Question

As a microbiologist, you are investigating an outbreak of food poisoning at a local taco joint "Salmonella Bell". You take a sample of the guacamole and make a 1/10 serial dilution. You plate 0.1 ml of each dilution on a nutrient agar plate and find that the 1/1000 dilution has 100 CFUs of a microorganism that you identify as Salmonella enterica.

A) Based on the CFUs that you counted, how many microorganisms were in the original guacamole sample?

B) The guacamole was made at 11am and you conducted your dilutions at 4pm. If Salmonella enterica has an approximate doubling time of 25 minutes, how many Salmonella cells was the guacamole initially contaminated with at 11am? (Hint: N_Final­ = N_initial 2ⁿ)

Answer 1

A.

CFU per mL = # colonies / (dilution × volume plated in mL)

# colonies = 100

Dilution = 1/1000 = 10^-3

Volume plated = 0.1 mL

= 100 / (10^-3 × 0.1) = 100 × 10^4 = 10^6 CFU per mL

B.

# generations, n = total time / doubling time

Total time = 11 am to 4 pm = 5 hours = 5 × 60 = 300 minutes

Doubling time = 25 minutes

n = 300/25 = 12

At 4 pm, we had 10^6 CFU per mL. This is N final.

10^6 = N initial 2^12

N initial = 10^6/2^12 = 10^6/4096 = 244.14

Initially, around 245 cells were present.

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