Experimental Design

Experiment 1

Problem Statement:

To what extent does the concentration of bacteria in a medium affect the concentration of bacteria remaining in the medium as a result of the product’s UV light sterilization?

Variables:

Independent Variable: Concentration of bacteria (CFU/mL)
Dependent Variable: Concentration of bacteria remaining in the medium (CFU/mL)
Control Variables:
- Type of bacterium
- Number of bacteria in each colony
- Type of medium the bacteria are in
- The volume of petri dish
- UV light type
- Duration of UV light emission

Groups:

Experimental: Concentrations of bacteria at 10-2 CFU/mL, 100 CFU/mL, 102 CFU/mL, 104 CFU/mL
Control: Concentration of bacteria at 1 CFU/mL

Materials:

Compound light microscope
Ten Petri dishes
UV lamp (- bulbs)
Halogen light bulb
Micrococcus luteus bacteria
Two 1 cm cuvettes
Water
Timer
Proper clothing and equipment (dark, polyester or nylon, covering all skin)

Procedure:

Set up the experiment in an isolated medium with limited concentrations of other microbial species.
Attach the - bulb into the UV lamp to produce the proper UV(-) light needed, turn on the lamp.
Supply the first bacterial concentration (10-2 CFU/mL) of Micrococcus luteus into one Petri dish.
Place the Petri dish under the UV lamp and set the timer for 10 minutes.
After the 10 minutes are complete, remove the Petri dish and record the bacterial concentration remaining in the dish with the microscope using a method called “optical density.”
Pour 5 mL of water into the Petri dish and mix the cultures until they are fully dissolved in the water.
Pour the solution into one 1 cm thick cuvette and pour 5 mL of water into another cuvette.
Replace the UV bulb with the Halogen light bulb and shine the light into the cuvette filled with the base medium of water. Record the amount of light that is received by the cuvette.
Shine the light through the cuvette filled with the solution and record the amount of light that passes through.
Repeat steps 3-5 with the other logarithmically proportioned bacterial concentrations with two trials for each concentration. There should be 10 trials overall.

Experiment 2

Problem Statement:

How does the time of exposure of the ultraviolet radiation on bacterial cultures in a medium affect the concentration of bacteria remaining in the medium?

Variables:

Independent Variable: Time of exposure (min)
Dependent Variable: Concentration of bacteria remaining in the medium (CFU/mL)
Control Variables:
- Type of bacterium
- Number of bacteria in each colony
- Type of medium
- The volume of the petri dish
- Type of UV light
- The concentration of bacteria in each petri dish

Groups:

Experimental: 10 min, 20 min, 30 min, 40 min, 50 min, 60 min of exposure
Control: 0 min of exposure

Materials:

Compound light microscope
Fourteen Petri dishes
UV lamp (- bulb)
Micrococcus luteus bacteria
Timer
Proper clothing and equipment (dark, polyester or nylon, covering all skin)

Procedure:

Set up the experiment in an isolated medium with limited concentrations of other species and other factors that may influence the results of the experiment.
Attach the (-) bulb into the lamp to produce the proper UV(-) light needed, turn on the lamp.
Place the bacterial concentration (1 CFU/mL) of Micrococcus luteus into one Petri dish.
Record the concentration of bacteria in the Petri dish with the microscope for the control group result (0 min) using optical density.
Take a new Petri dish with 1 CFU/mL of bacteria.
Place the Petri dish under the lamp for the first duration of exposure (10 min) and set the timer for 10 minutes.
After the 10 minutes have elapsed, remove the Petri dish and record the bacterial concentration remaining in the Petri dish with the microscope.
Repeat steps 5-7 for the other durations of exposure of the ultraviolet radiation with two trials for each duration. There should be 14 trials overall.

Experiment 3

Problem Statement:

What type of ultraviolet radiation would be most effective in eliminating the greatest concentrations of bacteria in a medium?

Variables:

Independent Variable: Type of UV radiation
Dependent Variable: Concentration of bacteria remaining in the medium (CFU/mL)
Control Variables:
- Type of bacterium
- Number of bacteria in each colony
- Type of medium the bacteria are in
- The volume of the petri dish
- The duration of the UV light emission
- The energy density (µJ/cm3) of UV light
- The intensity (µW/cm2) of the UV light

Groups:

Experimental: UVA, UVB, UVC rays
Control: No UV light

Materials:

Compound light microscope
Twelve Petri dishes
UV lamp
Bulbs (BL368 UVA, Fluorescent UVB, GERMICIDAL UVC)
Micrococcus luteus bacteria
Timer
Proper clothing and equipment (dark, polyester or nylon, covering all skin)

Procedure:

Set up the experiment in an isolated medium with limited concentrations of other species and other factors that may influence the results of the experiment.
Attach the first bulb (BL368) into the lamp to produce the proper UVA light needed, turn on the lamp.
Place the bacterial concentration (1 CFU/mL) of Micrococcus luteus into one Petri dish.
Record the concentration of bacteria in the Petri dish with the microscope for the control group result (no light) using optical density.
Take a new Petri dish with 1 CFU/mL of bacteria.
Place the Petri dish under the lamp emitting UVA light and set the timer for 10 minutes.
After the 10 minutes have elapsed, remove the Petri dish, and record the bacterial concentration remaining in the Petri dish with the microscope.
Repeat steps 5-7 for the other types of ultraviolet radiation, conducting three trials for each ultraviolet type (including no light). There should be 12 trials overall.