Question

83. A microbiologist is interested in picking pure colonies of Salmonella typhi and Escherichia coli. Which culture media would you recommend him to use to grow each type of bacteria respectively?                                                                            

A. Phenyl Ethanol Agar (PEA) for both

B. Salmonella-Shigella agar & Eosin Methylene Blue agar (EMB)

C. Bismuth Sulfite agar & MacConkey agar

D. Bismuth Sulfite agar & Eosin Methylene Blue agar (EMB)

84. The results of the Kirby-Bauer method are influenced by all of the following EXCEPT:

A. Rate of diffusion of the antibiotic                                 

B. Thickness of the agar

C. 0.5 McFarland Standard for concentration uniformity

D. Bacteria are susceptible to the antibiotic used regardless of the diameter of the zone of inhibition obtained post incubation

85. A positive indole test indicates cleavage of indole from what substrate?          

A. Glucose      

B. Sucrose     

C. Tyrosine        

D. Tryptophan

Answer 1

Answer 83

Option C - Bismuth Sulfite agar & MacConkey agar

For Salmonella selective isolation, Bismuth sulphate agar us best for the reason that it inhibits all other gram negative bacteria to a large extent and selectively support Salmonella typhi and other salmonellae. The colonies of Salmonella typhi will be those with black metallic sheen due to production of H2S.

MacConkey Agar by far is best for growing and selectively isolating Escherichia coli. Easy to prepare and differentiating lactose fermenters from non-fermenters, E. coli give pink colonies on MacConkey's agar that are easy to identify.

[By the way, other combinations of media in other options can also be used depending on the type of sample and expected load of these bacteria to get proper isolation,]

Answer 84

Option D - Bacteria are susceptible to the antibiotic used regardless of the diameter of the zone of inhibition obtained post incubation

This statement is NOT TRUE because in Kirby-Bauer method the zone of inhibition around the disc needs to be accurately measured to say if the organism is sensitive, resistant or intermediate between the two. There is a minimum requirement of the zone in mm to be measured to render bacteria sensitive, depending on MIC.

Answer 85

Option D - Tryptophan

Indole test is based on ability of some bacteria to decompose amino acid tryptophan to indole by enzyme tryptophanase. Indole is produced by reductive deamination of tryptophan and presence of indole in the media after incubation is tested by using Kovac's reagent / Ehrlich's reagent.