Classes of disinfectants and their MOA

 Classes of disinfectants and their MOA

Intended learning objectives

At the end of this lecture, the student will be able to:

       Classify the different types of disinfectants with examples

       Explain the mechanism of the different classes of disinfectants

       List the applications of various disinfectants

Classes of disinfectants

       Phenols and phenolics





       Heavy Metals and Their Compounds

       Surface-Active Agents

      Soaps and Detergents

      Acid-Anionic Sanitizers

       Quaternary Ammonium Compounds

Phenols and phenolics

       Lister was the first to use phenol (carbolic acid) to control surgical infections in the operating room

       Rarely used as an antiseptic or disinfectant because it irritates the skin and has a disagreeable odour


       A molecule of phenol that has been chemically altered to reduce its irritating qualities or increase its antibacterial activity in  combination with a soap or detergent

       Mechanism of action: Injuring lipid-containing plasma membranes, which results in leakage of cellular contents.

       Phenolics remain active in the presence of organic compounds, are stable, and persist for long periods after application.


       Derivatives of phenol that contain two phenolic groups connected by a bridge


                Hexachlorophene and triclosan


The active germ-killing ingredient in Dettol is in fact 4-chloro-3,5-dimethylphenol, also known by its non-systematic name para-chloro-meta-xylenol or PCMX, 


       Broad spectrum of activity

       Mode of action:  primarily affecting bacterial cell membranes


                                chlorhexidine – frequently used for microbial control on skin and mucous membranes

       Alexidine is a similar biguanide and is more rapid in its action


  1. Iodine

       Iodine is active against all kinds of bacteria, many endospores, various fungi, and some viruses

       Iodine impairs protein synthesis and alters cell membranes

       Iodine is available as a tincture

       An iodophor is a combination of iodine and an organic molecule, from which the iodine is released slowly.

                Eg. Betadine (povidone iodine)

2.       Chlorine

       Used as a gas or in combination with other chemicals

       Its germicidal action is caused by the hypochlorous acid (HOCl) that forms when chlorine is added to water

Chlorine compounds used as disinfectants:

       Calcium hypochlorite (bleaching powder)

       Sodium hypochlorite

       Chloramines - combinations of chlorine and ammonia


       Effectively kill bacteria and fungi but not endospores and nonenveloped viruses

The mechanism of action:

       Protein denaturation

       Disrupt membranes and dissolve many lipids, including the lipid component of enveloped viruses


       Recommended optimum concentration of ethanol is 70%

       Pure ethanol is less effective

       Isopropanol (rubbing alcohol)

Heavy Metals and their compounds

       Heavy metals  like silver, mercury, and copper can be biocidal or antiseptic

       Oligodynamic action: ability of very small amounts of heavy metals, especially silver and copper, to exert antimicrobial activity

Mechanism of action: When the metal ions combine with the sulfhydryl groups on cellular proteins, denaturation results

       Silver-impregnated dressings that slowly release silver ions have proven especially useful against antibiotic - resistant bacteria

       A combination of silver and the drug sulfadiazine,  Silver sulfadiazine  is used as a topical  cream in the treatment of burns

       Surfacrine (silver iodide) new antimicrobial for application to surfaces, either animate or inanimate


       Mercuric chloride: has broad spectrum of activity

       Primarily bacteriostatic

Copper in the form of copper sulfate used chiefly to destroy green algae

Zinc In the form of zinc chloride is a common ingredient in mouthwashes, and Zinc pyrithione is an ingredient in anti-dandruff shampoos

Surface-Active Agents

Soaps and Detergents

       Help in mechanical removal of microbes through scrubbing

Acid-Anionic Sanitizers

       Negatively charged portion (anion) of the molecule, reacts with the plasma membrane of microbes

       Nontoxic, non-corrosive, and fast acting

Quaternary Ammonium Compounds (Quats)

       Cationic surface active agents are most commonly used

       Their antimicrobial action is due to the cation

       Strongly bactericidal against gram positive, fungicidal, amoebicidal, and virucidal against enveloped viruses

       Mechanism of action: They probably affect the plasma membrane

       Two popular quats are benzalkonium chloride and cetylpyridinium chloride

       Strongly antimicrobial, colorless, odorless, tasteless, stable, easily diluted and nontoxic

       Organic matter interferes with their activity, and they are rapidly neutralized by soaps and anionic detergents


Phenol and Phenolics

       Disruption of plasma membrane

       Denaturation of enzymes

Biguanides (Chlorhexidine)

       Skin disinfection especially for surgical scrubs


       Protein denaturation and lipid dissolution


       Iodine inhibits protein function and is a strong oxidizing agent

       Chlorine forms the strong oxidizing agent hypochlorous acid, which alters cellular components

Heavy Metals and Their Compounds

       Denaturation of enzymes and other essential proteins

Surface· Active Agents

       Mechanic al removal of microbes through scrubbing

       Enzyme inhibition

       Protein denaturation

       Disruption of plasma membranes

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