Download lec#7 by Marwa Al-Awasa

Title of Lecture: Control of
microbial growth
Date of Lecture: 30-9-2014
Sheet no: 8
Refer to slide no. : 14-32
Written by: Marwa AL-Awasa
Physical Methods of sterilization/ disinfection
 Low Temperature
- Refrigeration
-Refrigeration is used in day to day life to preserve food by keeping it in the
refrigerator at a temperature of 2o to 8oC.
-It has a bacteriostatic effect which means it doesn’t kill bacteria instead it
reduces the metabolic rate of most microbes so they can't reproduce or
produce toxins with some exceptions like Listeria monocytogenes which can
survive even lower temperature.
-it's not really a sterilizing method but it preserves food for a long period of
time.
- Freezing
-in terms of the rate freezing is done by two methods; a very quick and
sudden one which doesn’t kill most microbes and known as Flash
Freezing , and a second method which allows the formation of ice crystals
within the bacteria itself thus disrupting the bacterial cell structure and this
method of freezing is the Slow Freezing.
- in Slow freezing, all of the constituents of the bacterial cell including the
bacterial proteins are disrupted But the rate at which this disruption occurs
differs from one type of microbes to another for example: Over a third of
vegetative bacteria may survive 1 year while most parasites are killed within
a few days of freezing at -21 oC .
- freeze-drying (lyophilization): used in preserving plasma products and
many of the powdery materials which are used in experiments and
laboratory work .this powder is sterilized ,freezed and placed in a vacuum
which will absorb all of the moist and then we don’t even need to freeze
them any longer because they are now considered Sterilized.
 Filtration
Some fluids that are used in medicines and injections can't be
autoclaved because they are heat sensitive materials which will be
broken down and spoiled upon heating that’s why they are sterilized.
We can filtrate gas and liquid and this is seen in operation rooms
where we need to sterilize the air by passing it through membrane
filter. Also we can filter fluids from microbes by passing them
through a membrane.
Membrane filters comes with pores of different sizes:
 0.22 and 0.45um Pores
Filters most bacteria with exception of spirochetes, mycoplasmas and
viruses because they are slender, small and they measure less than
0.45.
 0.01 um Pores: Retain all viruses and some large proteins.
 Ultrasonification
Its subjecting objects to an Ultra-sound which is a very high
frequency sound that can't be hear.
This sound produces shacking effect that disrupts and kill the
bacteria. Another use of this method is cleaning by shacking dust
particles.
 Desiccation /‫التنشيف‬
The removal of water which prevents bacterial growing and
reproducing. This method can be good for preserving some
food like fish but isn't a considerable method of sterilization.
Bacteria vary in its sensitivity and susceptibility to
desiccation:
Neisseria gonorrhea: it can survive for a very short time
outside the body, very sensitive to drying and it can only
survive for 1 hr. even in the lab it needs a special media to
be culture on.
Mycobacterium tuberculosis: May survive several months
in dust and if you inhale that dust you might get infection
which means that it's not always necessary to catch the
infection from a coughing infected individual.
Viruses: they are of 2 types enveloped and naked the
enveloped one which has plasma membrane around it can't
live for too long outside the body while naked viruses are
more resistant.
Clostridium spp. and Bacillus spp.: May survive
decades because they produce endospores which are
known to be very resistant and can live without the
presence of water.
 Radiation
Bacteria can be killed depending on the type of radiation
"wavelength of spectrum" used ( gamma rays are shorter in
wavelength than x-rays which are in turn shorter than UV and
these are less than 1 nm.
The Ionizing radiation (Gamma and x-rays ) these produce a
lot of energy and dislodge electrons from atoms and alter the
shape of DNA of exposed objects/ bacteria.
x-rays are not used in sterilization in contrast; gamma rays
are used to sterilize medical equipment that can't be heated.
Some risks are associated with the use of radiation for
sterilizing food like cancer, In addition it penetrates human
tissues and may cause genetic mutations.
 Ultraviolet light
-People who live in hot weather or continuously are exposed
to the sunlight (UV) have a high incident of developing skin
cancer.
-UV might damage eyes and skin.
-good for sterilizing surfaces, it doesn’t penetrate & it's non
ionizing like gamma rays.
-Damages DNA by producing thymine dimers, which
Cause mutations.

Microwave Radiation
- Shakes water particles within food under heat to kill bacteria.
- Only Heats food which is moist.
- Endospores are dry and will not be affected with microwave ,
unless their was some sort of liquid (water).
- Solid things will not be heated evenly.
Chemical methods of sterilization/ disinfection
There is NO universal disinfectant, but if we had such a
disinfectant it must have these qualities:
1.
2.
3.
4.
5.
Destroys all forms within a practical period of time.
Not irritant, allergenic or toxic.
Makes effective contact and not bypassed by dilution.
Readily soluble in water .
Chemically stable, no disagreeable odor, non discoloring and leaves no
stains.
6. Not expensive.
Important Factors
1) Characteristics of microorganisms (chemical make up), some microbacteria
are very resistant to disinfection because it contains fatty materials and
debris.
2) Concentration of disinfectant
3) PH of disinfectant (neutral to slightly acidic) affects degree of ionization
4) Interfering matter: contaminating matters , dirt , organic materials that’s why
we should clean things before disinfecting them.
5) Time: a disinfectant should work in a reasonable period of time.
Types of Disinfectants
 Phenols and Phenolics
- The first disinfectant to be used.
- Phenol (carbolic acid) was first used by Lister as a
disinfectant for some medical equipments but its rarely used
nowadays because of two reasons:
1- It is skin irritant.
2- It has unpleasant smell.
Phenolics are chemical derivatives of phenol
- Cresols: Derived from coal tar (Lysol).
- Bisphenols (Hexachlorophene): Effective against grampositive bacteria like staphylococci and streptococci. Used in
nurseries. Excessive use in infants may cause neurological
damage.
- Destroy plasma membranes and denature proteins.
- Advantages: Stable, persist for long time after application,
and remain active even in the presence of organic compounds
(dirt).
 Alcohol
-
Kill bacteria, fungi, but not endospores or naked viruses.
Act by denaturing proteins and disrupting cell membranes.
Evaporate, leaving no residue.
Used to mechanically wipe microbes off skin before injections or blood
drawing.
- Not good for open wounds, because it causes proteins to coagulate.
- Ethanol (Ethyl alcohol): Optimum concentration is 70%.it should be used
in the diluted form in which water makes it easier to penetrate. the absolute
form (100%) cant penetrate into the bacteria .
- Isopropanol: Rubbing alcohol. Better disinfectant than ethanol. Also
cheaper and less volatile BUT its not used as much as Ethanol!
 Halogens
-Mainly Iodine and chlorine but Not Bromine.
-iodine is usually mixed with ethyl alcohol and used as a disinfectant for the
skin and used in operations.
- Combines with the amino acid tyrosine in proteins and denatures proteins.
-stains skin and clothing.
-irritating: some people might be allergic to iodine and might initiate an
inflammatory response when their skin is exposed to it.
- Iodophors: Compounds with iodine that are slow releasing, take several
minutes to act.
-Chlorine is a gas that’s soluble in water and forms hypochlorus acid.
- Used to disinfect drinking water, pools, and sewage.
- Chlorine is used in house cleaners such as hypex.
-disadvantages of chlorine that its irritant , cause bleaching of clothes and its
corrosive that’s why it's not used with metals.
 Surfactants
-They interfere with the surface tension of molecules.
- they have hydrophobic and hydrophilic groups , one will react with fat an
the other with water
A-Soup
-its more used for cleaning and rinsing and not disinfecting.
-it doesn’t kill germs but it removes them (degerms skin mechanically).
B-detergents
-Interact with cell membrane of microorganisms.
- Two types: anionic and cationic.
 Alkylating agents
- They Substitute alkyl groups for the hydrogen of reactive groups of enzymes,
nucleic acids, and proteins.
- Ethylene oxide is a Chemosterilizer, colorless, soluble in water, Explosive
and toxic.
- Ethylene oxide action is influenced by concentration of gas, relative
Humidity, time of exposure, and temperature.
(The higher the gas concentration the less the time of exposure should be).
- Used to sterilize heat sensitive items like a heart machine or a kidney dialysis
machine.