The activity of bacteriophages is easily assayed. Bacteria are spread on a solid nutrient agar plate. Under appropriate growth conditions, the bacteria will form a cloudy layer completely covering the dish. In microbiology, this is called a lawn, as opposed to isolated spots, or colonies, of bacteria. If bacteriophages are introduced to the lawn of bacteria, the phages will infect single cells. Once the phages have reproduced, they will lyse the cells and the progeny phage will infect the surrounding bacterial cells. In a relatively short period of time, the phages will have lysed all the bacteria within a radius of the initial infection, causing a clear, circular spot to form in the lawn of bacteria. This is called a plaque.
A researcher working with bacteriophages set up the following experiment:
Bacteria were spread on agar plates and incubated at 370C for 24 hours. A lawn was clearly visible and no plaques formed.
The same strain of bacteria was mixed with a preparation of lambda bacteriophage. The mixture was spread on agar plates and incubated for 24 hours at 370C. An average of twenty plaques formed on the bacterial lawn on each plate.
The same strain of bacteria was spread on agar plates and then exposed to UV light. After incubation at 370C for 24 hours, lawns grew and approximately fifty plaques formed on each plate.
Before the phage life cycle was understood, researchers called the results from condition 3 autolysis. They thought an enzyme in the bacteria caused its own destruction and the destruction of surrounding cells. Which of the following would lend support to the viral theory and help to disprove autolysis?
|A||Within a week, all the bacterial cells in condition 3 die.|
|B||Viral particles can be purified from the plates in condition 3.|
|C||Exposure of bacteria to X rays also causes plaques to form.|
|D||With or without exposure to UV light, no bacteria grow at 40C.|
Once induction occurs, viral particles are produced that can go on to infect other bacterial cells. Therefore, the isolation of bacteriophage from cultures would not support the theory of autolysis. Answer A is possible in the realm of either theory, C would not distinguish between the two, and D has nothing to do with either theory, as bacteria that grow well at 370C will not grow at 40C.