4 phage was used for obtaining 20 plaques?

4. What is the phenol coefficient of a disinfectant, if the dilution of disinfectant that kills an organism in 10 minutes is 1/20 and dilution of phenol that kills the same organism in 10 minutes is 1/200.

5. You are given 4 set of bacteria, and you decide to perform Citrate test on the 4 bacteria. You inoculate the bacteria in its required media, and incubate it at 37C for 48 hours. After 48 hours, you observe the following set of results for the 4 bacteria.

Bacteria Color of the medium

A Green

B Blue

C Blue

D Green

What do the color changes showcased by citrate media tell us about bacteria’s ability to form NaHCO3 ?

6. In case of nitrate reduction test, you add Zn dust to the inoculated nitrate broth, and you get no color change, which set of chemical species might be possibly present in the medium? Consider a condition where you get a red color change of the nitrate broth, what kind of enzyme is present in case of bacteria under investigation?

7. A bacteria was inoculated in Glucose, Sucrose, Lactose and Maltose fermentation broth. It was incubated for 48 hours at 37C. You obtain the following set of results for the 4 given broths.

Fermentation broth Color change of the broth

Glucose Yellow with bubble

Maltose Fuchsia

Sucrose Yellow

Lactose No color change or bubble

Based upon the following set of results, what can you say about the bacteria’s fermentation/utilization pattern with reference to the 4 sugars

Answer 1

1) MR-VP tests are based on the detection of specific breakdown products of carbohydrate metabolism. The Voges-Proskauer (VP) test is used to determine if an organism produces acetylmethyl carbinol from glucose fermentation. MR-negative organisms further metabolize the initial fermentation products by decarboxylation to produce neutral acetyl methylcarbinol (acetoin), which results in decreased acidity in the medium and raises the pH towards neutrality (pH 6.0 or above).

2) HEA contains prtoeose, peptone, yeast extract, bile salts, sodium thiosulphate, ferric ammonium citrate, carbohydrates, dyes

a Salmonella

b Salmonella or Shigella

c rapid lactose fermentors like coli

3)

4)To calculate for phenol coefficient, the number indicating the degree of dilution of the disinfectant in which it kills the microorganism within a given time is divided by the number indicating the degree of dilution of phenol in which the latter kills the microorganism in the same period of time and under the same conditions. A phenol coefficient that is greater than 1 indicates that the disinfectant is more effective than phenol. In contrast, a phenol coefficient that is lower than 1 means the disinfectant is less effective than phenol.

Here phenol coefficient is = 20/200 = 0.1

5) Color change from green to blue shows the ability of bacteria to produce NaHCO₃  because it shows bacteria ability to utilize citrase which breaks down citrate to oxaloacetate and acetate. Oxaloacetate is further broken down to pyruvate and carbon dioxide (CO₂). Production of sodium bicarbonate(NaHCO₃) as well as ammonia (NH₃) from the use of sodium citrate and ammonium salts results in alkaline pH.

6) The addition of the zinc reduced the nitrate to nitrite, and the nitrite in the medium formed nitrous acid, which reacted with sulfanilic acid. The diazotized sulfanilic acid that was thereby produced reacted with the α-naphthylamine to create the red complex.

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