Ultraviolet (UV) radiation is commonly used to sterilize surfaces by damaging microbial DNA. In a laboratory experiment, cultures of Bacillus subtilis vegetative cells and their endospores are exposed to UV light for various time intervals. It is observed that vegetative cells are killed after 5 minutes of exposure, while endospores survive even after 20 minutes. Explain how the life stage of Bacillus subtilis affects its sensitivity to UV radiation and why endospores are more resistant than vegetative cells.
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Lab 2-12: The Effect of Ultraviolet Radiation on Microbial Growth With the exception of 0 minutes, the lids of the petri plates were removed. Side A was exposed to UV light. Side B was covered with a paper index card. Plates were incubated for 24 hours, and then growth was assessed on the plates: 0 = no growth; 1 = individual colonies; 2 = sparse confluent growth; 3 = heavy confluent growth. 1. Which organism survived the longest exposure to UV light? EXPLAIN IN DETAIL WHY YOU SUPPOSE IT DID. 2. What is the general relationship between bacterial death and UV exposure time? 3. Why were you told to remove the plate covers prior to exposing them to UV? 4. What was the purpose of covering half of the plate with a paper index card when exposing the plate to UV? 5. What specific wavelengths are most detrimental to bacteria? EXPLAIN WHY. 6. Which phase of bacterial growth, logarithmic or stationary, would Bacillus subtilis show the best survival to UV light? EXPLAIN WHY. 7. Give a specific example of how UV light is commonly used to disinfect within the healthcare setting.
Sana R.
(4 points) What properties of endospores do you think help them to endure more UV radiation than non-sporulating bacteria? (8 points) Please explain why a human sunbathing too much and too long without sunblock is bad for their skin? While eukaryotic cells are different from prokaryotic cells, draw correlations from the knowledge in this experiment to what could be happening to the skin cells of a sunbather (Hint: central hypothesis). (4 points) While the cells were spread across the whole plate, how was only part of the plate exposed to the UV light? Explain in terms of wavelength what a material has to do in order to block UV light? (example Hints: UV glasses vs. polarized glasses, thick blanket vs clear plastic)
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Suggest two explanations to account for the growth of the bacteria in the exposed portions of the two plates. Do the incubation conditions (light or dark) make a difference in how these bacteria grow after exposure to UVC radiation? How can you tell? What might be the cytochemical basis for this difference? Bacterial endospores are supposed to be resistant to UVC radiation. Describe the class data that either supports or refutes this statement.
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