The Lethal Effects of Ultraviolet Light
Although certain wavelengths of light are essential to some bacteria, particularly photosynthetic bacteria, sunlight is normally injurious to most bacteria. This effect is primarily due to ultraviolet (UV) radiation present in sunlight. By definition, ultraviolet light includes those electromagnetic radiations that fall in the wavelength band from 10 to 400 nanometers (nm). Thus, it bridges the gap between the X-rays and the shortest wavelengths of visible light. The mechanism by which ultraviolet radiation kills microorganisms is not completely understood. Data suggest that nucleic acids, in particular DNA, and to some extent proteins, undergo chemical and physical changes when exposed to UV light. These changes lead to cellular dysfunction and death.
Within the UV spectrum, wavelengths between 240 and 300 nanometers exhibit the greatest bactericidal efficiency. For this reason, low pressure UV lamps with an output of 254 nm are widely used to reduce the microbial population in hospital operating rooms, aseptic filling rooms in the pharmaceutical industry, public waiting rooms, culture transfer chambers, and other enclosed environments. The effectiveness of UV radiation is limited by the fact that it has very little penetrating power and, unless the microorganisms are directly exposed, they are likely to escape destruction. Furthermore, the type of organism to be killed must be known so that the proper time and intensity of exposure can be adjusted. Endospores are highly resistant to
UV light, and are thus not as easily killed as non-spore-formers. However, the vegetative cell of sporeformers will be destroyed just as effectively as that of comparable non-spore-formers.
In this exercise, test organisms will be exposed to a germicidal UV lamp for intervals of time from 5 seconds to 1 minute.
Materials:
1. Broth cultures of test organisms
3. Hockey puck spreader
4. Tryptic Soy Agar plates
5. Set of paper masks
6. Germicidal ultraviolet light source
7. Beaker of disinfectant
8. 50- or 100-ul pipet with sterile tips
Procedure:
1. Use 50-ul pipet to transfer 50 ul broth culture to labeled Tryptic Soy Agar plates and spread with flame-sterilized “hockey puck“. Be sure to cover the entire surface.
2. The ultraviolet light source is set up in the front of the room. DO NOT LOOK DIRECTLY AT THE LAMP AS SEVERE EYE DAMAGE CAN RESULT. Before proceeding to the lamp, study the next procedure carefully.
3. You are to expose each plate, WITH ITS COVER REMOVED, to varying intervals of ultraviolet radiation based on chart provided.
4. Incubate the plates as directed until the next laboratory session.
5. At the next laboratory session, note the amount of growth in each plate. Compare results obtained from different organisms/groups.