Question

9. Suppose you ingest 20 cells of Salmonella typhimurium in a chicken taco. Some of the bacteria are killed as they pass through the stomach but 10 viable cells survive and reach the intestine. Suppose further that Salmonella generation time in the intestine is 2.7 hours and that you will feel sick when there are 1,000,000 bacteria. How much time will elapse before you feel sick? Show your work. 10. You are doing a viable cell count on a bacterial culture and you perform ten-fold serial dilutions as shown in Figure 4.14 of your text. You count 130 colonies on a plate that was spread with 0.05 ml of a 106 dilution of the original culture. How many viable cells per mililiter are there in the undiluted bacterial culture? Show your work

Answer 1

9. Generation time= t/3.3 log B/Bo

B= Final number of bacteria= 1,000,000

Bo= Initial number of bacteria= 10

G=2.7 hrs

Salmonella only start dividing when they enter the intestine, target organ.

2.7 hrs= t/3.3 log (1,000,000/10)

2.7 hrs= t/3.3 log 10⁶/10= t/3.3 * (6-1)= t/3.3 X 5

2.7= t/ 16.5

t= 2.7 * 16.5= 54.45 hours

10. Number of colonies= 130

Dilution = 10^-6

Dilution Factor=10⁶

Volume plated = 0.05 mL

CFU/mL= Number of colonies on Plate X Dilution Factor/Volume spread plated

CFU/mL= 130 X 10⁶/0.05= 2600 X 10⁶= 2.6 X 10⁹ cells/mL