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BIOL 411
Muhammad Suleman
Kamran Rasool
Fatima Amjad
Aysha Imtiaz
BIOL 411
Physical and Chemical Control of Microbes
Physical and Chemical Control of Microbial Growth
Background:
In the 19thcentury, the surgery for several diseases was very risky and dangerous as well as
more prone to getting infections. This was so because surgery was not performed under aseptic
conditions. The operating room, the surgeon's hands, and the surgical instruments were laden
with microbes, which caused high levels of infection and mortality.
Surgeons in the mid-1800s often operated wearing their street clothes, without
washing their hands. They frequently used ordinary sewing thread to suture wounds, and stuck
the needles in the lapels of their frock coats in between patients. Surgical dressings were often
made up of surplus cotton or jute from the floors of cotton mills. It was against this background
that French scientist Louis Pasteur demonstrated that invisible microbes caused disease. Lister
used a solution of carbolic acid (phenol), which was sprayed around the operating room by a
handheld sprayer. But surgeons were very slow for adopting the aseptic techniques which are
essential for avoiding the infections and for controlling the microbial growth.
Introduction:
Control of microbial growth may involve two ways:
Killing microbes
Suppress the microbial growth by targeting the inner components
The control of microbial growth may involve sterilization, disinfection, antisepsis,
sanitization, or degerming. Sterilization is the destruction of all forms of microbial life, with
particular attention to bacterial spores. Disinfection and antisepsis both refer to destruction of
microbial pathogens, although some organisms, such as bacterial spores, may remain
alive. Disinfection refers to the destruction of pathogenic organisms, while antisepsis refers to
that destruction on a living object, such as the skin surface.
Cellular targets of control:
For controlling the microbial growth or for killing them, it is necessary to target the inner
cellular components of bacteria to which any physical or chemical agent can be applicable.
Followings are cellular targets for controlling microbial life:
Cell wall
Cell membrane
Proteins
Nucleic acids (DNA and RNA )
Physical Control
Thermal death time
The thermal death point is “shortest period of time to kill a suspension of bacteria or
bacterial spores at a prescribed temperature and under specific conditions”.
Thermal death point
Thermal death point “is the lowest temperature at which all microorganisms in a
particular liquid will be killed in ten minutes”.
Decimal reduction time
Decimal reduction time is the” time in minutes that it takes for 90% of a given
population of microorganisms to be killed at a given temperature”
Physical Methods:
Physical methods controlling the growth of microorganisms are basically divided into
Heat methods and non heat methods. Heat methods are those in which different temperature
conditions along with the Different pressure is applied and non heat methods involve the use of
techniques other than the temperature.
Heat
Cold temperature
Desiccation
Radiations
Filtration
Heat:
Two types of heats are used to control the growth of the microbes.
1-Dry Heat
2-Moist heat
Dry Heat:
Dry heat reacts with the proteins and oxidizes the proteins .This oxidation damages the
tertiary structure of the proteins and as result proteins become inactive. Microbes lose their
source of energy and eventually death of microbes occurs. Dry heat can also denature the DNA.
Dry heat is used for the sterilization of those apparatus that would not be damaged by the heat
i.e. Petri plated and other glass ware apparatus powders, petroleum products, sharp
instruments).
Dry heat is used in different instruments as microbicidal. Dry heat sterilization technique
requires longer exposure time (1.5 to 3 hours) and higher temperatures than moist heat
sterilization. Bunsen burner and dry heat oven are best example for the use of the dry heat .
Dry Heat Oven:
Dry heat ovens are used to sterilize items
that might be damaged by moist heat or that are
impenetrable to moist heat. In dry heat oven Heat
at 160‐170 Celsius degree is used for 2 hours to kill
microbes and sterilizes the apparatus. Dry heat
ovens kill the End spores by oxidation or
denaturation .it is used on glassware and
instruments
Can’t be used on liquid media, cloth, plastics, or articles wrapped in paper.
Bunsen burner
Bunsen burner is common laboratory
instrument that is used for the sterilization
purposes in the microbiology labs. The destruction
of the microbes is carried out by the Bunsen burner.
The temperature of the Bunsen burners varies from
the 160 degree Celsius to several hundred Celsius
degree. Transfer needle is sterilized by using the
Bunsen burner.
Moist Heat
Figure 2: Bunsen burner
Figure 1: Dry Oven
Heat is provided in the form of the steam under specific pressure. This steam has
advantage over the dry heat in having more penetration power, rapid heating and moisture
present in the abundance. Steam causes the protein coagulation that eventually results in the
death of the microbes.
Autoclave:
Moist heat is used in the autoclave. Autoclave is a double-
jacketed steam chamber equipped with devices which permit the
chamber to be filled with saturated steam and maintained at a
designated temperature and pressure for any period of time. It is not
the pressure that kills the organisms but the temperature of the steam
high pressure device that uses the steam to kill the microbes. It is
commonly used instrument. The optimal conditions for the working of
the autoclave are 1210C &15Psi pressure for 15 minutes.
Figure 3 Autoclave
Pasteurization:
Pasteurization is technique used to kill reduce the microbial number to minimum level
with our affecting the quality of milk products. In this technique high temperature for short
time is given to the milk products. The mechanism involves the treatment of milk products at
72oC for 15 sec this is called flash method. In case of batch method we provide 63-66degree
Celsius for about 30 minutes. Ultra temperature method involves the treatment of milk
products at 134 degree Celsius for 1 to 2 sec.
Boiling water
In this method different instruments are sterilized by boiling in water at 100 degree
Celsius for about 30 minutes. Most of the non spore forming bacteria is killed by the boiling
method.
Cold temperature:
Cold temperature acts as the micro biostatic which means low temperature slows down
the growth of the microorganisms. Two Basic techniques are used to control the micro
organisms.
Refrigeration:
Food material that is prone to spoilage is preserved for short period of time at 4 degree
Celsius. T his temperature does not kill the microbes but it slows down the metabolic activity of
the microbes.
Freezing:
Freezing is done to preserve thing for longer period of time .long term storage is done
by freezing the material at -20 degree Celsius or by using liquid nitrogen that has the
temperature at -80 OC to -196oC. Freezing temp do not allow the microbes to grow .
Desiccation:
Desiccation is gradual removal of water from cells of microbes. It is a very important
method to control microbial growth. This method has following characteristics:
It leads to metabolic inactivation of cell. As all metabolic reactions require water to
continue, it is very difficult for cell to carry out metabolic reactions in the absence of
water. As a result, cells shrink and ultimately die.
Different techniques are used to remove water. Usually water loss is due to osmosis. So
we treat food with salts and as a result foods dry and water is drawn from cells of
microbes present in food.
Another method for desiccation is lypholization. In this method, food is quickly frozen
and then subjected to drying to remove water. It is an effective way to preserve foods
and microbial cultures as growth of microbes is stopped.
In desiccation, mostly microbes go into dormant stage which means whenever microbes
find water, they will start to grow again. This makes it an ineffective method to control
microbes
Filtration:
Filtration is physical removal of microbes present in air or water by
passing through a membrane. This membrane traps most of the
bacteria and microbes to keep air or liquid microbes free. This
method has following characteristics:
This method is used for heat sensitive liquids to remove
microbes. Hospital isolation units and research laboratories
use specialized filters to filter air. In research labs, mostly
HEPA filters are used to filter microbes
Membranes used in this method are usually made up of
cellulose acetate and their pore size is precisely monitored.
Figure 4: Filtration
Radiation:
Radiation is another physical non heat method to control microbes. In this method,
microbes are subjected to radiations and different molecular abnormalities occur in microbial
cells which lead to cell death. Basically radiation is high energy released from atomic activities
and has a very high speed that it can penetrate to surfaces and cause destruction of various
parts of cells. There are basically two types of radiations:
Ionizing radiation:
They have very high penetrating power. It causes chemical changes in organelles and
subject cells to toxic substances. They also break DNA backbone and disrupt DNA
structure.
Examples include gamma rays and X-rays
Mostly used to sterilize surgical instruments and medical supplies and sometimes food
products but their dosage is precisely controlled as they can affect other cells than
microbes.
Figure 5: Ionizing Radiations
Non-ionizing Radiations:
They have less penetrating power. Mostly these
radiations cause formation of T-T dimers in the DNA
of cell. Thus they disrupt DNA structure and cause cell
death.
Example is UV rays.
They are mostly used to sterilize surfaces, air and
water. These radiations are used in biosafety cabinets
to kill microbes.
Types of Chemical control
Halogens:
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. It is available in a tincture (in solution with alcohol) or an iodophor (combined with an
organic molecule).
The action of chlorine is based on the formation of hypochlorous acid (HOCl) when chlorine
is added to water.
Good oxidizing agent.
Figure 6: Non- Ionizing Radiations
Chlorine is used as a disinfectant in gaseous form (Cl2 ) or in the form of a compound,
such as calcium hypochlorite, sodium hypochlorite (NaOCl, Clorox), sodium
dichloroisocyanurate, and chloramines.
Used to disinfect drinking water and swimming pools.
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. It can also denature proteins
and enzyme inactivation.
Qualities:
Not inactivated by organic compounds
Stable for long periods
Persist for long periods after application
Good for disinfecting things like pus, saliva, and feces
Alcohols :
Alcohols participate by denaturing proteins and dissolving lipids.
Not good for wound disinfection because proteins coagulate and form a protective coat
around bacteria.
In tinctures, they enhance the effectiveness of other antimicrobial chemicals.
Aqueous ethanol (60-95%) and isopropanol (lower concentrations required, typically 62 -
65%) are used as disinfectants. Alcohol evaporates quickly and leaves no residue behind.
Denaturation requires H2O, which is why aqueous preparations are better than pure.
70% is best ethanol concentration.
Hydrogen Peroxide :
It is used as antiseptic for the treatment of minor cuts and also as bleaching agent.
When it is placed on injured part , hydrogen per oxide bubbles due to release of an enzyme ,
which break it down into water and oxygen.
Detergents and soaps :
It disrupts the cell membrane and used as skin antiseptics and disinfectants .They
decrease the surface tension between microorganisms and surfaces, and in this way they help
in cleansing of the surface. Soaps emulsify the oily film on the body surface, carrying the oils,
debris, and microorganisms away in a degerming action. The
cationic detergents are quaternary ammonium compounds. They solubilize the cell
membranes of microorganisms.
Heavy Metals :
A number of heavy metals have antimicrobial ability. For instance :
silver is used as silver nitrate in the eyes of newborns to guard against infection by
Neisseria gonorrhea. It is also used to cauterize wounds.
Copper is used as copper sulfate to retard the growth of algae in swimming pools, fish
tanks, and reservoirs.
Zinc is useful as zinc chloride in mouthwashes and as zinc oxide as an antifungal agent
in paints. The heavy metals are believed to act by combining with sulfhydryl groups on
cellular proteins.
Aldehydes:
Two aldehydes, formaldehyde and glutaraldehyde inactivate microbial proteins by cross
linking the functional groups in the proteins.
Formaldehyde gas is commonly used as formalin, 37 % solution of formaldehyde gas. It
is widely employed for embalming purposes.
Glutaraldehyde is used as a liquid to sterilize hospital equipment. However, several
hours are required to destroy bacterial spores.
Works Cited "Chapter 11: Physical & Chemical Control of Microbial Growth." Microbiology. 08 May 2015
<https://sites.google.com/site/rccmicrobiology/chapter-11-physical-chemical-control-of-
microbial-growth>.
Gantarm. "Physical and Chemical Control of Microbes." Florida International University. 07 May
2014 <http://faculty.fiu.edu/~gantarm/Ch.%2011%20Control%20of%20Microbes.html>.
Harcourt, Houton Miflin. "Physical Methods of Control." Cliff Notes. 07 05 2015
<http://www.cliffsnotes.com/sciences/biology/microbiology/control-of-microbial-
growth/physical-methods-of-control>.