Chemical Germicides Overview

Using Chemicals to Destroy Microorganisms and Viruses

• Germicidal chemicals are used to disinfect or sterilize.
• These chemicals react irreversibly with vital proteins, DNA, cytoplasmic membranes, or viral envelopes.
• Mechanisms of action are often poorly understood.
• Chemicals are generally less effective than heat.
• They are suitable for treating large surfaces and heat-sensitive items.
• Some chemicals can be used as antiseptics.
• Some chemicals have weak germicidal but bacteriostatic activity; thus, they can be used as preservatives.

Selecting the Appropriate Germicide

• Toxicity: Germicides are often toxic to humans and the environment. Weighing the risk versus the benefit is crucial. EPA's "Safer Choice" label identifies less hazardous products.
• Activity in the Presence of Organic Matter: Hypochlorite (bleach) is easily inactivated by organic material, but phenolics can tolerate some organic matter.
• Compatibility with Material: Liquid disinfectants are not always compatible with all materials. Gaseous alternatives are sometimes necessary. Hypochlorite can corrode some metals and rubber.
• Residue: Some germicides leave toxic or corrosive residues, requiring thorough rinsing of sterilized items with sterile water.

Potency of Germicidal Chemicals

• Formulations commonly contain one or more antimicrobial agents.
• FDA regulates chemicals used in processing medical devices.
• EPA regulates disinfectants.
• Germicides are grouped by potency.
  • Sterilants (Sporocides): Destroy all microbes, including endospores and viruses. Useful on heat-sensitive instruments.
  • High-level disinfectants: Destroy all viruses and vegetative cells, but not endospores.
  • Intermediate-level disinfectants: Kill vegetative cells, including mycobacteria, fungi, and most viruses, but not all endospores.
  • Low-level disinfectants: Removes vegetative bacteria, fungi, and enveloped viruses but not mycobacteria, naked viruses, or endospores.

Classes of Germicidal Chemicals

Alcohols

• Aqueous solutions of 60-80% ethanol/isopropanol rapidly kill vegetative bacteria and fungi but not endospores or naked viruses.
• Coagulate enzymes and proteins, damage lipid membranes.
• 70% ethanol solutions are more effective than pure alcohol due to water solubility of proteins and thus denaturation.
• Used as antiseptics to degerm skin and as disinfectants.
• CDC recommends alcohol-based hand sanitizers for health personnel. More effective than soap and convenient.

Aldehydes

• Include glutaraldehyde, orthophthalaldehyde (OPA), and formaldehyde.
• Glutaraldehyde: destroys microbes and viruses by inactivating proteins and nucleic acids.
• A 2% solution of alkaline glutaraldehyde is one of the most widely used liquid sterilants.
• Destroys all forms of microbial life, including endospores and viruses.

Orthophthalaldehyde (OPA)

• A new type of disinfectant being studied as an alternative to glutaraldehyde.

Formaldehyde

• Used as a gas or an aqueous solution (formalin).
• An extremely effective germicide that kills all forms of life within minutes.
• Used to kill bacteria and inactivate viruses for use in vaccines and to preserve biological specimens.
• Irritating vapors and suspected carcinogen.

Biguanides

• Include chlorhexidine (most effective in antiseptic products like skin creams, disinfectants, and mouthwashes).
• Low toxicity and act by reacting with proteins.
• Kill vegetative bacteria, fungi, and some enveloped viruses.

Ethylene oxide

• An extremely useful gaseous sterilizing agent that destroys all microbes, including endospores and viruses.
• Used in a special chamber to sterilize heat-sensitive items.
• Carcinogenic in nature.

Halogens

• Include chlorine and iodine, which work by oxidizing proteins and other cell components.
• Chlorine: Destroy all types of microorganisms and viruses. Chlorine-releasing compounds are used for disinfecting waste liquids and surfaces.

Chlorine Dioxide (ClO2)

• A powerful oxidizing agent, increasingly used to disinfect drinking, wastewater, and swimming pools.
• Does not react with organic compounds to form trihalomethanes or other toxic chlorinated products.
• Liquid ClO2 decomposes readily, allowing on-site generation.

Iodine

• Unlike chlorine, does not reliably kill endospores but used as a disinfectant
• Tincture of iodine or iodophores are used as disinfectants or antiseptics.
• Some Pseudomonas species survive in concentrated solutions of iodophores.

Metal Compounds

• Too toxic for medical use.
• Silver is one of the few metals still used as a disinfectant, often as silver sulfadiazine cream for preventing infection of severe burns.
• Some silver-containing bandages are in use.

Ozone (O3)

• A powerful oxidizing agent.
• Increasingly used as an alternative to chlorine for disinfecting drinking water and wastewater.

Peroxygens

• H2O2 and peracetic acid are powerful oxidizing agents and can be used as sterilants.
• Readily biodegradable and less toxic.
• H2O2 is less effective as an antiseptic because of breakdown by catalase
• Peracetic acid is widely used to disinfect and sterilize medical devices.

Phenolic Compounds (Phenolics)

• Phenol (carbolic acid) is important historically.
• Includes triclosan and hexachlorophene, which are relatively non-toxic.
• Active ingredients in Lysol.
• Destroy cytoplasmic membranes and denature proteins.
• Wide range of activity and reasonable cost.
• Remains effective in the presence of detergents and organic contaminants.
• Triclosan is in many personal care products (toothpaste, lotions, soaps).
• Hexachlorophene is effective against Staphylococcus aureus but limited use due to neurological damage.

Quaternary Ammonium Compounds (Quats)

• Cationic detergents, relatively non-toxic.
• Used to disinfect food preparation surfaces.
• Inactivated by anionic soaps and detergents.
• Widely used to disinfect inanimate items and preserve non-food substances.
• React with membranes to destroy vegetative bacteria and enveloped viruses.
• Not effective against endospores, mycobacteria, and naked viruses.

Preservation of Perishable Products

• Preventing microbial growth extends shelf life.
• Chemical preservatives:
  • Non-food items use germicides. Mouth washes contain quaternary ammonium compounds.
  • Food items use weak acids (benzoic acid, sorbic acid, propionic acid). Low pH inhibits most bacteria.
  • Nitrates/nitrites are used to preserve meats by inhibiting endospores and growth of Clostridium botulinum. Conversion to nitrosoamines poses a potential health hazard.
• Low-temperature storage:
  • Slows enzymatic reactions.
  • Freezing stops all microbiological activity.
• Reducing water availability:
  • Adding salt or sugar creates a high solute environment, causing plasmolysis that prevents microbial growth.

Next Lecture

• Read chapter 20: Antimicrobial medications.

Test your knowledge on the action, limitations, and characteristics of various chemical germicides. This quiz covers essential concepts related to germicidal chemicals and their applications in sterilization processes. Understand the critical factors in selecting appropriate germicides and their interaction with organic matter.