Control by Heat and UV and Unknown Identification

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Control of microbial growth refers to the inhibition and prevention of growth of microorganisms. Its application is a significant process in various industries such as agriculture, medicine, and food production and preparation. Microorganisms get controlled by two ways namely killing or inhibiting growth using physical or chemical agents. The agent used for the killing of microorganisms gets known as a cidal agent, whereas the agent for inhibiting growth gets referred to as a static agent.

UV light is a type of radiant energy that is used in controlling microorganisms. UV light has a wavelength ranging between 100 and 400 mm with the energy at approximately 265 nm most destructive to bacterial cells. When microorganisms get subjected to UV light, cellular DNA absorbs the energy, and adjacent thymine molecules link together kinking the double helix and disrupting DNA replication. The destroyed organism can no longer generate vital proteins or reproduce, thus quickly dies.

UV light effectively minimizes the microbial population where direct exposure takes place. UV light gets used to limit airborne or surface contamination in a hospital room, pharmacy, morgue, toilet facility, and also a food service operation (Elander, 2005). UV light from the sun may also be significant in controlling microorganisms in the air and the upper layers of the soil, although it may not be effective against all bacterial spores. UV Light is also sometimes used to sterilize vaccines, serum and drinking water. However, the use of UV light has some disadvantages. UV light can effectively penetrate dirt films, glass, water and other substances, and also can damage the eyes and prolonged exposures can lead to skin burns and cancer (Sumbali & Mehrotra, 2009).



  • A gram-positive coccus
  • A gram-negative rod


  • Acquire 2 Trypticase Soy Agar (TSA) plates. One plate will get inoculated with a gram-positive coccus and the other plate with a gram-negative rod.
  • Label the two plates with name, lab time, and organism
  • Divide the bottom of the plate in half using a labeling marker.
    • Label one side of the plate control.
    • Label the other side of the plate UV Exposure.
  • Inoculate a sterile swab into the culture.
  • Plate the organism by making three ziga zag lines on the agar surface
    • Make the first ziga zag line.
    • Rotate plate 450.
    • Make the second ziga zag line.
    • Rotate plate 450.
    • Make the third ziga zag line.
    • A “lawn” of bacteria will get obtained at this stage.
  • UV Light Exposure
    • Wear safety glasses during the procedure.
    • Do not look into the UV light source.
    • Keep the light source pointed away from you and other students.
    • Remove the lid from half of the plate.
    • Keep the other half covered with the plate.
    • Expose the plate for 5 minutes.
  • Incubate the plates at 350C for 48 hours.

Unknown Identification


Microorganisms get identified for various reasons. The reasons include knowing the causative agent of a disease or the pathogenic organisms in a patient so as to treat the illness appropriately and knowing the correct microorganisms for use in making certain foods or antibiotics. The applications in the clinical setting include a request by the clinical lab microbiologist to determine the organism present in a patient’s blood, or quality control microbiologist may want to understand the identity of contaminant food sample (Alcamo & Pomerville, 2004). Another application is that a drug company may wish to determine bacteria from the soil during the search of new antibiotics, or a water quality microbiologist may wish to identify the bacteria in water as part of a regular monitoring program.


At the beginning of the laboratory test, the lab instructor handed each student a numbered test tube that contained unknown bacteria (a gram-positive coccus and a gram-negative rod). The test label for this case was 201. Isolating techniques were first performed on the bacterium first to determine the bacterium present. Laboratory techniques and procedures got used throughout the experiment. Isolation streak got conducted using an inoculating loop and dry technique, and also nutrient agar plates got incubated at a room temperature for the next 24 hours.

After 24 hours of incubation, the nutrient agar got examined for bacterial growth of two different colonies. Two different cultures got observed on a nutrient agar plate. So as to continue with the identification, the cultures got isolated as separate organisms, using inoculating streak technique and incubated for the next class period. The outcome that was expected was supposed to indicate a gram-positive coccus and a gram-negative rod bacterium that was growing on the two different media. Once the organism gets confirmed to be pure-gram positive, the remainder of the colony gets transferred to a nutrient agar slant.

Inoculation of the isolated gram-negative and gram-positive unknowns on to the right microbiological media gets performed so as to identify their identity. A gram stain on bacterial growth from each agar slant is done to confirm that the cultures are pure. The dichotomous key gets followed to determine the identity of the sample. The tests and results of the laboratory experiment got recorded.


Alcamo, E., & Pomerville, J. C. (2004): Alcamo’s Laboratory fundamental of microbiology.

Elander, R. P. (2005): Alcamo’s Fundamentals of Microbiology. By Jeffrey C Pommerville. Sudbury (Massachusetts): Jones and Bartlett Publishers. $99.95. xxx+ 1001 p+ A1–A23+ G1–G24+ I1–I32; ill.; index. ISBN: 0‐7637‐0067‐3. 2004. The Quarterly Review of Biology, 80(2).

Sumbali, G., & Mehrotra, R. S. (2009): Principles of Microbiology. Tata McGraw-Hill.

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