Chemical agents are used to control microbial growth in foods, industries, and hospitals. They work by inhibiting or killing microorganisms. An ideal antimicrobial chemical agent is soluble, stable, nontoxic to humans, homogeneous, and effective at low concentrations against a broad spectrum of microbes. Common chemical agents used include phenols, alcohols, halogens, heavy metals, quaternary ammonium compounds, aldehydes, and gases. They kill microbes through mechanisms like protein denaturation, cell membrane damage, and inhibition of essential metabolic processes. Selection of the appropriate agent depends on the target microbes and environmental conditions.

1. Control of
Microbial Growth-
Chemical Methods
By
Debomitra Dey

2. WHY CHEMICAL AGENTS?
o To control growth of microbes in mainly in Food, Industries,
Hospitals, etc.
o Chemical agents have the ability to inhibit growth and
metabolism of microorganisms or kill them.
o No single chemical agent is capable of destroying all microbes,
they specifically work against certain microbes or situation.
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3. CHARACTERISTIC OF AN IDEAL ANTIMICROBIAL
CHEMICAL AGENT
1. ANTIMICROBIAL ACTIVITY - at low concentration, should have a
broad spectrum of antimicrobial activity.
2. SOLUBILITY- for effective use should be soluble in water or other
solvents.
3. STABILITY- while dissolved in solvent or water changes should be
minimal, shouldn’t result in loss of germicidal action.
4. NONTOXICTO HUMANS AND ANIMALS- compound should be
lethal to microbes and not humans.
5. HOMOGENEITY- Preparation must be uniform so active
ingredients are present in each application, mixtures may lose
homogeneity.
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6. NON-COMBINATIONWITH EXTRANEOUS ORGANIC MATERIAL- Many
disinfectants have affinity for proteins or other organic material, in that case very
little disinfectant will be available for action.
7. TOXICITYTO MICROBESAT ROOMTEMPERATURE-The compound should
effectively work at room temperature.
8. CAPACITYTO PENETRATE- Upon penetration maximum germicidal action is
observed.
9. NON-CORRODINGAND NON-STANING- Should not rust or disfigure metals nor
stain or damage fabric.
10. DEODOURIZING ABILITY-The disinfectant should either have a pleasant smell or
should be odourless
11. DETERGENTCAPACITY- a disinfectant which is also a detergent has cleansing
effect and also improves effectiveness.
12. AVAILABILITY-Compound should be available in large quantity and reasonable
price.

5. o STERILIZATION- The process of destroying all forms of microbial
life.
o DISINFECTANT- Agent that kills growing forms of microbes,
resistant to spore forms
o ANTISEPTIC- Usually associated with substance applied to body,
prevents growth of microbes by inhibiting growth and metabolism
o SANITIZER- Kills 99.9% of growing bacteria, applied to inanimate
objects.
o GERMICIDE- Kills growing bacteria, not necessarily resitant spores.
Used for control of all kinds of microbes
o BACTERICIDE- Agent that kills bacteria.
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DEFINITIONS

6. o BACTERIOSTATIS- condition wherein growth of bacteria is
prevented. Agents that in common can microorganisms are
called microbistatic agents.
o ANTIMICROBIAL AGENT- Inhibition of growth, with
reference to specific group of organisms they are termed as
antibacterial or antifungal.
Some antimicrobial agents are used to treat infections and
they are called chemotherapeutic agents.
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7. SELECTION OF A CHEMICAL AGENT
FOR PRACTICAL APPLICATION
o NATURE OF MATERIALTO BETREATED- the antimicrobial
agent must be compactible with the material to which it is
applied.
o TYPE OF MICROORGANISM- agents selected should be
known to be effective against type of organism to be
destroyed.
o ENVIRONMENTAL CONDITIONS- factors like pH,
temperature, time, concentration, presence of extraneous
organic matter have an effect on the rate and efficiency of
antimicrobial action.
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8. MAJOR GROUP OF ANTIMICROBIAL
AGENTS
o PHENOL AND PHENOLIC COMPOUNDS
o ALCOHOLS
o HALOGENS
o CHLORINE AND CHLORINE COMPOUNDS
o HEAVY METALS ANDTHEIR COMPOUNDS
o DYES
o DETERGENTS
o QUATERNERY AMMONIUM COMPOUNDS
o ALDEHYDES
o GASEOUS AGENTS
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9. PHENOL AND PHENOLIC COMPOUNDS
o Very effective disinfectant.
o 5% kills rapidly vegetative cells, spores are more resistant.
o Phenolic substances maybe either Bactericidal or
Bacteriostatic.
o Derivative of Phenol- Hexylresorcinol, marketed in solution of
glycerin and water is a strong surface tension reductant, has
high bactericidal activity.
o Mode of Action- Depends upon the concentration and the
microbial cell exposed. Results in precipitation of cell protein,
inactivation of enzymes and leakage of amino acids from
cells.
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10. ALCOHOLS
o Ethyl alcohol, in concentration from 50-90% effective against
vegetative or non-spore forming cells. Usually 70% is used in
practical application. Propyl & Isopropyl 40-80% are bactericidal.
o Methyl alcohol is less bactericidal, besides highly poisonous
o Higher alcohol like- propyl, butyl,etc. more germicidal
o 60% conc effective against viruses, depending upon amount of
extraneous protein material, this in turn protects virus.
o Mode ofAction- Alcohols are protein denaturants, they are
solvents for lipid, may damage lipid complexes in cell membrane;
dehydrating agents , this attributes to bacteriostatic condition.
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11. HALOGENS
o Iodine is the oldest and most effective germicidal agent, it has
been used in the form of tincture iodine.
o Iodophors- possessgermicidal characteristics of Iodine, have
advantages of non-staining and low irritant properties.Mixture
of Iodine with surface-active agents that act as carriers and
solubilizers for Iodine. For eg. Polyvinyl pyrrolidone (PVP).
o Iodine effective against all kinds of bacteria, possess sporicidal
activity. Sporicidal effect depends upon amount of organic
matter and extent of dehydration. Highly fungicidal, and to
some extent virucidal.
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oMode ofAction- Oxidising agent, irreversibly oxidize thus,
inactivate metabolic compound such as proteins with SH
groups. Also suggested that action may involve halogenation
of tyrosine units of enzymes and other cellular proteins
requring tyrosine for activity.

13. CHLORINE AND COMPOUNDS
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o Either used as gas or in chemical combination, widely used
disinfectants.
o Disinfectants- Hypochlorites, Chloramines- used as
disinfectants, sanitizing agents
o Mode ofAction- The antimicrobial action is due to Oxygen
released during the reaction, strong oxidizing agent,
destroys the cellular constituents, hence kills the
microorganism.
o Killing of microbe is also due to direct combination of Cl
with protein of cell membranes and enzymes.

14. HEAVY METALS AND THEIR COMPOUNDS
o Most heavy metals either alone or in combination with
certain compounds have detrimental effect upon
microorganisms.
o Mode ofAction-The antimicrobial activity is due to the
reaction between the cellular proteins and the heavy metals
along with their compounds, results in inactivation of
microbes. For example: Mecuric chloride, attacks the
enzymes which contain SH groups. High concentration of
salts of heavy metals coagulate proteins resulting in cell
death or damage.
o Examples- Mecuric oxide, Copper Sulfate, Silver nitrate,
Metaphan (Mercury + Organic acid)
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15. DYES
o Two types of Dyes are used
o Triphenylmethane dyes- Malachite green, CrystalViolet.
Crystal violet will inhibit Gram positive cocci, and 10 times the
amount is required for inhibition of E.coli. Mode of action is
the inhibitory effect by interfering in cellular oxidation
process. Used in media preparation for detection of E.coli in
public health microbiology.
o Acridine dyes- Dyes used are Acriflavine,Tryptoflavine.They
exhibit selective inhibition Staphylococci and Gonococci. Used
for treatment of burns.They also possess little fungicidal
activity.
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16. DETERGENTS
o Detergents are surface tension depressants or wetting agents.
o Chemical detergents are classified as- Cationic, Anionic, Non-
Ionic
o Cationic detergents are considered more germicidal than
anionic detergents.
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17. QUATERNARY AMMONIUM COMPOUNDS
o Cationic detergent class are Quaternary ammonium salts
o The bactericidal power is exceptionally high against Gram
positive bacteria, as compared to G-ve.
o They are used in dairy, egg and fishing industries to control
microbial growth on surface of equipment and the
environment.
o Mode of action- they have damaging effect on protein
denaturation, interference with glycolysis, membrane
damage. Additionally, alter permeability of cell.
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18. ALDEHYDES
▪ Formaldehyde and Glutaraldehyde, both are highly microbicidal
and have the ability to kill spores.
▪ Formaldehyde- Formaldehyde solution in sterilization of
instruments. In gaseous form can be used for disinfection and
sterilization of enclosed area.
– Vaporization of formaldehyde into enclosed area can kill vegetative cells
quickly as compared to spores (at specific temperature and time)
– Mode of Action- Combines readily with vital organic nitrogen compounds
like proteins and nucleic acids this accounts for the antimicrobial action.
o Glutaraldehyde- 2% solution exhibits wide spectrum of
antimicrobial activity. Effective against vegetative bacteria.
Fungi, and spores of Fungi and Bacteria.
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19. GASEOUS AGENTS
o Sterilization by means gaseous agents like Formaldehyde and
Ethylene Oxide is effective, can be used to sterilize an enclosed
area.
o Ethylene oxide is generally used sterilizing heat or moisture
sensitive materials.
o Spores show little resistance to destruction by this agent.
o Mode of action- Alkylation reactions with organic compounds
such as enzymes and other proteins, alkylation is the
replacement of active H with CH2CH2O
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20. REFERENCE
▪ Microbiology by Michael Pelczar 5th edition,
Pg:488-504
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