Bacteriology Microbiology

1. 1
Microbial control
Chapter 7
bhoushaymi@gmail.con

2. 2
• Sterilization: Removal of all microbial life.
• Commercial Sterilization: Food in cans is subjected to enough heat for a
complete sterility, for destroying Clostridium botulinum endospores, which
can produce a deadly toxin in food.
• Disinfection: Removal of pathogens (chemical, ultraviolet radiation, boiling
water, or steam).
• Antisepsis: Removal of pathogens on living tissue (use chemical
disinfectant).
• Degerming: Removal of microbes from a limited area, such as alcohol use
for the skin around an injection site or before receiving an injection.
• Sanitization: Lower microbial counts on eating utensils. May be done with
high temperature washing or by dipping into a chemical disinfectants.
Terminology

3. 3
• Treatment that cause outright death of microbes have suffix -cide,
meaning kill.
– Biocide/Germicide: Kills microorganisms (exception endospores)
– Fungicide: kills fungi
– Virucide: inactivates virus.
• Treatments only inhibit the growth or multiplication of bacteria have
suffix –stat or stasis.
– Bacteriostasis: Inhibiting, not killing, microbes.
• Sepsis: refers to microbial contamination as in a septic tanks for
sewage treatment. (toxic conditions resulting from the growth and
spread of bacteria in blood and tissue.
• Asepsis: is the absence of significant contamination.
• Aseptic: an object or area is free of pathogens.
– Aseptic technique is important in surgery techniques to prevent
microbial contamination of wounds.

4. 4
The rate of microbial death
Bacterial populations die at a constant logarithmic rate when heated
or when treated with antimicrobial chemicals.
Example: suppose a
population of 1 million
microbes has been
treated for 1 min,
and 90 % of the
population has died;
the population now is
100,000 microbes.
If the population is
treated for another
min, 90% of those
microbes 100,000
will die, so we left
with 10,000 microbes
alive, etc…1

5. 5
• Number of microbes
• Environment influence:
– The presence of organic matter often inhibits
the action of chemical antimicrobials. In
hospital, the presence of organic matter in blood
vomit, or feces influence the selection of
disinfectants.
– The nature of suspending medium is also a
factor in heat treatment. Fats and proteins are
especially protected, and a medium rich in these
substances protects microbes which will then
have a higher survival rate.
• Heat is more effective under acidic conditions.
• Time of exposure
• Microbial characteristics
Effectiveness of antimicrobial treatment depends on:
The effect of high or low initial
load of microbes. If the rate of
killing is the same, it will take
longer to kill all members of
larger population than a smaller
one. This is true for both heat
and chemical treatments.

6. 6
We examine the ways various agents actually kill or inhibit
microbes (the effects of microbial control agents on cellular
structure).
• Alternation of membrane permeability
– Damage to the lipids or proteins of the plasma membrane by
antimicrobial agents causes cellular content to leak into the
surrounding medium and interferes with growth of the cell.
• Damage to proteins
– Denaturation of protein
• Damage to nucleic acids
Actions of Microbial Control Agents

7. 7
When selecting methods of microbial control, consideration must be
given to effects on things besides the microbes.
Heat
Heat appears to kill microorganisms by denaturing their enzymes.
Heat resistance varies among different microbes;
• These differences can be expressed through the concept of thermal
death point.
– Thermal death point (TDP): the temperature required to kill all the bacteria
in liquid culture in 10 min.
• Another factor to be considered in sterilization is the length of time
required.
– Thermal death time (TDT): the length of time required to kill all the bacteria
in liquid culture at a given temperature.
• A third concept related to bacterial heat resistant is the Decimal
reduction time (DRT): The time (in minutes) required to kill 90% of a
bacterial population at a given temperature; also called D value.
Physical Methods of Microbial Control

8. 8
• Moist heat: killing microorganisms by the coagulation of proteins
(denaturation), which is caused by breakage of the hydrogen bonds that
holds the proteins in three dimensional structure (example: egg white
frying).
– One type of moist heat sterilization is boiling, which kills vegetative form of
bacterial pathogens.
– Reliable sterilization with moist heat requires temperature above boiling water.
• It can be achieved by steam under pressure in an autoclave. The higher the
pressure the higher temperature.
Pressure 15 psi 121ºC
Pressure 20 psi 126ºC
psi: pressure per square inch

9. 9
• Pasteurization reduces spoilage organisms (lowering microbial number) and
pathogens
• In the classic pasteurization treatment of milk, the milk was exposed to a
temperature (to eliminate pathogenic microbes, and to lower the microbial
numbers, which prolongs milk’s good quality under refrigeration) of about:
– 63°C for 30 min
– High-temperature short-time (HTST) 72°C for 15 sec
• Milk also can be sterilized by something quite different from
pasteurization by Ultra-high-temperature (UHT) so that it can be stored
without refrigerator: 140°C for <1 sec
• The concept of equivalent treatments: Different methods that have the
same effect on controlling microbial growth.
• Many relatively heat resistant (thermoduric) organisms survive
pasteurization, but these are unlike to cause disease or cause refrigerated
milk to spoil.
Pasteurization

10. 10
• One of the simplest method of Dry Heat Sterilization:
– Flaming
– Incineration (an effective way to sterilize and dispose of contaminated
cups, bags, dressing)
– Hot-air sterilization (temperature of about 170ºC maintained for nearly
2 hours ensures sterilization.
Dry Heat Sterilization
Hot-air Autoclave
Equivalent treatments 170˚C, 2 hr 121˚C, 15 min
• Filtration
– removes microbes
• Low temperature
– inhibits microbial growth
Refrigeration Deep freezing Lyophilization
• High pressure
– denatures proteins

11. 11
• Desiccation
– In the absence of water, microorganisms cannot grow or
reproduce but remain viable for years. Then when water is made
available to them, they can resume their growth and division.
– For example: The gonorrhea bacterium can withstand dryness
for only about an hour, while the tuberculosis bacterium can
remain viable for months. Viruses are normally resistant to
desiccation, but they are not resistant as bacterial endospores,
some of which survive for centuries.
• Osmotic pressure:
– The use of high concentrations of salts and sugars to preserve
food.
– causes plasmolysis

12. 12
Radiation
• Damages DNA
• Radiation has various effects on cells, depending on its wavelength, intensity,
and duration. Radiation that kills microorganisms (sterilizing radiation) is of
two types: ionizing and nonionizing.
– Ionizing radiation: (X rays, gamma rays or high energy electron beams),
have a wavelength shorter than nonionizing radiation, less than about 1nm.
– Nonionizing radiation: (UV) has a wavelength longer than that of ionizing
radiation, ususlalay greater than about 1nm.
– Microwaves: microorganism can be killed by heat; not especially
antimicrobial (do not have much direct effect on microorganism)