Question

1. Which of the following aspects of the Kirby-Bauer test must be standardized? A. the depth of agar in the test plate B.the turbidity/opacity of the broth culture used to inoculate the plate C.the concentration of the antibiotic to be tested D.all of the above

2. How may the number of bacteria in a tube be quantified? a)O.D. measured in a spectrophotometer. b)Comparison to a McFarland turbidity standard. c)Serial dilution and colony counting. d)All of the above e)None of the above

3. Organism A produces a zone of inhibition of 20 mm with penicillin, and organism B produces a zone of 10

mm in the same test. Which organism is more sensitive to penicillin? a)A b)B c)There is not enough information to tell

4. When performing an identification of an unknown organism what would be the first action you should

perform? a)Perform a subculture of the specimen to a broth b)Produce a lawn of culture on a nutrient agar plate using the specimen c)Produce a streak plate for isolated cfu

5. What tool is unique to microbiology? a)Pipette b)Bunsen burner c)Loop d)Swab

Answer 1

1. (CORRECT ANSWER) OPTION C

(EXPLANATION FOR CORRECT ANSWER) Kirby-Bauer test determines the minimum concentration of antibiotic required to inhibit the growth of microorganism. So standardization of the concentration of antibiotics to be used is the pre-requisite for this test.

(EXPLANATION FOR INCORRECT ANSWER) OPTION A It is recommended that the dept of the agar plate should be 4mm for testing the sensitivity of antibiotics.

(EXPLANATION FOR INCORRECT ANSWER) OPTION B Whether the culture to be used for the Kirby-Bauer test is turbid or opaque does not affect the test.

(EXPLANATION FOR INCORRECT ANSWER) OPTION D Only one option is correct as mentioned above.

2. (CORRECT ANSWER) OPTION D

(EXPLANATION FOR CORRECT ANSWER) The first three options are correct as explained below.

(EXPLANATION FOR INCORRECT ANSWER) OPTION A Optical density is used to calculate the number of bacteria at a wavelength of 600nm. This wavelength is used as it does little or no harm to the bacteria. It works on the principle of light scattering.

(EXPLANATION FOR INCORRECT ANSWER) OPTION B The number of bacteria in a tube can be calculated upon comparison with the Mc Farland turbidity standard. The standards are prepared by mixing specific amounts of barium chloride and sulphuric acid. The resulting product is barium sulphate that produces turbidity. The bacterial culture is compared with these standards. If it is turbid, then it is diluted. If it is less turbid the more bacteria are added.

(EXPLANATION FOR INCORRECT ANSWER) OPTION C It is practically impossible to count the number of microorganisms in a sample. Hence the sample is diluted subsequent times and plated. Each colony is equivalent to a single microorganism and the Colony-forming units can be determined.

(EXPLANATION FOR INCORRECT ANSWER) OPTION E The first four options are correct as explained above.

3. (CORRECT ANSWER) OPTION A

(EXPLANATION FOR CORRECT ANSWER) Organism A is more sensitive to penicillin. There exists an inverse relationship between the size of the zone of inhibition and the sensitivity of antibiotics. Larger the size of the zone of the inhibition more is the sensitivity and vice-versa. Here is an organism, A has a large zone of inhibition which is 20mm in this case.

(EXPLANATION FOR INCORRECT ANSWER) OPTION B

Organism B has a zone of inhibition of 10mm which is less than that of organism A. As larger the size of Zoe of inhibition more is the sensitivity. Thus organism B is less sensitive than Oragnsim A towards penicillin.

(EXPLANATION FOR INCORRECT ANSWER) OPTION C

All the required information to answer the question is mentioned as explained above.

4. (CORRECT ANSWER) OPTION C

(EXPLANATION FOR CORRECT ANSWER) CFU stands for the Colony-forming unit. This unit measures only the viable colonies of microbes. The viable colonies when the streaked result in the isolation of single colonies can then be identified, studied or tested.

(EXPLANATION FOR INCORRECT ANSWER) (OPTION A) Subculturing is often used to isolate a microbe from a pool containing different microorganisms.

(EXPLANATION FOR INCORRECT ANSWER) (OPTION B) a lawn of culture is obtained when all the individual colonies on a Petri plate merge to give a mat-like appearance. In this case, single colonies are not obtained. and hence it is not possible to identify the unknown organism.

5. (CORRECT ANSWER) (OPTION C)

(EXPLANATION FOR CORRECT ANSWER) Loops are used for the transfer of a small amount of inoculum from a culture of microorganisms. Earlier the loop used to be made of tungsten, nichrome, or platinum. Nowadays plastic ones are used.

(EXPLANATION FOR INCORRECT ANSWER) (OPTION A) Pipettes are a common laboratory tool used to particularly pick up small volumes such as 0.1, 0.5, 10, 20, 100, 200 micrometers, etc respectively.

(EXPLANATION FOR INCORRECT ANSWER) (OPTION B) Bunsen burner is common lab equipment. It is used for dissolving certain solutions that require high temperatures. In other words, it is used to provide heat.

(EXPLANATION FOR INCORRECT ANSWER) (OPTION D) Swabs are absorbent material used for the application of medication or removal of material from an area.