The Control of Microbial Growth


10. Differentiate between antiseptics and disinfectants.

11. List five general principles for choosing an antiseptic and/or a disinfectant.

12. List and describe three methods for the evaluation of antiseptics and/or disinfectants.

13. Define phenol coefficient and use dilution and understand how to interpret data from both kinds of tests.

14. Describe the filter paper method of antiseptic/disinfectant evaluation.

15. List and describe eleven chemical methods of microbial control, give their mode of action and their applications.

16. Identify those chemicals that may be used in surgical hand scrubs.

17. Explain why chlorine is antimicrobial.

18. Identify the concentration of ethyl alcohol that is the most effective against bacteria.

19. List the heavy metals used in microbial control.

20. List the chemicals that are used to control microbial growth in foods.

21. List the disinfectants that are used as a gas.

Chemical Methods of Microbial Control

Chemical agents are used on living tissue (as antiseptics) and on inanimate objects (as disinfectants).

Few chemical agents achieve sterility.

Principles of Effective Disinfection

Properties of the disinfectant – how it works determines what it will be effective against.

Concentration of the disinfectant - requires proper hydration.

The presence of organic matter can act as a buffer

The degree of contact with microorganisms – if the surface needs cleaning or is porous microorganisms can escape contact with the disinfectant.

Temperature should also be considered, since increased temperatures usually enhance the efficacy of disinfectants.

Evaluating a Disinfectant

Phenol coefficient – compares activity of a disinfectant to phenol, was once the standard to evaluate the effectiveness of a disinfectant.

The current standard is the American Official Analytical Chemist's use-dilution test. In this test bacterial (Salmonella choleraesuis, Staphylococcus aureus, and Pseudomonas aeruginosa) survival in the manufacturer’s recommended dilution of a disinfectant is determined.


In the disk-diffusion method, a disk of filter paper is soaked with a chemical and placed on an inoculated agar plate; a clear zone of inhibition indicates effectiveness.

Types of Disinfectants

Phenol and Phenolics

Phenolics are derivatives of phenol that have been altered to reduce irritating qualities or increase antimicrobial activity when combined with detergents.

Phenolics exert their action by injuring plasma membranes.


Cresols are phenolics that are derived from coal tar.

O-phenylphenol is a cresol that is the the main ingredient in Lysol.


Bisphenols have two phenolic groups and include triclosan (over the counter) and hexachlorophene (prescription).

Hexachlorphene is used in pHisoHex

Triclosan is used in soaps, toothpaste, and incorporated into plastic kitchenware.




Some halogens (iodine and chlorine) are used alone or as components of inorganic or organic solutions.

Iodine may combine with certain amino acids to inactivate enzymes and other cellular proteins.

Iodine is available in a tincture (in solution with alcohol) or an iodophor (combined with an organic molecule).

The germicidal action of chlorine is based on the formation of hypochlorous acid (HOCl) when chlorine is added to water.


Alcohols exert their action by denaturing proteins and dissolving lipids.

In tinctures, they enhance the effectiveness of other antimicrobial chemicals.

Aqueous ethanol (60-95%) and isopropanol (lower concentrations required, typically 62 - 65%) are used as disinfectants. Alcohol evaporates quickly and leaves no residue behind.

Heavy Metals and Their Compounds

Silver, mercury, copper, and zinc are used as germicidals.

They exert their antimicrobial action through oligodynamic action. When heavy metal ions combine with sulfhydryl (—SH) groups, proteins are denatured.


Used as 1% silver nitrate solution to prevent opthalmia neonatorum before being replaced by antibiotics. Ironically, silver impregnated wound dressings are protective against antibiotic-resistant bacteria.

Silver sulfadiazine - topical cream for burns.

Surfacine - water-insoluble silver iodide in a polymer carrier, persistent for at least 13 days, can be used on animate and inanimate objects.

Silver nanoparticle infused commericial products - plastic food storage containers keep food fresher, minimizes odors in athletic clothing.


Primarily bacteriostatic but broad range of activity. Some question of toxicity (har) and autism (BS).

Mercuric chloride is one formulation that was once used to treat syphylis. It is so toxic that poisoning was common, the symptoms of which were often confused with those of syphilis.

Other antiseptic mercury based compounds include the former parental favorites Mercurochrome and Merthiolate.

Mercurochrome is a trade name for merbromin, a compound containing mercury and bromine.

Merthiolate is a trade name for thimerosal, a compound containing mercury and sodium, which is was also available as a tincture.

They both "sting" upon application and the merthiolate tincture (as I remember) feels such that it conjures the thought that the application of battery acid with a red-hot poker would be preferable.

Thimerosal is still in use but the primary applications of mercury use are industrial and as an inhibitor of mildew in paint.


Copper sulfate is used to control green algae growth in ponds, pools, reservoirs and fish tanks and copper compounds are sometimes used in paint to prevent mildew.


Zinc chloride is used in some mouthwashes, zinc pyrithione in antidandruff shampoos.


Surface -Active Agents

Surface-active agents decrease the surface tension among molecules of a liquid; soaps and detergents are examples.

Soaps have limited germicidal action but assist in the removal of microorganisms through scrubbing.

Acid-anionic detergents are used to clean dairy equipment.

Quanternary Ammonium Compounds (Quats)

Quats are cationic detergents attached to NH4+.

By disrupting plasma membranes, they allow cytoplasmic constituents to leak out of the cell.

Quats are most effective against gram-positive bacteria.

Inactivated by anions, soaps, detergents, and organic material.

Effectiveness comparison



Chemical Food Preservatives

SO2, sorbic acid, benzoic acid, and propionic acid inhibit fungal metabolism and are used as food preservatives.

Good to prevent mold growth in acidic foods (molds grow well at low pH).

Nitrate and nitrite salts prevent germination of Clostridium botulinum endospores in meats and preserve red color.


Nisin and natamycin are antibiotics used to preserve foods, especially cheese.

Not used for treatment of disease.


Aldehydes such as formaldehydes and glutaraldehyde exert their antimicrobial effect by inactivating proteins.

They are among the most effective chemical disinfectants.

Chemical Sterilization


Ethylene oxide is a gas commonly used for sterilization.

Penetrates most materials and kills all microorganisms (and destroys endospores) by cross linking proteins and nucleic acids.

Requires long exposure times and is explosive and toxic in pure form but works well at low (room) temperatures

Chlorine dioxide is a commonly used gas in water treatment to prevent or reduce formation of carcinogens during chlorination; although it is far more stable in aqueous solution, it has also been used (in gaseous form) to fumigate enclosed building areas contaminated with anthrax endospores.


A state of matter that occurs when a gas is excited (commonly with an electromagnetic field) to produce a mixture of nuclei with various charges and free electrons.

Good for sterilizing plastic surgical instruments and long hollow tubes with a small diameter.

Done in a chamber where conditions are generated by an electromagnetic field in a vacuum. The chemical that forms the plasma is often H2O2. The free radicals generated kill vegetative cells and endospores and the process can be carried out at a low temperature. But expensive.

Supercritical Fluids

At ordinary pressures (1 atmosphere or so) substances above their critical temperature are gases and under their critical temperature become liquids. If you increase the pressure enough a substance will become both gas-like and liquid like; it will expand to fill a container and still have the density of a liquid.

These supercritical liquids make great solvents and are used in a number of industrial applications (decaffienating coffee, dry-cleaning, etc.).

It turns out that they can be used for food preservation and are now used to sterilize tissues (bones, ligaments, tendons) that are to be surgically implanted.

CO2 is often used; it requires a pressure of 73 atmospheres (a fair amount of pressure) but can be done at a temperature of roughly 31º C.

Peroxygens (Oxidizing Agents)

Peroxygens exert their effect by oxidizing molecules inside the cell.

Hydrogen peroxide (H2O2) - not a good antiseptic because it is rapidly detoxified by catalase released by injured cells in a wound but does make a good disinfectant on non-living surfaces.

At high concentrations it is sporocidal and overcomes the catalase/peroxidase of aerobes and facultative anaerobes. Is used in the food industry for aseptic packaging and for contact lens disinfection.

Can be used as a gas but doesn't penetrate as well as ethelene oxide and can't be used to sterilize liquids and textiles.

Peracetic acid - very good liquid disinfectant, highly sporocidal, isn't affected much by the presence of organic material, and leaves no toxic by products behind.

Used for sterilizing food processing and medical equipment and is approved for washing fruits and vegetables.

Ozone (O3) - highly reactive, forms when oxygen is passed through an electrical field.

Good for eliminating odors and tastes; used to supplement chlorination of water (more effective than chlorine but unstable in water) and to keep Hemmingway's from smelling like last week's socks.

Benzoyl peroxide - over the counter acne medication