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.
After incubating the plates from condition 2 for an additional 5 days, the plates became totally clear. What is the most likely explanation for this result?
|A||Newly produced phages infected all bacteria on the dish, thus killing everything.|
|B||The bacteria died off due to lack of nutrients.|
|C||The bacteria protected themselves from the phages by slowing their growth.|
|D||The phage forced all the bacteria to go into the lysogenic cycle and stop growing.|
All the bacteria were infected by the phage and lysed. The other conditions, whether true or not, deal with growth or death of bacteria and would not cause lysis of the bacteria. Therefore, the plate would remain cloudy. Only lysis causes the plates to appear clear.