Session 8 - Control & Destruction

• Microbial control is essential in order to prevent the unwanted microbial contamination, transmission of diseases and infection, stop decomposition and spoilage of food.
• The effects of microorganisms can often be controlled by simply limiting or inhibiting growth. Microorganisms are controlled by means of physical agents and chemical agents.
• Sterilization – the removal or destruction of all living microorganisms. It is one of the example of physical agent. It is the killing or removal of all microorganisms (including viruses).
• Decontamination – the treatment of an object or surface to make it safe to handle. As a result, it needs wiping off to remove fragments before using.
• Disinfection – a process that directly targets pathogens although it may not eliminate all microorganisms. It requires agents called disinfectants that actually kill microorganisms or severely inhibit their growth.
• Sepsis refers to the presence of pathogens in blood or tissues, whereas asepsis means the absence of pathogens.
• Antisepsis is the prevention of infection.
• Microbial control means to kill or to inhibit the growth of microorganisms.
• Heating is the most common method used for killing microbes, including the most resistant forms, such as endospores.
• Liquids or gases can be sterilized by filtration.
• Physical methods are used extensively in industry, medicine, and the home.
• Physical methods of microbial control can be classified into three classes – heat, radiation, and filtration.
• Heat – the most widely used method of physically treating an object or substance to render it sterile.
• There are two factors that determine the effectiveness of heat sterilization: time and temperature.
• Higher temperature and shorter time are required to kill pathogens.
• Decimal Reduction Time is quantified by the time required for a 10-fold reduction in the viability of a microbial population at a given temperature.
• Thermal death time is the time it takes to kill all cells at a given temperature. It is greatly affected by population size.
• Types of Heat:

  1. Moist Heat
  2. Dry Heat
• Moist heat – has more penetrating power and inhibits growth or kills cells more quickly than does dry heat.
• Types of Moist Heat:

  1. autoclave
  2. pasteurization
• Autoclave – sealed heating device that uses steam under pressure to kill microorganisms.
• In autoclaves, it is not the pressure inside the autoclave that kills the microorganisms but the high temperatures that are achieved when steam is placed under pressure.
• Steam under a pressure of 15 PSI, which yields a temperature of 121 °C for 15 mins.
• Pasteurization uses heat to significantly reduce rather than totally eliminate the microorganisms found in liquids, such as milk.
• Types of pasteurization:

  • Flash Pasteurization
  • Ultrahigh Temperature (UHT) pasteurization
• Flash Pasteurization – 71°C for 15 seconds, after which it is rapidly cooled.
• Ultrahigh Temperature (UHT) pasteurization - requires heat treatment at 135°C for 1–2 sec and actually sterilizes the milk such that it can be stored at room temperature for long periods without spoilage.
• Dry heat – effective sterilization of metals, glasswares, some powders, oils and waxes.
• Types of Dry Heat:

  1. Incineration (burning)
  2. Flaming
• Incineration (burning) – effective means of destroying contaminated disposable materials; intense heat ignites and reduces microbes to ashes and gas; limited to metals and heat-resistant glass materials.
• Flaming – accomplished by holding the end of the loop or forceps in the yellow portion of the gas flame.
• Ultraviolet (UV) Radiation - Useful for disinfecting surfaces and air.
• Ionizing radiation - electromagnetic radiation of sufficient energy to produce ions and other reactive molecular species from molecules with which the radiation particles collide.
• Filtration - Used to separate cells, larger viruses, bacteria, certain microbes from liquids or gases in which they are suspended.
• FIltration is a unique method of sterilization as it removes contaminants instead of killing them.
• Cotton plug in test tube, flask, pipette is a good filter for preventing the entry of microbes.
• Types of Filters:

  1. Depth filters
  2. Membrane filters
  3. Nucleopore filters
• Depth filters – important in biosafety applications such as in a biological safety cabinet.
• Membrane filters – most common filters used for liquid sterilization.
• Nucleopore filters – commonly used to isolate specimens for scanning electron microscopy (SEM).