07 Chemical and physical control of microbes

Physical and Chemical Control of Microorganisms

Introduction

• Sterilization

  • Definition: The removal of all life forms, including viruses.

  • Bacterial endospores are the most resistant biological entities.

  • Example: Deinococcus radiodurans is highly resistant to radiation.

Physical Control with Heat

Thermal Death

• Effects of Heat

  • Denaturation of cellular macromolecules.

  • Dehydration of microbial cells.

  • Thermal death time: Time taken for a specific population of microorganisms to be killed at a specific temperature.

  • Thermal death point: Minimum temperature at which a species begins to die.

Heat Sterilization Methods

Direct Flame

• Utilizes a Bunsen burner for sterilizing items such as inoculating loops and the mouths of culture tubes.

• Incineration is used for diseased or contaminated animal carcasses.

Hot-Air Oven

• Operates at dry heat, typically up to 160°C.

• Effectively sterilizes powders, water-free oily materials, and glassware.

• Must remove organic materials to prevent insulation against dry heat.

Boiling Water

• Utilizes moist heat at 100°C.

• Denatures proteins and other macromolecules.

• Does not truly sterilize; bacterial endospores, some fungal spores, and resistant viruses can survive.

Autoclave

• Employs moist heat under pressure to reach 121°C at 15 psi.

• Effective for sterilizing liquids, glassware, metal items, and contaminated fabrics.

• Limitations include melting of some plastics and breakdown of certain chemicals.

Fractional Sterilization

• Materials exposed to steam for 30 minutes over three consecutive days.

  • Day 1: Bacteria are killed; surviving spores germinate when cooled.

  • Day 2: New germinated bacteria are killed.

  • Day 3: Any remaining bacteria are killed.

Pasteurization

• Not a sterilization process; reduces population of bacteria in foods that lead to spoilage and disease.

• Bacterial spores are not affected by this method.

  • Holding method: Heat milk at 62.9°C for 30 minutes to eliminate harmful bacteria.

  • Flash pasteurization: Heat milk for 15 seconds at 71.6°C.

  • Ultrapasteurization: Heat for 3 seconds at 82°C.

Physical Control by Other Methods

Filtration

• Microorganisms are trapped on a porous membrane, and the filtrate is sterile if all microbes are captured.

  • Inorganic filters: Glass fibers, porous porcelain.

  • Organic filters: Diatomaceous earth.

  • Membrane filters: Cellulose acetate, polycarbonate, nylon.

  • HEPA filters: Capture particles with diameters greater than 0.3 µm in the air.

Ultraviolet Radiation

• Non-ionizing radiation with a wavelength of 100-400nm, which is bactericidal.

  • Induces thymine dimers in DNA when absorbed, hindering replication by DNA polymerase.

Other Types of Sterilizing Radiation

• X-rays and gamma rays: Short wavelength radiation that causes ionization of molecules, especially DNA.

  • Used for sterilizing plastics, some drugs, and biochemicals.

  • Irradiated foods and sterilization of mail post-2001 anthrax attacks.

Other Physical Methods for Food Preservation

• Drying: Eliminates moisture to inhibit microbial growth; used for meats, fruits, and cereals.

• Salting: Increases osmotic pressure on microorganisms.

• Low Temperatures: Refrigeration and freezing retard growth; can resume on thawing.

• Freeze Drying (Lyophilization): Removes water under vacuum while frozen.

General Principles of Chemical Control

Disinfection

• Different from sterilization as it targets only pathogenic organisms.

• Disinfectants: Decontaminate inanimate objects.

• Antiseptics: Kill microorganisms on body tissues.

• Bactericidal agents kill bacteria, whereas bacteriostatic agents inhibit further growth.

Properties of Antiseptics and Disinfectants

• Can kill or impede microbial growth.

• Nontoxic to humans or animals.

• Soluble in water with a good shelf life.

• Effective in diluted form and work quickly.

Important Chemical Agents

Halogens

• Oxidize proteins; effective in gaseous or soluble form (e.g., chlorine).

• Chlorine: Effective at low concentrations; used in bleach and chloramines for wound antisepsis.

• Iodine: Used as a wound antiseptic; forms iodophors (iodine-detergent complexes) like Betadine.

Phenol and Phenolic Compounds

• Used as disinfectants since Lister in the 1860s; active against Gram-positive bacteria by denaturing proteins.

• Disadvantages: expensive, caustic to skin, pungent odor.

• Cresols and bisphenols (two phenol molecules) are commonly used in disinfection.

Heavy Metals

• Interfere with metabolic activity.

  • Mercury: Typically mercuric chloride, used in skin disinfectants.

  • Copper: Used as an algicide (copper sulfate).

  • Silver: Silver nitrate used as antiseptic drops for infants.

Alcohols

• Denature proteins and disrupt membranes; effective for skin antiseptics and disinfecting surgical instruments.

• Common solutions include 50-80% ethanol and isopropyl alcohol.

Aldehydes

• Inactivate proteins and nucleic acids via alkylation.

  • Formaldehyde: Used as a sterilizing agent effective against viruses but leaves a residue.

  • Glutaraldehyde: Very effective for sterilizing surgical instruments and optical equipment.

Ethylene Oxide

• Used to sterilize materials such as paper and rubber; carcinogenic and requires closed container use.

Hydrogen Peroxide

• Damages cellular components and produces oxygen free radicals; used for disinfecting wounds and plastics.

Soaps and Detergents

• Soaps: Fatty acids with NaOH or KOH; mechanical removal of bacteria and creates alkaline conditions on the skin.

  • Detergents: Disrupt surface tension, perturb microbial membranes:

    • Anionic detergents: Negatively charged, not highly effective against bacteria.

    • Cationic detergents: Positively charged and react with phospholipids.

    • Nonionic detergents: No net charge, effective in disrupting membranes.

Acids

• Used as preservatives and treatments for infections.

  • Common examples: Benzoic acid, salicylic acid, lactic acid.