Microbial Control Methods

Microbial Control

• Sterilization eliminates all microbial life forms, including vegetative cells, spores, and viruses, from surfaces, media, or articles, potentially except for prions.
• Commercial sterilization uses sufficient heat to kill Clostridium botulinum endospores in canned foods.
• Antisepsis destroys vegetative pathogens on living tissues
• Disinfection kills vegetative pathogens on inanimate objects.
• Decontamination removes pathogenic microorganisms from objects to make them safe to handle.
• Degerming removes microbes from a limited area, for example, skin around an injection site.
• Sanitizing or sanitation lowers the bacterial count of objects to a safe public health level.
• Cleaning physically removes visible soil, dust, or foreign material through manual or chemical processes.
• Sepsis indicates the presence of microbial contamination, while asepsis indicates its absence.
• Bacteriostatic agents inhibit bacterial growth and reproduction.
• Bactericidal agents kill bacteria.

Aseptic vs. Clean Techniques

• Aseptic techniques aim to eliminate germs entirely.
• Clean techniques aim to reduce the number of germs whenever possible.

Microbial Death Rate

• When subjected to heat or antimicrobial chemicals, bacterial populations die at a constant rate.
• A microbial death curve plots this constant death rate logarithmically as a straight line.

Factors Influencing the Effectiveness of Antimicrobial Agents

• The Effectiveness of antimicrobial agents rely on factors like
  • Number of microbes
  • Environmental influences
  • Exposure time
  • Microbial characteristics
  • Presence of organic matter (blood, feces, vomitus)
  • Suspending medium nature

Actions of Microbial Agents

• Microbial agents can alter membrane permeability
• Damage proteins
• Damage nucleic acids.

Physical Methods of Microbial Control

• Heat is the most reliable and popular method to destroy microbes.
• Microbes are killed above their maximum growth temperature
• Metabolism is inhibited below minimum growth temperature.
• Thermal Death Time (TDT) is the minimum time needed to kill a microorganism population at a specific temperature and condition.
• Thermal Death Point (TDP) is the temperature at which a microorganism dies within a given time.
• Decimal Reduction Time (DRT) is the time in minutes required to kill 90% of a population at a given temperature.
• Moist heat sterilization works by breaking hydrogen bonds in bacterial proteins.
  • Boiling kills vegetative forms of bacterial pathogens, viruses, fungi, and some spores after 10 minutes.
  • Autoclaving uses steam under pressure (15 psi at 121°C for 15 minutes) to kill all organisms except prions, considered the most dependable method.
    • It is important to ensure steam directly contacts the materials being sterilized.
    • Aluminum should not be used as wrapping, trapped air should be removed, and mineral oil or petroleum jelly should be avoided due to preventing moisture penetration.
  • Pasteurization includes:
    • Holding Method (63°C for 30 minutes followed by rapid cooling to 13°C).
    • Flash Process (72°C for 15 seconds followed by rapid cooling to 13°C or below; store at low temperatures to retard microbial growth).
    • UHT (Ultra-High Temperature) – 140°C for 3 seconds
• Dry heat sterilization kills by oxidation.
  • Methods:
    • Flaming
    • Incineration
    • Hot-air sterilization requires 160°C for 2 hours and is used for items.
• Filtration uses screen-like materials with small pores to retain microorganisms from liquid or gas.
  • High-efficiency particulate air (HEPA) filters remove almost all microorganisms larger than 0.3 µm.
• Low temperatures
  • Ordinary refrigeration (0-7°C) is bacteriostatic, reducing metabolic rate to prevent reproduction or protein synthesis.
  • Rapid subfreezing temperatures render microbes dormant.
    • Deep-freezing (-50°C to -90°C) renders microbes dormant
    • Lyophilization (freeze-drying) quickly freezes and removes water through sublimation.
  • Slow freezing can harm bacteria due to ice crystal formation.
• Osmotic Pressure
  • High concentrations of salt and sugar preserve food by creating a hypertonic environment.
  • Molds and yeasts can grow in low moisture, high osmotic pressure, and acidic conditions.
• Radiation affects cells differently based on its type:
  • Ionizing - Shorter wavelengths, High energy, Fast duration
  • Non-ionizing - Longer wavelengths, Less energy, Slow duration
• Ionizing Radiation
  • Works by ionizing water to form hydroxyl radicals and damaging DNA, potentially causing nonlethal mutations.
  • High-energy electron beams are used to sterilize pharmaceuticals and medical supplies, and certain classes of postal mail.
• Non-ionizing Radiation
  • UV light at 260 nm is most effective for killing microorganisms by causing thymine dimers that inhibit DNA replication.
    • Used in germicidal lamps in hospitals, clinics, and cafeterias.
    • Features include low penetration and potential damage to human eyes and skin with prolonged exposure.
  • Visible Light
    • 470 nm wavelength kills methicillin-resistant Staphylococcus aureus.
    • Sunlight contains UV radiation and has antimicrobial effects but can be blocked by pigments produced by bacteria.
  • Microwaves
    • Kill most vegetative pathogens through heat generated by moisture.
• Other Methods:
  • High Pressure denatures bacterial proteins while preserving flavor in liquids.
  • Desiccation prevents microbial growth and reproduction but does not necessarily kill them.

Chemical Methods of Microbial Control

• Few chemical agents achieve sterility.
• Antiseptics are regulated by the FDA
• Disinfectants are regulated by the EPA
• Mode of Action of Disinfectants includes:
  • Protein coagulation
  • Cell membrane disruption
  • Removal of free sulfhydryl groups
  • Inhibition of respiration.
  • Loss of membrane integrity.
• The Disk Diffusion Method is used to evaluate the effectiveness of disinfectants.
• Phenol and Phenolics
  • Phenol (Carbolic acid) was first used by Joseph Lister for surgical infections, acting as both an antiseptic and disinfectant, but irritates the skin and has a disagreeable odor.
  • Phenolics (phenol derivatives) injure lipid-containing plasma membranes
    • Effective against Mycobacteria.
    • It is also a Suitable for disinfecting pus, saliva, and feces.
  • Cresol is a phenolic derived from coal tar.
  • O-polyphenol is a very important cresol, the main ingredient of Lysol.
  • Bisphenols (phenol derivative) contain two phenolic groups connected by a bridge.
    • Hexachlorophene is used in surgical and hospital microbial control procedures and to control infections in nurseries.
    • Triclosan, found in antibacterial soaps and toothpastes, inhibits an enzyme needed for fatty acid biosynthesis.
• Biguanides affect bacterial cell membranes.
  • Effective against gram-positive and gram-negative bacteria, except Pseudomonas.
  • Chlorhexidine is used for microbial control on skin and mucous membranes, known as the best biguanide.
• Halogens
  • Chlorine
    • Is an oxidizing agent widely used as a disinfectant.
    • Forms bleach when added to water.
    • Broad-spectrum, but not sporicidal.
  • Iodine
    • Is one of the oldest and most effective antiseptics.
    • It works by impairing protein synthesis and altering cell membranes.
    • Preparations include tincture and iodophor
• Alcohols
  • Effectively kill bacteria and fungi but not endospores and nonenveloped viruses.
  • They work by denaturing proteins, disrupting membranes, and dissolving lipids.
    • Ethyl alcohol (ethanol) is most effective at 70%.
    • Isopropyl (isopropanol) is slightly superior as an antiseptic or disinfectant and is less volatile and expensive.
• Heavy metals and their compounds
  • Silver is used as an antiseptic in 1% silver nitrate solutions to treat ophthalmia neonatorum.
  • Silver-sulfadiazine is a topical cream for burns.
  • Plastic food containers are infused with silver for freshness.
• Surface-acting agents include:
  • Soaps and Detergents for degerming.
  • Acid-anionic sanitizers for cleaning food-processing facilities.
• Chemical Food Preservatives
  • Sulfur dioxide to disinfect wine.
  • Sorbic acid and sodium benzoate to prevent molds in acidic foods.
  • Calcium propionate to prevent molds and Bacillus bacteria in bread.
• Aldehydes and Chemical Sterilant:
  • Aldehydes inactivate proteins by cross-linking functional groups; glutaraldehyde sterilizes surgical instruments.
• Plasma sterilization
  • Involves vaporizing hydrogen peroxide in low-temperature, low-pressure plasma, and then generates free radicals - effective for heat-sensitive materials.
• Oxidizing Agent: Hydrogen peroxide
  • Is effective against anaerobic bacteria and good for cleaning wounds.