Microbial Control Methods Unit-II

Control of Microbial Growth
Microbiology
Unit-II
Muhammad Iqbal
Lecturer
KMU
M. Iqbal, KMU 1
Objectives
4 Define Key words

4 Discuss Physical and Chemical microbial
control method
4 Discuss the importance of microbial control
M. Iqbal, KMU 2
Key Terms
Sterilization: The destruction of all forms of microbial

life (including endospores) by physical or chemical
method.
Disinfection: Reducing the number of pathogenic
microorganisms to the point where they no longer
cause diseases. Usually involves the removal of
vegetative pathogens.
Disinfectant: An agent applied to inanimate
objects to be disinfected.
Antiseptic: A chemical agent applied to skin or living
tissues to limit microbial growth.
M. Iqbal, KMU 3
Sanitization: Treatment of food-handling
equipment used to meet public health standards
and minimize chances of disease transmission.
E.g: Soap & hot water.
Sepsis: (Gk word: decay or putrid.) The presence of
bacterial contamination.
Asepsis: The absence of bacterial contamination.
Aseptic techniques: used to prevent microbial
contamination of any clinical procedure and
products of pharmaceutical and food industry to
keep standards.
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M. Iqbal, KMU
Bacteriostatic Agent: (stasis: to stop)
An agent that inhibits the growth of bacteria.
Germicide: An agent that kills certain
microorganisms.
u Bactericide: An agent that kills bacteria.
u Virucide: An agent that inactivates viruses.
u Fungicide: An agent that kills fungi.
u Sporicide: An agent that kills bacterial endospores and
fungal spores.
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4 Basic principles of controlling microbial
growth. These apply to all methods of control of
microbial growth : 1. agent used must be able to
affect the micro-organisms directly; 2. the item
must be cleaned so as to remove extraneous soil;
3. moisture is essential for the action of chemical
agents; 4. killing of micro-organisms is not
instantaneous, and the time required depends upon
: a. the nature of the organism b. the nature of the
agent used c. the numbers of organisms present d.
the temperature M. Iqbal, KMU 6
Cont….
4 Gramm positive organisms are more susceptible to disinfectants and antiseptics

than Gramm negative organisms. Pseudomonads are resistant to chemical
agents, and will actively grow in some disinfectants and antiseptics. They can
also survive in simple saline solutions, and are resistant to many antibiotics.
They are a major cause of problems in hospitals as they are opportunistic
pathogens in the absence of the normal flora of the body, i.e. when the normal
flora has been suppressed during abti-biotic therapy. Micro-organisms are also
more susceptible to chemical agents during their growth, but with an increase in
age comes an increase in resistance. Endospores are much more resistant than
vegetative cells, e.g. spores of C. Botulinum will survive after 5½ hours of
boiling. Heat is more effective under acidic conditions, as are some chemical
agents. Anti-microbial agents work by altering the permeability of the cell
membrane, causing leakage of the contents and affecting growth. Other agents
work by damaging the proteins or nucleic acids within the cell. If these proteins
are destroyed then the cell is unable to synthesise proteins, leading to cell death.
M. Iqbal, KMU 7
Physical Methods of Microbial Control
HEAT:
Dry Heat:
Direct Flaming: Used to sterilize inoculating loops and
needles.
u Incineration: Effective way to sterilize disposable items
(paper cups, dressings) and biological waste.
Filtration: Removal of microbes by passage of a liquid or air

through a screen like material with small pores. Used to
sterilize heat sensitive materials like vaccines, enzymes,
antibiotics, and some culture media.
. 8
M. Iqbal, KMU
Moist Heat: Kills microorganisms by coagulating their

proteins.
In general, moist heat is much more effective than dry
heat.
u Boiling: Heat to 100oC or more. It Kills vegetative forms of
bacterial pathogens, almost all viruses, and fungi and their
spores within 10 minutes or less. Endospores and some
viruses are not destroyed quickly. However brief boiling
will kill most pathogens.
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Moist Heat:
Reliable sterilization with moist heat requires
temperatures above that of boiling water.
u Example of moist heat is Autoclave, a Chamber
which is filled with hot steam under pressure. This is
a Preferred method of sterilization for non-heat-
sensitive materials in which temperature of steam
reaches 121oC at 15 psi pressure.
M. Iqbal, KMU 10
Moist Heat (Continued):

u Pasteurization: It is used to reduce microbes responsible for
spoilage of beer, milk, wine, juices, etc.
u Classic Method of Pasteurization: Milk is heated to 62oC for 30
minutes.
u High Temperature Short Time Pasteurization (HTST): Milk is
exposed to 72oC for 15 seconds.
u Ultra High Temperature Pasteurization (UHT): Milk is treated at
140oC for 3 seconds and then cooled very quickly in a vacuum
chamber.
u Advantage: Milk can be stored at room temperature for several
months. M. Iqbal, KMU 11
Refrigeration: Temperatures from 0 to 7oC. It exerts

bacteriostatic effect. Reduces metabolic rate of most microbes
so they cannot reproduce or produce toxins.
u Freezing: Items are protected by stopping the growth of
microbes at temperatures below 0oC.
u Lyophilization: (Greek, and means "made solvent-loving“).
Lyophilization, commonly known as freeze-drying, is a way of
freezing and then drying something in a vacuum.
Cells and some foods are preserved in this way.
M. Iqbal, KMU 12
Physical Methods of Microbial
Desiccation: It is the process of removal of

water. In the absence of water, microbes
cannot grow or reproduce, but some may
remain viable for years. After water becomes
available, they start growing again.
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Osmotic Pressure: The use of high concentrations

of salts and sugars in foods is used to increase the
osmotic pressure and create a hypertonic
environment.
Note:
Yeasts and molds are resistant to high osmotic pressures.
Staphylococci spp. that live on skin are fairly resistant
to high osmotic pressure.
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Radiation: Three types of radiation kill microbes:

1. IonizingRadiation: Gamma rays, X rays, electron beams, or
higher energy rays. Have short wavelengths (less than 1
nanometer).
Dislodge electrons from atoms and form ions.
Cause mutations in DNA and produce peroxides.
Used to sterilize pharmaceuticals and disposable medical
supplies. Food industry is interested in using ionizing
radiation.
Disadvantages: Penetrates human tissues. May cause genetic
mutations in humans.
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M. Iqbal, KMU
2. Ultraviolet light (Nonionizing Radiation):

Wavelength is longer than 1 nanometer. Damages DNA by
producing thymine dimers.
Used to disinfect operating rooms, nurseries, cafeterias.
Disadvantages: Damages skin, eyes. Doesn’t penetrate
glass, and cloth.
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4 Chemical methods of destruction. Very few
chemical agents actually achieve sterility, but they
do reduce the microbial population to safe levels,
destroying pathogens. Chemical agents should be
selected so as to kill the organisms as quickly as
possible. The following table illustrates some of
the commonly used chemical agents.
M. Iqbal, KMU 17
Chemical Methods of Microbial Control
u Phenol (carbolic acid): was first used by Lister as a

disinfectant.
Destroys plasma membrane and denatures proteins.
Rarely used today because it is a skin irritant and has strong
odor.
Advantages: Stable, persist for long times after applied, and
remain active in the presence of organic compounds.
M. Iqbal, KMU 18
Chemical Methods of Microbial Control
Halogens: Effective alone or in compounds.

A. Iodine:
u Tincture of iodine denatures proteins.
u It Stains skin and clothes.
B. Chlorine:
u Used to disinfect drinking water, pools, and sewage.
u Chlorine is easily inactivated by organic materials.
u When mixed in water, forms hypochlorous acid:
Cl2 + H2O ------> H+ + Cl- + HOCl

Hypochlorous acid
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M. Iqbal, KMU
Chemical Methods of Control
Types of Disinfectants
Alcohols:
u Kill most bacteria, fungi, but not endospores.
u Act by denaturing proteins and disrupting cell
membranes.
u Evaporate, leaving no residue.
u Used to mechanically wipe microbes off skin before
injections or blood drawing.
u Not good for open wounds, because cause proteins
to coagulate.
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Heavy Metals:
u Heavy metals and their compounds are microbicidal.
u Include copper, selenium, mercury, silver, and zinc.
u The property of heavy metal to exert biocidal effect is called
Oligodynamic action.
A. Silver:
u 1% silver nitrate used to protect infants against gonorrheal
eye infections until recently.
B. Copper
u Copper sulfate is used to kill algae in pools and fish tanks.
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C. Selenium
u Kills fungi and their spores. Used for fungal infections.
u Also used in dandruff shampoos.
D. Zinc
u Zinc chloride is used in mouthwashes.
u Zinc oxide is used as antifungal agent in paints.
M. Iqbal, KMU 22
Oxidizing Agents:
A. Ozone:
u It is Used to disinfect water.
u Helps neutralize unpleasant tastes and odors.
u More effective killing agent than chlorine,
but less stable and more expensive.
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B. Hydrogen Peroxide:
u Used as an antiseptic.
u Not good for open wounds because quickly broken down by
catalase enzyme present in human cells.
u Effective in disinfection of inanimate objects.
u Sporicidal at higher temperatures.
u Used by food industry and to disinfect contact lenses.
C. Benzoyl Peroxide:
u Used in acne medications.
M. Iqbal, KMU 24
Importance of Control of Microbial Growth
Control of microbial growth means to kill or inhibit

the growth of microorganisms. Control of growth
usually involves the use of physical or chemical
agents which either kill or prevent the growth of
microorganisms.
The control of microbial growth is necessary in
many practical situations, and significant advances
in medicine, agriculture, and food science have
been made through the study of microbiology.
M. Iqbal, KMU 25
4 Gluteraldehyde (2% solution)
4 Formalin (37% solution)
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Importance of Microbial Control (Cont…)
Microbial control is very important in the following different

aspects.
*Controls infectious diseases.
*Minimizes cross and nosocomial infections
*Prevents surgical complications
*Decreases morbidity and mortality
*Prevents food from spoilage
*Food can be preserved for months
*Improves and ensures the safety of agricultural products
*Ensures the provision of safe drinking water to public
M. Iqbal, KMU 27

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Control of Microbial Growth

4 Define Key words

Sterilization: The destruction of all forms of microbial

4 Gramm positive organisms are more susceptible to disinfectants and antiseptics

Filtration: Removal of microbes by passage of a liquid or air

Moist Heat: Kills microorganisms by coagulating their

Moist Heat (Continued):

Refrigeration: Temperatures from 0 to 7oC. It exerts

Desiccation: It is the process of removal of

Osmotic Pressure: The use of high concentrations

Radiation: Three types of radiation kill microbes:

2. Ultraviolet light (Nonionizing Radiation):

u Phenol (carbolic acid): was first used by Lister as a

Halogens: Effective alone or in compounds.

Cl2 + H2O ------> H+ + Cl- + HOCl

Control of microbial growth means to kill or inhibit

Microbial control is very important in the following different

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