Control of Microbial Growth: Effect of Ultraviolet Light

Control of Microbial Growth: Effect of Ultraviolet Light

Instructions
1. Complete the virtual lab in MH Connect.
2 Write your lab report in essay format. Your report must include the following in 1-2 pages:
Introduction
Hypothesis
Methods
Results
Conclusion

Key Concepts
Several different approaches can be used to remove, destroy, or inhibit the growth of microbes. The choice of approach usually depends on whether body tissues or inanimate objects are involved and the extent to which control is necessary.
Physical control methods include the use of heat, cold, irradiation, and mechanical removal. An example is the use of heat to sterilize an inoculating loop.
Chemical control methods involve the use of antimicrobial chemicals like alcohol or bleach. Some are appropriate for skin, like alcohol, while bleach would not be safe on human skin but could be used on inanimate objects.
Sterilization is the destruction of all microbial life, including viruses and endospores. Either something is sterile or it is not. There is no in-between.
Not all control methods need to create a sterile environment to be useful. Reducing the number of harmful microbes to a safe level can be sufficient. Disinfection or antisepsis creates a safe environment. The distinction between them has to do with whether the material involved is an object or body tissues. Antiseptics are safe to use on tissues; disinfectants are not.
All control methods target a specific part of a microbe. Targets include the cell wall, cell membrane, nucleic acid and protein synthesis, proteins, and specific metabolic pathways.
Most microbial cells are susceptible to basic control methods, but some organisms have the ability to resist these methods. Endospore forming organisms for example, are highly resistant to normal methods so they require more extreme methods to sterilize a material.

Microbes have different levels of resistance to control methods

The electromagnetic spectrum is the range of electromagnetic radiation; that is, energy that travels in waves and displays different properties based on its wavelength.
Ultraviolet light includes wavelengths at the lower end of the electromagnetic spectrum, between 100 and nearly 400 nanometers.
Ultraviolet Light Is a Type of Electromagnetic Radiation
When UV radiation is applied to cells, it causes the formation of abnormal bonds in the DNA helix. This can interfere with DNA replication, reduce the expression of important genes and potentially cause death of the cell.
UV light causes the formation of thymine dimers. In the backbone of DNA a pair of adenine nucleotides would bind to a pair of thymine nucleotides on the opposite strand. Instead, the two thymine nucleotides form a bond to each other rather than the opposing adenine nucleotides.
Ultraviolet Radiation Damages the DNA of Microbial Cells
UV radiation has several uses as a disinfecting tool:
Disinfection of air and surfaces in medical and research facilities
Disinfection of clear liquids
Commercial uses in food processing facilities
Disinfection of personal items like cell phones
The use of UV radiation does not work well in all situations. UV radiation is only effective on surfaces with direct exposure and mostly clear liquids. UV light does not penetrate solid surfaces or cloudy liquids.

Overview
Many lab simulations encourage the growth of microbes under various conditions to study their characteristics or determine what makes them grow the most efficiently. In many other cases, both in the lab and daily life, there are reasons to control the growth of microbes.
Without adequate control methods, lab results may show contamination, food would spoil more quickly, water and beverages would be undrinkable, and we would have less ability to prevent and treat infectious disease.
In this simulation, you will learn methods that can be used to control the growth of microbes and ways to test the effectiveness of these methods.
UV light damages DNA and limits the growth of bacteria under certain conditions. This lab simulation will test the effects of UV light exposure for various lengths of time to determine how much time is necessary to control growth.

Before you begin
In this simulation, a uniform layer of growth, called a lawn, will be created on multiple agar plates.
After a bacterial lawn is created, half of the plate will be exposed to UV light for different lengths of time. Inhibition of growth will be detected when the plate shows a difference between each half, as seen here:
Plate exposed to UV light for 20 seconds
A card is placed on the plate to shield the right half and prevent exposure to UV light. The organisms are unaffected and free to grow and form a lawn.
Make sure you are familiar with the following terms:
+ Agar media
+ Agar plate
+ Antisepsis
+ Aseptic technique
+ Bacterial lawn
+ Colony
+ Colony Forming Units (CFUs)
+ Culture
+ Disinfection
+ Heavy growth
+ Incubate
+ Inoculum
+ Intermediate growth
+ Isolation
+ Lawn
+ Liquid broth culture
+ Petri dish
+ Pure culture
+ Scant (or scanty) growth
+ Sterile
+ Sterilization
+ Vegetative cell
+ Zone of inhibition
UV light can be damaging to skin cells and the eyes with prolonged, unprotected exposure. Safety glasses, long sleeves and gloves will protect from this damage.
Remember that while this is a safe simulation, you should practice good laboratory hygiene protocols when translating this learning to the benchtop. This includes following aseptic technique procedures, proper handwashing, and the cleaning of benchtop surfaces as you complete similar exercises in the lab setting.

Inoculate plate with S. aureus bacterial sample
Label agar plate with marker
Turn on Bunsen burner
Pick up cotton applicator
Pick up bacterial culture tube. Remove cap. Briefly heat mouth of tube in Bunsen burner flame to reduce contamination
Insert cotton applicator into tube to acquire bacterial sample
Briefly heat mouth of tube, replace cap, and return tube to rack
Use cotton applicator to swab agar surface
Dispose of cotton applicator in waste bin
Turn off Bunsen burner
Phase 2: Expose plates to UV light
15 seconds exposure
1 minute exposure
5 minutes exposure
Select Incubate to incubate plates for 24 hours at 37°C
Phase 3: Observe growth
Observe growth of each plate and record observations and image in Lab Data
Phase 4: Graph and analyze
Select variable for X axis
Select variable for Y axis
Select graph type
Create graph
Save graph. Repeat steps to create additional graphs if needed
Phase 5: Apply what you have learned
Select your answer to the question
Phase 6: Save Lab Data
Relevant Lab Data is available to be saved for personal reference. Data will be available if you return to this laboratory simulation