Question 1

A physician sends a stool sample to your lab,and wants to know if there are lactose fermenting microbes in the sample.How might you determine if these microbes are present or not from this mixed-microbe specimen?
A)Streak the sample for isolation on Thayer-Martin agar (which contains lactose and particular antibiotics for selectivity).
B)Streak the sample for isolation on a blood agar plate (which contains lactose AND red blood cells that enrich the culture for iron).
C)Streak the sample for isolation on a MacConkey agar plate (which contains lactose and a pH indicator that turns pink when acid byproducts are present).
D)None of the above would work-there's no way to reliably determine this feature from the specimen given.

Accepted Answer

Streaking the sample for isolation on a MacConkey agar plate would be the best choice. The plate contains lactose and a pH indicator that turns pink when acid byproducts are present, indicating the presence of lactose fermenting microbes. Thayer-Martin agar is selective for certain pathogens and would not necessarily show lactose fermentation, and blood agar plates would not show a clear indication of lactose fermentation either.

Question 2

You are in charge of water quality for your city's water treatment plant.Of the methods at your disposal,which will be the most efficient and cheapest method of determining the number of viable bacteria in the water coming out of your plant?
A)Direct counts using a microscope and a counting chamber.
B)Using a Coulter counter machine.
C)Performing serial dilutions of your samples and doing spread plate counts.
D)Using membrane filtration followed by placing the membrane in a growth medium for colony counts after incubation.
E)Biochemical analysis of secondary metabolites in the water that are given off by bacteria.

Accepted Answer

Using membrane filtration followed by placing the membrane in a growth medium for colony counts after incubation is the most efficient and cheapest method of determining the number of viable bacteria in the water coming out of the plant. This method allows for the growth of individual bacterial colonies on a membrane filter, which can then be counted and used to determine the number of viable bacteria present in the water sample. It is a simple, reliable, and cost-effective method that provides accurate results for water quality testing.

Question 3

You take absorbance readings on a spectrophotometer across a 6-hour culture of
A)Clearly,something is wrong with the spectrophotometer and it isn't measuring the correct values.Perhaps something is on the detector,making it register falsely high absorbance numbers.
B)When we establish a growth curve,we should actually plot the log of the number of viable cells vs.time.However,a spectrophotometer can only measure absorbance.Absorbance is NOT the same as the number of viable cells.Many of the cells in the tube are most likely dead,but the machine can't discriminate between a live cell and a dead one.This keeps the absorbance high even into the death phase.
C)It'll happen-E.coli just grow slowly.We haven't gotten to the end of the stationary phase yet.Be patient!
D)Perhaps there's a big smudge of something on the tube that is blocking some of the light.This would lead to an elevated reading for every timepoint after the smudge was placed on the tube.
E)coli cells growing in tryptic soy broth (TSB).Your absorbance readings clearly indicate a lag phase,a log phase,and a stationary phase.You come back in and take readings at 8,10,12,14,and 16 hours,but the absorbance number remains the same.Shouldn't it start coming down as the closed batch culture enters death phase? What's the most likely thing that is happening?

Accepted Answer

The absorbance readings cannot distinguish between live and dead cells, leading to higher readings even into the death phase.

Answer: C 
 E. coli might grow slowly, and it could take longer to reach the death phase.

Therefore, the best choices are B and C.

Quiz 4: Dynamics of Prokaryotic Growth

A physician sends a stool sample to your lab,and wants to know if there are lactose fermenting microbes in the sample.How might you determine if these microbes are present or not from this mixed-microbe specimen?

You are in charge of water quality for your city's water treatment plant.Of the methods at your disposal,which will be the most efficient and cheapest method of determining the number of viable bacteria in the water coming out of your plant?

You take absorbance readings on a spectrophotometer across a 6-hour culture of