CHAP6:CONTROL OF MICROBIAL GROWTH

• Microbial control


  The prevention of transmission of disease and infection, prevention of contamination or growth by undesirable microorganisms, and prevention of deterioration and spoilage of materials by microorganisms
• Salting, smoking, pickling, and drying foods and exposing food, clothing, and bedding to sunlight were prevalent practices among early civilizations
• The Greeks and Romans burned clothing and corpses during epidemics, and they stored water in copper and silver containers
• Burning wood releases formaldehyde, which could have acted as a disinfectant; herbs, perfume, and vinegar contain mild antimicrobial substances
• Relative resistance of microbial forms


  • Highest resistance: Bacterial endospores
  • Moderate resistance: Protozoan cysts; some fungal sexual spores (zygospores); some viruses. In general, naked viruses are more resistant than enveloped forms. Among the most resistant viruses are the hepatitis B virus and the poliovirus. Bacteria with more resistant vegetative cells are Mycobacterium tuberculosis, Staphylococcus aureus, and Pseudomonas species.
  • Least resistance: Most bacterial vegetative cells; ordinary fungal spores and hyphae; enveloped viruses; yeasts; and trophozoites
• Sterilization


  Destruction of all forms of microbial life including endospores, which are the "most resistant form". This is most often done with heat. There are no degrees of sterilization: an object is either sterile or not. A few chemicals called sterilants can be classified as sterilizing agents because of their ability to destroy spores.
• Commercial sterilization


  Heat treatment that kills endospores of Clostridium botulinum, the causative agent of botulism, in canned food. Does not kill endospores of thermophiles, which are not pathogens and may grow at temperatures above 45°C.
• Disinfection


  Destruction of pathogenic organisms on an inanimate (lifeless) object, such as a table-top. This process destroys vegetative pathogens but not bacterial endospore.
• Examples of disinfection


  • Applying a solution of 5% bleach to examining table
  • Boiling food utensils used by a sick person
  • Immersing thermometers in an isopropyl alcohol solution between use
• Antisepsis


  Destruction of pathogenic organisms on a living object, such as the skin surface.
• Examples of antisepsis


  • Preparing the skin before surgical incisions with iodine compounds
  • Swabbing an open root canal with hydrogen peroxide
  • Ordinary hand washing with a germicidal soap
• Sanitization


  Reduction in the number of pathogens to a level deemed safe by public health guidelines. A sanitizer is a compound (e.g., soap or detergent) that is used to perform this task.
• Examples of sanitization


  • Air sanitization with ultraviolet lamps reduces airborne microbes in hospital rooms, veterinary clinics, and laboratory installations
• Degerming


  Physical removal of microorganisms by using such things as soaps or detergents. This process usually involves scrubbing the skin or immersing it in chemicals, or both. It also emulsifies oils that lie on the outer cutaneous layer and mechanically removes potential pathogens from the outer layers of the skin.
• Examples of degerming procedures


  • Surgical hand scrub
  • Application of alcohol wipes to the skin
  • Cleansing of a wound with germicidal soap and water
• Germicide


  Any chemical agent that kills microorganisms
• Bactericide


  An agent that destroys bacteria
• Fungicide


  One that kills fungi
• Virucide


  One that kills viruses
• Sporocide


  An agent that kills bacterial endospores of fungal spores
• Bacteriostatic agent


  Prevents the further multiplication of bacteria without necessarily killing all that are present
• Sepsis


  Comes from Greek for decay or putrid; indicates bacterial contamination
• Asepsis


  Absence of significant contamination
• Aseptic techniques


  Used to prevent contamination of surgical instruments, medical personnel, and the patient during surgery. Also used to prevent bacterial contamination in food industry.
• The more microbes present, the more time it takes to eliminate population
• Endospores are very difficult to destroy. Vegetative pathogens vary widely in susceptibility to different methods of microbial control.
• Factors influencing the effectiveness of antimicrobial treatment

  • Organic matter (blood, feces, etc.) often interferes with chemical antimicrobials, and also to a lesser extent with heat treatment. Any medium containing fats or proteins tends to protect bacteria (inhibit antimicrobials)
  • pH - heat is more effective in an acid pH
  • Time of Exposure - Chemical antimicrobials and radiation treatments are more effective at longer times. In heat treatments, longer exposure compensates for lower temperatures
• Alteration of membrane permeability

  The plasma membrane regulates the entry of materials into and the exit of wastes out of the cell. Damage to the plasma membrane causes leakage of cell contents into the surroundings, killing the cell or at least preventing cell division.
• Death in microbes is loss of the ability to reproduce
• Mode of action of surfactants on the cell membrane

  Surfactants inserting in the lipoidal layers disrupt it and create abnormal channels that alter permeability and cause leakage both into and out of the cell.
• Damage to proteins


  Enzymes and other proteins are essential for cell function. Hydrogen bonds hold proteins in the characteristic 3-dimensional shape required for their functions. Heat and certain chemicals break these bonds and the shape is lost, which is called denaturation.
• Modes of action affecting protein function

  • The functional native state is maintained by bonds that create active sites to fit the substrate. Some agents denature the protein by breaking all or some secondary and tertiary bonds, resulting in complete unfolding or random bonding and incorrect folding. Some agents react with functional groups on the active site and interfere with bonding.
  • Covalent bonds, which are also part of protein structure, may be broken by chemicals or heat even though they are stronger than hydrogen bonds.
• Damage to nucleic acids

  DNA and RNA carry the cell's genetic information and function in protein synthesis. Damage to these by heat, radiation, or chemicals usually kills the cell.
• Heat


  Most common method, effective, least expensive. Denatures (coagulates) cell proteins (enzymes). Two forms: moist heat, dry heat.
• As a rule, higher temperatures (exceeding the maximum) are microbicidal, whereas lower temperatures (below the minimum) tend to have inhibitory or microbistatic effects.
• Sterilization by moist heat


  Moist heat occurs in the form of hot water, boiling water, or steam (vaporized water). In practice, the temperature of moist heat usually ranges from 60 to 135°C. Adjustment of pressure in a closed container can regulate the temperature of steam. Moist heat kills microorganisms by denaturation and coagulation of proteins.
• Boiling


  98-100°C/10 min. Inactivates most vegetative cells, not heat-resistant forms (endospores, viruses & bacterial toxins). Used for drinking water, canning jars, etc.
• Steam sterilizer


  Usually, Koch's or Arnold's steam sterilizer is used for heat-labile substances that tend to degrade at higher temperatures and pressure, such as during the process of autoclaving. (Except Thermophiles)
• Autoclave


  Steam under pressure, temperature steam 15 psi pressure/121°CI5- 20 min. Destroys all forms sterilization. In addition to coagulating proteins, causes hydrolysis. Used for media, surgical instruments, etc.
• Uses of autoclave


  • It is a good method to sterilize heat-resistant materials, such as glassware, cloth (surgical dressings), rubber (gloves), metallic instruments, liquids, paper, some media, and some heat-resistant plastics.
  • It is also useful for sterilization of heat-sensitive items, such as plastic Petri plates that need to be discarded.
  • It is useful for sterilization of materials that cannot withstand the higher temperature of the hot-air oven.