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


In the use-dilution test, bacterial (S. choleraesuis, S. aureus, and P. 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.

Phenol coefficient – compares activity of a disinfectant to the activity of phenol.



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.

  • Can also denature proteins – enzyme inactivation


  • Not inactivated by organic compounds
  • Stable for long periods
  • Persist for long periods after application
  • Good for disinfecting things like pus, saliva, and feces

Cresol is a phenolic that is derived from coal tar.


O-phenylphenol is the main ingredient in Lysol.





Bisphenols such as triclosan (over the counter) and hexachlorophene (prescription) are widely used in household products.


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 are used as disinfectants.


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.



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.

Quaternary 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.


Gaseous Chemosterilizers


Ethylene oxide is the gas most frequently used for sterilization.


It penetrates most materials and kills all microorganisms by protein denaturation.


Peroxygens (Oxidizing Agents)


Ozone peroxide, and peracetic acid are used as antimicrobial agents.


They exert their effect by oxidizing molecules inside the cell.