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

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


       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)


       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

compounds used as disinfectants:

       Calcium hypochlorite (bleaching

       Sodium hypochlorite

       Chloramines – combinations of
chlorine and ammonia


       Effectively kill bacteria and fungi
but not endospores and nonenveloped viruses

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

       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

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

Surface-Active Agents

and Detergents

       Help in mechanical removal of
microbes through scrubbing


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

       Nontoxic, non-corrosive, and fast

Quaternary Ammonium Compounds

       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

       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


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

Metals and Their Compounds

Denaturation of enzymes and other essential proteins

Active Agents

Mechanic al removal of microbes through scrubbing

Enzyme inhibition

Protein denaturation

Disruption of plasma membranes

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