What does it mean that the cultures used in the Ames test are“auxotrophic”? Explain the necessity of starting with a strain that is mutated in this experiment.

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Question #1
Generation times for four different bacterial species were calculated in the 6 media listed in the table. All media were prepared with pure distilled water, inoculated with the indicated species, and incubated aerobically in the light at room temperature. Compare and contrast the growth requirements of the four bacteria listed above relative to the glucose, amino acid, and mineral content of each culture medium.


Question #2

The results below were obtained from a use-dilution test of two disinfectants. Cultures were inoculated into tubes with varying concentrations of disinfectants and incubated for 24 hr at 20°C, then each tube was sub-cultured into nutrient media without any disinfectant.(Hint: -static disinfectants don’t kill the bacteria, the cells can start to grow again when removed from the disinfectant)
(+ = growth; – = no growth)

In the table, what is the minimal bacteriostatic concentration of each disinfectant? Which compound is bactericidal? At what concentration?


Question #3

Assume that you are responsible for decontaminating materials in a large hospital. How would you sterilize each of the following? Briefly justify your answers.


  1. a mattress used by a patient with bubonic plague
    b. intravenous glucose-saline solutions
    c. used disposable syringes
    d. tissues taken from patients


Question #4

What does it mean that the cultures used in the Ames test are“auxotrophic”? Explain the necessity of starting with a strain that is mutated in this experiment.

Question #5

Scientists are concerned that bacteria will be resistant to all antibiotics within the next decade. Using your knowledge of genetics and bacterial growth, describe how bacterial populations can develop drug resistance in such a short time frame.

Question #6

Characteristic A B C C E
Morphology Rod Rod Coccus Coccus Rod
Motile Yes Yes No No Yes
Gram Reaction +
Glucose Utilization Oxid. Ferm. Ferm. Ferm. None
% G+C 58-70 50-51 30-40 47-52 58-70
Cytochrome oxidase Present Absent Absent Present Present

Use the information given in the table to answer the following question. Which organisms are most closely related? On what did you base your answer? DNA from which organisms will probably hybridize?

Question #7

One of the advantages of some newly developed rapid identification tools is that pure cultures are not needed. Why is a pure culture necessary for biochemical tests such as the Enterotube, but not for DNA probes?

Question #8

Explain the rationale for using molecular/genetic data for classification, in the place of morphology.

Question #9 

Bacteroides and Escherichia are both gram-negative rods found in the large intestine. Why are they classified in different phyla? (Hint: see your response to Q#8)

Question #10

How is the replication of the viral genome of retroviruses e.g.HIV, unique among the viruses?

Question #11
The figure shows a typical bacterial growth curve with the y-axis indicating the log of the number of bacteria and the x-axis indicating the time of culture growth. The labels a – d mark the four phases of a typical bacterial growth curve.  

  1. Name and identify what is occurring in each of the four growth phases (a-d) in terms of bacterial growth and/or death.
  1. During which of the four growth phases (a-d) would bacteria be most susceptible to antibiotics and why?


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