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.