Question

QUESTION 10

1. A bacterial colony growing on a blood agar plate exhibits a clear zone around the colony. What does this indicate?

   		a. Antibiotic resistance
   		b. Alpha hemolysin
   		c. Beta hemolysin
   		d. Gamma hemolysin
   		e. Antibiotic susceptibility

5 points   

QUESTION 11

1. The presence of E. coli in drinking water indicates

   		a. Fecal contamination
   		b. Possible fecal contamination
   		c. That it is from a free-living source
   		d. Presence of fecal pathogens
   		e. None of the above

5 points   

QUESTION 12

1. Staphylococcus aureus growing on mannitol salt agar plate would show this result.

   		a. Purple colonies
   		b. Red colonies
   		c. Clear colonies
   		d. The plate appears yellow
   		e. The plate appears red

5 points   

QUESTION 13

1. A broad spectrum is

   		a. An antibiotic that works only on gram positives
   		b. An antibiotic that works equally well on gram positives and gram negatives
   		c. Is a disinfect that kills all bacteria
   		d. A chemical that causes thymine dimers

5 points (Extra Credit)   

QUESTION 14

1. A cell count showed these results. What is the cell concentration in cells/ml?

   First counting square- (20 clear cells 10 blue cells)

   Second counting square- (15 clear cells 5 blue cells)

   Third counting square- (30 clear cells 15 blue cells)

   Fourth counting square- (10 clear cells 5 blue cells)

   Fifth counting square- (15 clear cells 10 blue cells).

   The cell count was prepared using ½ sample ½ dye.

   		a. 1.8x104 cells/ml
   		b. 3.6x105 cells/ml
   		c. 9.0x104 cells/ml
   		d. 1.8x105 cells/ml
   		e. 3.6x104 cells/ml

Answer 1

10. c.A clear zone around a bacterial colony in blood agar indicates beta hemolysin as it has completely lysed the red blood cells and the hemoglobin. In alpha, hemoglobin is partially lysed and in gama there is no lysis or clear zone.

11.a. The presence of E.coli indicates fecal contamination and water and the possible presence of pathogens.

12.d. When Staphylococcus aureus is grown on MSA , the plate turns yellow as it can tolerate the high salt concentration and can also ferment mannitol to release acidic by-products that turn the pH indicator phenol red in the media from red to yellow.

13.b. A broad spectrum antibiotic works equally well on gram positves and gram negatives.

14. Average number of live cells per grid =(20+15+30+10+15)/5 =18

Dilution =Volume of sample/volume of sample +dye =1/2

Volume of grid in hemocytometer =0.1*0.1*0.01cm3=10-4cm3

Number of cells/ ml = Average number of live cells /(Volume of grid*dilution) =1.8*2/10-4=3.6*104 cells/ml

So option e is correct.