Exercise 8

TURNING MICROBES ON AND OFF: CONTROL OF MICROBIAL GROWTH


Sterilization: kills all microorganisms

Disinfection: reduces the amount of microorganisms


Disinfectants do not necessarily sterilize an object because resistant spores and a few microorganisms remain.


The use of a microbial growth control method depends on the type of material to be sterilized and its volume or size.


Physical methods include:

  • changing the osmotic pressure of the microbial environment

  • employment of heat such as pasteurization, dry heat sterilization, moist heat sterilization, Tyndallizaton, and radiation


Moist Heat under Pressure

  1. Obtain three nutrient agar flasks with 50ml NA sterilized at 15psi or 121C at different lengths of time: Flask A: 15mins, Flask B: 10mins, Flask C: 5mins. An unsterilized nutrient agar flask will serve as the control

  1. Directly incubate the flasks overnight at 37C. Observe growth in the flask after 24hrs


Osmotic Pressure (effect of Sodium Chloride)

  1. Obtain 5 nutrient broth tubes with different concentrations of sodium chloride. Inoculate each with a loop-full of bacterial cultures.

  2. Obtain another set of tubes then inoculate with a yeast culture. Do the same to another set of tubes and observe growth after 2 to 7 days.


Effect of Ultraviolet Radiation

  1. Obtain a 24hr Culture of Serratia marcecseens and five nutrient agar plates. Label them A, B, C, D, E.

  2. Spread plate 0.1 ml of the culture on each plate. Immediately cover plate A with foil and incubate at 30C for 24-48hrs This will serve as control

  3. Expose plates B-E (top dish removed) at different lengths of time (10s, 30s, 60s 120s) to UV radiation via UV lamp under 50 mJ/cm squared within a laminar hood or mini-isolation chamber. Position plates about 4 inches from the lamp. Eliminate light from working area.