Sterilization and Disinfection

• The lecture covers the control of microorganisms
• Students are expected to understand the importance of controlling microorganisms, different control mechanisms, antimicrobial agents and microbial resistance mechanisms.

Importance of Controlling Microorganisms

• Microorganisms can easily cause contamination, infection, or decay
• Control is necessary to remove or destroy them from consumed materials or areas
• Microbial control is important because it allows us to control infectious diseases, minimize infections, prevent food spoilage and ensure safe drinking water

Microorganism Control

• Microorganisms are controlled using antimicrobial agents
• Antimicrobial agent - an agent that kills or inhibits the growth of microorganisms
• Antimicrobial agents are either physical or chemical agents

Antimicrobial Agents

• Physical agents include:
  • Sunlight
  • Desiccation
  • Moist heat
  • Dry heat
  • Filtration
  • Radiation
  • Ultrasonic
• Chemical agents include:
  • Phenol
  • Halogens
  • Alcohol
  • Aldehydes
  • Metallic Salts
  • Oxidizing Agents

Microbial Resistance

• Microorganisms differ in their ability to withstand chemical and physical treatments due to cell wall composition and protective mechanisms
• Bacterial spores are the most resistant, followed by Mycobacteria, non-lipid viruses, fungi, bacteria, and lipid viruses
• Bacterial spores have coats rich in proteins, lipids, and carbohydrates and cores rich in dipicolinic acid and calcium
• Viruses containing lipid-rich envelopes are more susceptible to detergents and wetting agents

Sterilization

• Sterilization kills or destroys all forms of microbial life, including endospores in an object or media
• A sterile object is free of viable organisms, spores, and infectious agents
• Sterilant - any chemical agent used for sterilization
• Aseptic techniques are used to preserve sterility or prevent contamination
• Sterilization is mainly used in microbiology, surgery, and the food and drug industry to prevent contamination

Disinfection

• Disinfection reduces the number of pathogenic microorganisms to a point where they no longer cause diseases
• Disinfection usually involves the removal of vegetative pathogens and non-endospore forming pathogens
• Disinfectant - A chemical agent applied to inanimate objects
• Antiseptic - A chemical agent for disinfection applied to skin or living tissue
• Sepsis - presence of bacterial contamination
• Asepsis - absence of bacterial contamination

Germicides

• Bacteriostatic Agent - inhibits/stops the growth of bacteria, but does not necessarily kill them
• Germicide - an agent that kills certain microorganisms
  • Bactericide - kills bacteria
  • Virucide - inactivates viruses
  • Fungicide - kills fungi
  • Sporicide - kills bacterial endospores

Physical Methods to Control Microorganisms

• Physical methods discussed in the lecture are moist heat, dry heat, filtration, radiation

Moist Heat

• Moist heat kills microorganisms by coagulating their proteins.
• Moist heat is more effective than dry heat
• Boiling involves heating to 100°C or more
  • Boiling starts killing vegetative forms of bacterial pathogens, viruses/fungi, and spores within 10 minutes or less
  • Endospores and some viruses are not destroyed quickly, but brief boiling will kill most pathogens
  • Hepatitis virus can survive up to 30 minutes of boiling
  • Endospores can survive up to 20 hours or more of boiling
• Reliable sterilization requires temperatures above the boiling point of water
• An autoclave Chamber is filled with hot steam under pressure (121°C at 15 lbs pressure for 15 min)
  • A preferred method of sterilization for non-heat sensitive materials
• Pasteurization was developed by Louis Pasteur
  • Used to reduce microbes responsible for spoilage of beverages, milk, or juices
  • Classic Method - milk was exposed to 65°C for 30 minutes
  • HTST (High Temperature Short Time Pasteurization) - Milk is exposed to 72°C for 15 seconds
  • Ultra High Temperature Pasteurization (UHT) - Milk is treated at 140°C for 3 seconds and then cooled in a vacuum chamber
    • UHT Advantage - Milk can be stored at room temperature for several months
    • UHT Disadvantage - cause change in smell and taste and damage to proteins/vitamins

Dry Heat

• Dry heat kills microorganisms through oxidation effects
• Direct Flaming - sterilizes inoculating loops and needles
• Incineration - Effective for sterilizing disposable items (paper cups, dressings) and biological waste
• Hot-air sterilization - oven operated by electric power and mainly used for sterilizing glass wares
  • Flasks, tubes, pipettes are sterilized in microbiological laboratories.
  • The walls of the sterilizer are made of stainless steel or aluminum and devised to prevent heat conduction from inside to outside.

Filtration

• Filtration removes of microbes by passage of a liquid or air through a material with small pores
• Used for heat sensitive like vaccines, enzymes, antibiotics, and some culture media
• Membrane Filters: Uniform pore size, used in industry and research
  • 0.22 and 0.45um Pores: Filter most bacteria, but don't retain spirochetes, mycoplasma and viruses
  • 0.01 um Pores: Retain all viruses and some large proteins

Radiation

• Electromagnetic radiations are used for sterilization

• Electromagnetic radiations are two types:

• Ionizing Radiation: Gamma rays, X rays or electron beams with short wavelengths (less than 1 nanometer)

  • Dislodge electrons from atoms and form ions, produce peroxides

• Non-ionizing radiation: Cause mutations in DNA, forms pyrimidine dimers

  • Used to sterilize pharmaceuticals/disposable medical supplies; used in the food industry

• Disadvantages: Penetrates human tissues, can cause genetic mutations

Chemical Methods to Control Microorganisms

• The chemical agents that will be discussed in detail are phenol, halogens, alcohols, aldehydes, metallic salts, oxidizing agents

Phenol (carbolic acid)

• First used by Lister as a disinfectant
• Rarely used today because it is a skin irritant with a strong odour
• Destroys plasma membrane and denatures proteins
• Advantages: Stable, persist for long times after applied, and remain active in the presence of organic compounds.

Halogens

• Iodine:
  • Tincture of iodine was one of first antiseptics
  • Stains skin and clothes, irritating
  • Used as skin antiseptic in surgery
  • Ineffective against bacterial endospores
  • Example: Betadine
• Chlorine:
  • Mixed in water, forms hypochlorous acid
  • Used to disinfect drinking water, pools, and sewage
  • Sodium hypochlorite (NaOCI) is the active ingredient of bleach
  • Chloramines (NH2CI) consist of chlorine and ammonia and are less effective as germicides

Alcohol

• Kills most bacteria, fungi, but not endospores
• Act denaturing proteins and disrupting cell membranes
• Evaporates, leaving no residue
• Used to mechanically wipe microbes off skin before injections or blood drawing
• Not good for open wounds, cause proteins to coagulate

Aldehydes

• Formaldehyde gas:
  • Excellent disinfectant; commonly used as formalin, a 37% aqueous solution
  • Formalin was used extensively to preserve biological specimens and inactivate viruses and bacteria in vaccines
  • Disadvantage: Irritates mucous membranes, strong odour
• Gluteraldehyde:
  • Less irritating and more effective than formaldehyde
  • A few chemical disinfectants that is a sterilizing agent
  • Disinfect hospital instruments

Metallic Salts

• Heavy metals and their compounds are microbicidal:
  • Include silver, copper, selenium, zinc, and mercury
• Silver:
  • 1% silver nitrate used to protect infants against gonorrheal eye infections until recently
• Copper:
  • Copper sulphate is used to kill algae in pools and fish tanks
• Selenium:
  • Kills fungi and their spores, used for fungal infections, and in dandruff shampoos
• Zinc:
  • Zinc chloride is used in mouthwashes
  • Zinc oxide is used as antifungal agent in paints

Oxidizing Agents

• Ozone
  • Used to disinfect water, helps neutralize unpleasant tastes/odors
  • More effective as a killing agent than chlorine, but less stable and more expensive
• Hydrogen peroxide
  • Used as an antiseptic
  • Not good for open wounds because quickly broken down by catalase enzyme present in human cells
  • Effective in disinfection of inanimate objects; sporicidal at higher temperatures

Explore the differences between sterilization and disinfection methods, including moist heat, autoclaving, and chemical agents. Understand the mechanisms of microbial control and their effectiveness against various microorganisms. This lesson covers the primary goals and parameters of sterilization techniques.