Question

80. You set up a ten-tube Minimum Inhibitory Concentration test where the concentration of the antibiotic Rapamycin in each tube is a 1/2 dilution of the previous tube. Starting the dilution from tube 2, the concentration of Rapamycin added to tube 1 is 1280 µg/ml. After incubation, tube 6 was the first tube showing no turbidity, then the MIC of Rapamycin is?                                                                                                 

A. 1280 µg/ml                

B. 640 µg/ml                  

C. 40 µg/ml                

D. 20 µg/ml

81. The same procedure of Minimum Inhibitory concentration test as in question 80 was repeated with Ampicillin starting with a concentration of 6400 µg/ml in tube 1. Tube 6 was the first tube showing no turbidity. We can conclude that the bacterium:

A. Is more resistant to Ampicillin

B. Is more resistant to Rapamycin

C. Has same susceptibility to both antibiotics

82. Which of the following bacteria are incorrectly matched with their growth pattern in thioglycolate broth?                                                       

A. Pseudomonas aeruginosa-surface growth is extensive and no growth as you go deeper    in the agar

B. Clostridium histolyticum-growth is uniform from top to bottom of agar

C. Bacteroides fragilis-growth is observed deep at the bottom of the agar tube

D. Staphylococcus aureus- growth decreases gradually as you go deeper in the agar

E. All are correct

Answer 1

80. In the sixth tube we see no turbidity. It means that this tube has no growth of bacteria. So the concentration of antibiotic in 6 tube will be MIC. Dilution is 1/2. So the concentration of antibiotic is - 1280 , 640, 320, 160, 80 , 40 in respective tubes. So 40 micrograms/ml is the correct option.

81. Here everything applied is same as above question. Minimum inhibitory concentration of ampicillin will be - 200 micrograms/ml . So bacterium is more resistant to ampicillin as compared to rapamycin . More concentration of ampicillin is required for no growth of bacterium . So option 1 is correct.

82. Option b is incorrectly matched.
pseudomonas are obligate aerobic so growth is only at the surface. Clostridium are obligate anaerobes so growth is only at the bottom of the agar while the option states it is uniform which is incorrect . Bacteroides are also obligate anaerobes so growth is observed only at the bottom. Staphylococcus are facultative anaerobes so growth decreases as we go down tube.