MicroLab - Quiz (UV)

radiation

• an energy in the form of electro-magnetic waves or particulate matter, traveling in the air

types of radiation that kills microbes

• ionizing radiation

• non-ionizing radiation

• microwave radiation

ionizing radiation

• higher energy electromagnetic waves (gamma) or heavy particles (beta and alpha)

• high enough energy to pull electrons from orbit

• a type of radiation that is able to disrupt atoms and molecules on which they pass through, giving rise to ions and free radicals

• short wavelengths (less than 1 nanometer)

• dislodge electrons from atoms and forms ions

• causes mutations in DNA and produce peroxides

• used to sterilize pharmaceuticals and disposable medical supplies

• food industry is interested in using ionizing radiation

• disadvantages = penetrates human tissues which cause genetic mutations in humans

non-ionizing radiation

• lower energy electromagnetic waves

• not enough energy to pull electron from orbit, but can excite the electron

• irradiation can directly impair critical cell functions or components (DNA)

  • single-strand breaks = repairable in most cases

  • double-strand breaks = not repairable in most cases

• irradiation can indirectly form radiolytic product/free radicals from water (H, OH)

  • OH radicals are responsible for 90% of DNA damage

primary types of ionizing radiations

• gamma rays or photons

• X-rays or photons

• neutrons

non-ionizing radiation — ultraviolet light

• wavelength is longer than 1 nanometer

• damages DNA by producing thymine dimers, which causes mutations

• used to disinfect operating rooms, nurseries, cafeterias

• disadvantages = damages skin, eyes, doesn’t penetrate paper, glass, and cloth

ultraviolet radiation

• defined as the portion of the electromagnetic spectrum between x-rays and visible light

  • example: between 40 and 400 nanometer

• can be divided into:

  • UVC - 220-290 nm

  • UVB - 290-320 nm

  • UVA - 320-400 nm

sources of UV radiation

• sun = the primary natural source of UV radiation

• artificial sources: 

  • tanning booths

  • black light

  • curing lamps

  • fluorescent light

  • incandescent light

  • some types of layers

UVC

• almost never observed in nature = absorbed completely in the atmosphere

• germicidal lamps are designed to emit UVC radiation because of its ability to kill bacteria

• in humans, the UVC is absorbed in the outer layer of the epidermis

• accidental exposure can cause:

  • corneal burns (welders flash)

  • snow blindness = severe sunburns to the face

UVB

• typically the most destructive form of UV radiation because it has enough energy to produce photochemical damage to cellular DNA but not enough to completely absorbed by the atmosphere

• UVB is needed for synthesis of vitamin D

• harmful effects:

  • erythema (sunburn)

  • cataracts

  • development of skin cancer

UVA

• the most commonly encountered type of UV light

• initial exposure to UVA has a darkening (tanning) effect on the skin

• atmospheric ozone absorbs very little of this part of UV spectrum

• needed by humans for synthesis of vitamin D

• over-exposure to UVA has been associated with toughening of the skin, suppression of immune system, and cataract formation

• UVA is often called black light

• most phototherapy and tanning booth use UVA lamps

approximate doses for radiation needed to kill various organisms

• higher animals = 0.005 to 0.10

• insects = 0.01 to 1

• non-spore forming bacteria = 0.5 to 10

• bacterial spores = 10 to 50

• viruses = 10 to 200

microwave radiation

• wavelength ranges from 1 millimeter to 1 meter

• heat is absorbed by water molecules 

• may kill vegetative cells in moist food

• bacterial endospores, which do not contain water, are not damaged by microwave radiation

• solid foods are unevenly penetrated by microwaves

• trichinosis outbreaks have been associated with pork cooked in microwaves

pH

• the pH of each batch of Mueller-Hinton agar should be checked when the medium is prepared

• the agar medium should have a pH between 7.2 and 7.4 at room temperature

• if the pH is too low, certain drugs will appear to lose potency while other agents may appear to have excessive activity

• if the pH is too high, the opposite effects can be expected

moisture

• just before use, excess surface moisture is present, the plates should be placed in an incubator (35C) or a laminar flow hood at room temperature with lids ajar until excess surface moisture is lost by evaporation (usually 10-30 mins) 

• the surface should be moist, but no droplets of moisture should be apparent on the surface of the medium or on the petri dish covers when the plates are inoculated

inoculum density

• usually optimal results are obtained with an inoculum size that produces near confluent growth

timing of disc application

• if the plates after being seeded with the test strain, they are left at room temperature for periods longer than the standard time, multiplication of the inoculum may take place before the discs are applied

• this causes a reduction in the zone diameter and may result in a susceptible strain being reported as resistant

temperature of incubation

• susceptibility tests are normally incubated at 35C for optimal growth

• if the temperature is lowered, the time required for effective growth is extended and larger zones result

• at higher temperatures, the entire culture appears to be susceptible

incubation time

• most techniques adopt an incubation period of between 16-18 hours

size of plate, depth of agar medium, and spacing of antibiotic discs

• susceptibility tests are usually carried out with 9-10 cm plates and no more than 6-7 antibiotic discs on each plate

• if larger numbers of antibiotics have to be tested, two plates, or one 14 cm diameter plate is to be preferred 

• excessively large inhibition zones may be formed on very thin media; the converse is true for thick media

• minor changes in the depth of the agar layer have negligible effect

• proper spacing of the discs is essential to avoid overlapping of the inhibition zones or deformation near the edge of the plate

potency of the antibiotic discs

• the diameter of the inhibition zone is related to the amount of drug in the disc

• if the potency of the drug is reduced owing to deterioration during storage, the inhibition zone will show a corresponding reduction in size

methods of antimicrobial susceptibility testing

• diffusion

• dilution

• diffusion and dilution