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.
What happened in condition 3?
|A||Phage contaminated the dishes causing infection and lysis of the bacteria.|
|B||The nutrients in the agar were insufficient to support bacterial growth.|
|C||The bacteria were probably a lysogenic strain induced by UV light.|
|D||The bacteria were not healthy and died off when exposed to UV light.|
The control in this experiment (condition 1) rules out answers A and B. If UV light killed the bacteria, as suggested in D (and as can happen, depending on conditions), then all the bacteria would die. (Although it is possible that only a small percentage would die, it would be highly unlikely that this lethality would show a plaque pattern.) C is the only plausible answer, and we know that UV light can induce lysogenic strains to become lytic.