Killing by Ultraviolet Light (UV Irradiation)
Read the exercise in your lab manual and complete the exercises that follow.
Ultraviolet (UV) light is often used as a physical method of control to prevent bacterial growth. UV light used to prevent or control the growth of bacteria must have a wavelength between 200 and 290 nm in order to be most effective. UV light within this range induces mutations in the DNA of the bacteria exposed. Thymine and/or cytosine nucleotides are rearranged by UV light, causing them to form dimers (thymine base pairing with other thymines or cytosine base pairing with another cytosine). This form of base pairing causes the DNA to become unreadable by replication or transcription machinery, rendering the DNA useless. An accumulation of these mutations quickly becomes lethal to the cell.
UV irradiation is observed by inoculating plates with bacteria and immediately exposing the plates to UV light. Half of the plate may be covered with an index card in order to observe later the difference between exposure and non-exposure to UV radiation. Bacillus is used in order to show UV resistance in endospore-forming bacteria. Bacillus will be compared to Serratia, a non endospore-forming bacteria. Four plates will be prepared, one control (no UV exposure) and three different lengths of UV exposure. The plates are then incubated to prepare for observation at a later time.
UV light is a part of sunlight and is a known cause of skin cancer. Human skin cells are constantly exposed to UV light from the sun and are susceptible to the same types of mutations as bacteria. Germicidal lamps are UV lamps in the 200-290 nm wavelength range used in restaurant kitchens, delicatessens and meat processing plants as a method to control and prevent food contamination by pathogens. Next time you go to the deli, look for the purple lamps!