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

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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>.