Physical and Chemical Agents for Microbial control

Controlling Microorganisms

• Physical, Chemical, and mechanical methods to destroy or reduce undesirable microbes in a given area (decontamination)

• Primary targets are microorganisms capable of causing infection or spoilage:

  • Vegetative bacterial cells and endospores

  • Fungal hyphae and spores, yeast

  • Portozoan trophozoites and cysts

  • Worms

  • Viruses

  • Prions

Relative Resistance of Microbes

• Highest resistance

  • Prions, bacterial endospores

• Moderate resistance

  • Pseudomonas sp.

  • Mycobacterium tuberculosis

  • Staphylococcus aureus

  • Protozoan cysts

• Least resistance

  • Most bacterial vegetative cells

  • Fungal spores and hyphae, yeast

  • Enveloped viruses

  • Protozoan trophozoites

Sterilization

A process that destroys all viable microbes, including viruses and endospores

Disinfection

A process to destroy vegetative pathogens, not endospores; inanimate objects

Antiseptic

Disinfectants applied directly to exposed body surfaces

Sanitization

Any cleansing technique that mechanically removes microbes

Degermation

Reduces the number of microbes through mechanical means

Microbial Death

• Hard to detect, microbes often reveal no conspicuous vital signs to begin with

• Permanent loss of reproductive capability, even under optimum growth conditions

Factors that affect microbial death rate

• Number of microbes

• Nature of microbes in the population

• Temperature and pH of environment

• Concentration or dosage of agent

• Mode of action of the agent

• Presence of solvents, organic matter, or inhibitors

Selection of microbial control methods

• Depends on the circumstance

• Does the application require sterilization?

• is the item to be reused?

• Can the item withstand heat, pressure, radiation, or chemicals?

• Is the method suitable?

• Will the agent penetrate to the necessary extent?

• Is the method cost and labor efficient and is it safe?

Cellular targets of physical and chemical agents

1. The cell wall

   • Cell wall becomes fragile and lyses

   • some antimicrobial drugs, detergents, and alcohol

2. The cell membrane

   • Loses integrity

   • Detergent surfactants

3. Protein and nucleic acid synthesis

   • Prevention of replication, transcription, translation, peptide bond formation, protein synthesis

   • Chloramphenicol, ultraviolet radiation, formaldehyade

4. Proteins

   • Disrupt or denture proteins

   • Alcohols, phenols, acids, heat

Methods of physical control

• Heat - moist and dry

• Cold temperatures

• Desiccation

• Radiation

• Filtration

Moist heat

• Lower temperatures and shorter exposure time

• Coagulation and denaturation of proteins

• Methods include:

  • Steam under pressure

  • Boiling water

  • Pasteurization

Dry heat

• Moderate to high temperatures (higher temperatures than moist heat)

• Dehydration, alters protein structure

• Incineration - flame or electrical heating coil

  • Ignites and reduces microbes and other subtances

• Dry ovens - 150-180^oC - coagulate proteins

Steam under pressure (sterilization)

• Autoclave

  • 15 psi/ 121^oC / 10-40 min

• Steam must reach surface of item being sterilized

• Item just not be heat or moisture sensitive

• Mode of action - denaturation of proteins, destruction of membranes and DNA

Boiling water

• Boiling at 100^oC for 30 minutes to destroy non-spore-forming pathogens

• Disinfection

Pasteurization

• Heat is applied to kill potential agents of infection and spoilage without destroying the food flavor or value

• 63^oC - 66^oC for 30 minutes (batch method)

• 71.6^oC for 15 seconds (flash method)

• Not sterilization

  • kills non-spore-forming pathogens and lowers overall microbe count

  • Does not kill endospores or many nonpathogenic microbes

Cold

• Microbiostatic - slows the growth of microbes

• Refrigeration 0-15^oC and freezing <0^oC

• Used to preserve food, media, and cultures

Desiccation

• Gradual removal of water from cells, leads to metabolic inhibition

• Not effective microbial control - many cells retain ability to grow when water is reintroduced

• Lyophilization - freeze drying; preservation

Ionizing radiation

• Deep penetrating power that has sufficient energy to cause electrons to leave their orbit, breaks DNA

  • Gamma rays, X-rays, cathode rays

  • used to sterilize medical supplies and food products

• Used in the preservation of food

Nonionizing radiation

• Little penetrating power so it must be directly exposed

• UV light creates pyrimidine dimers, which interfere with replication

• used for sterilizing air, water, or surfaces

Filtration

• Physical removal of microbes by passing a gas or liquid through filter

• Used to sterilize heat sensitive liquids and air in hospital isolation units and industrial clean rooms

Chemical agents in microbial control

• Disinfectants, antiseptics, sterilants, degermers, and preservatives

• Some desirable qualities of chemicals:

  • Rapid action in low concentration

  • Solubility in water or alcohol, stable

  • Broad spectrum, low toxicity

  • Penetrating

  • Noncorrosive and nonstaining

  • Affordable and readily available

Levels of Chemical Decontamination

• High-level germicides

  • Kill endospores

  • May be sterilants

  • Used for: Devices that are not heat sterilizable and intended to be used in sterile environments (body tissue)

• Intermediate-level

  • Kill fungal spores (not endospores), tubercle bacillus, and viruses

  • Used to disinfect devices that will come in contact with mucous membranes but are not invasive

• Low-level

  • Eliminate only vegetative bacteria, vegetative fungal cells, and some viruses

  • Used to clean surfaces that touch skin but not mucous membranes

Factors that affect germicidal activity of chemicals

• Nature of the material being treated

• Degree of contamination

• Time of exposure

• Strength and chemical action of the germicide

Germicidal Categories

1. Halogens

2. Phenolics

3. Chlorhexidine

4. Alcohols

5. Hydrogen peroxide

6. Aldehydes

7. Gases

8. Detergents and soaps

9. heavy Metals

10. Dyes

11. Acids and alkalis

Halogens

• Chlorine - Cl₂, hypochlorites (chlorine bleach), chloramines

  • Denaturate proteins by disrupting disulfide bonds

  • Intermediate level

  • Unstable in sunlight, inactivated by organic matter

  • Used for water, sewage, wastewater, inanimate objects

• Iodine - I₂, iodophors (betadine)

  • Interferes with disulfide bonds of proteins

  • Intermediate level

  • Milder medical and dental degerming agents, disinfectants, ointments

Phenolics

• Disrupt cell walls and membranes and precipitate proteins

• Low to intermediate level

  • Bactericidal, fungicidal, virucidal not sporicidal

  • Lysol

  • Triclosan - antibacterial additive to soaps

Chlorhexidine

• A surfactant and protein denaturant with broad microbicidal properties

• Low to intermediate level

• Hibiclens, Hibitane

• Used as skin degermign agents for preoperative scrubs, skin cleaning, and burns

Alcohols

• Ethyl, isopropyl in solutions of 50-95%

• Act as surfactants dissolving membrane lipids and coagulating proteins of vegetative bacterial cells and fungi

• Intermediate level

Hydrogen Peroxide

• Produce highly reactive hydroxyl-free radicals that damage protein and DNA while also decomposing to O₂ gas - toxic to anaerobes

• Antiseptic at low concentrations

• Strong solutions are sporicidal

Aldehydes

• Kill by alkylating protein and DNA

  • Glutaraldehyde in 2% solution (Cidex) used as sterilant for heat sensitive instruments

    • High level

  • Formaldehyde - disinfectant, preservative, toxicity limits use

    • Formalin - 37% aqueous solution

    • Intermediate to high level

Gases and Aerosols

• Ethylene oxide, propylene oxide

• Strong alkylating agents

• High level

• Sterilize and disinfects plastics and prepacked devices, foods

Detergents and Soaps

• Quaternary ammonia compounds (quats) act as surfactants that alter membrane permeability of some bacteria and fungi

  • Very low level

• Soaps - mechanically remove soil and grease containing microbes

Heavy metals

• Solutions of silver and mercury kill vegetative cells in low concentrations by inactivating proteins

• Oligodynamic action

• Low level

• Merthiolate, silver nitrate, silver

Dyes

• Aniline dyes are very active against gram-positive species of bacteria and various fungi

• Sometimes used for antisepsis and wound treatment

• Low level, narrow spectrum of activity

Acids and alkalis

• Low level of activity

  • Organic acids prevent spore germination and bacterial and fungal growth

  • Acetic acid inhibits bacterial growth

  • Propionic acid retards molds

  • Lactic acid prevents anaerobic bacterial growth

  • Benzoic and sorbic acid inhibit yeast