Transcript L4-Controlling Microbial Growth

Second part
CONTROLLING MICROBIAL
GROWTH IN THE ENVIRONMENT
Topic Outline
1. Basic Principles of Microbial Control
2. The Selections of Microbial Control
Methods
3. Physical Methods of Microbial Control
4. Chemical Methods of Microbial Control
Basic Principles of Microbial Control
1. Terminology of Microbial Control
• Sterilization – The removal or destruction
of all microbes, including viruses and
bacterial endospores, in or on an object.
• Aseptic – An environment or procedure
that is free of contamination by pathogens.
• Disinfection – The use of physical or
chemical agents known as disinfectants.
• Degerming – The removal of microbes
from a surface by scrubbing.
• Sanitization – The process of disinfecting
places and utensils used by the public to
reduce the number of pathogenic
microbes to meet accepted public health
standards.
• Pasteurization – The use of heat to kill
pathogens and reduce the number of
spoilage micoorganisms in food and
beverages.
2. Microbial Death Rates
• Definition – The permanent loss of
reproductive ability under ideal
environmental conditions.
• Technique for evaluating the efficacy of an
antimicrobial agent.
3. Action of Antimicrobial Agents
•
i.
-
-
Modes of action fall into two basic
catagories.
Alteration of cell walls (fungi) and
membranes (virus).
Give damage to a membrane’s proteins
or phospholipids and therefore allows
the cellular contents to leak out and
causes death.
Give damage to viral envelope that
interrupts the reproduction.
ii. Damage to proteins and nucleic acids.
- By breaking hydrogen and disulfide bonds
in three dimensional shape and resulting
in proteins denaturation. Proteins cease to
function.
- Genes of a cell or virus are composed of
nucleid acids. Disruption of nucleic acid
can produce fatal mutation.
The Selection of Microbial Control
Methods
1. Factors Affecting the Efficacy of
Antimicrobial Methods
i.
-
-
Site to be treated
Harsh chemicals and extreme heat
cannot be used on human, animals and
fragile objects.
To sterilized the utensils to be used on
the body to prevent infections.
ii. Relative susceptibility of microorganisms
- Often to select a method to kill the hardiest
microorganisms present, assuming that
method will kill more fragile microbes as
well.
- Germicides can be classified as high,
intermediate or low depending on their
effectiveness on inactivating or destroying
microorganisms.
- a. High-level kill all pathogens, including
endospores.
b. Intermediate-level kill fungal spores,
protozoan cysts, viruses and pathogenic
bacteria.
c. Low-level kill vegetative bacteria, fungi,
protozoa and some viruses.
iii. Environmental conditions
- Temperature: warm disinfectants work
better than cool ones.
- pH: some disinfectants more effective at
low pH.
- To clean objects before sterilization.
2. Methods for Evaluating Disinfectants
and Antiseptics
i.
-
Phenol coefficient
The first method used.
If >1.0 ; the agent is more effective than
phenol.
The larger the ratio, the greater the
effectiveness.
ii. Use-Dilution Test
The current standard test
The most effective agent is the one that
entirely prevents microbial growth at the
highest dilution.
iii. In-use test
- A more realistic method.
- Swabs are taken from actual objects
before and after application of disinfectant.
- More accurate determination of a given
disinfection agent for each specific
situation.
PHYSICAL METHODS OF MIROBIAL
CONTROL
1. Heat Related Methods
- High temperatures denature proteins, interfere
with the integrity of cytoplasmic membranes and
cell walls and disrupt the function and structure
of nucleic acids.
- Thermal death point: lowest temperature that
that kills all cells in a broth in 10 minutes.
- Thermal death time: the time it takes to
completely sterilize a particular volume of liquid
at a set temperature.
- Decimal reduction time (D): time required to
destroy 90% of the microbes in a sample.
How to calculate D?
• For Clostridium botulinum for example, D
value is 0.204 minute.
• The actual time required to reduce 1012
endospores to 1(100) endospore is 0.204 x
12 = 2.5 minutes.
• 12 is refer to 12-fold reduction.
i.
-
Moist heat
To disinfect, sanitize, sterilize and pasteurize
by denaturing proteins and destroying
cytoplasmic membranes.
More effective than dry heat because water is
better conductor of heat than air.
Methods: Boiling, Autoclaving, Pasteurization
and Ultrahigh-Temperature Sterilization.
ii. Dry heat
Denatures proteins and fosters the oxidation
of metabolic and structural chemicals.
For substances cannot be sterilized by boiling
or steam or materials can be damaged by
repeated exposure to steam.
• endospores are very heat resistance.They
are to survive heat that would rapidly kill
vegetative cells of the same species.
• a major factor in heat resistance is the
amount and state of water within the
endospores.
• during endospores formation, the
protoplasma is reduced to a minimum
volume as a result of the accumulation of
Ca2+-dipicolinic acid complexes.
• also the protection of spore DNA by small
acid-soluble proteins (SASP).
• this mixture forms a gel in the cytoplasm.
• a thick cortex then forms around the
protoplast core.
• contraction of the cortex results in a
shrunken, dehydrated protoplast with a
water content of only 10-30% of a
vegetative cells.
• high concentration of SASPs and low
water content will give high heat
resistance.
- A routine part of standard aseptic in
microbiology laboratory procedure .
- Requires higher temperatures for longer
times than moist heat.
2. Refrigeration and Freezing
- Tempt. is between 0oC and 7oC for
refrigeration and below 0oC for freezing.
- Will decrease microbial metabolism,
growth and reproduction.
- Because slow in chemical reactions and
unavailable of liquid.
3. Desiccation And Lyophilization
- Dessication or drying inhibits microbial
growth because the absent of water.
- Lyophilization- technique combining
freezing and drying to preserve microbes
and cells.
4. Filtration
- The passage of a fluid (liquid or gas)
through a sieve to trap and separate
particles (cells or viruses) from the fluid.
- To sterilize heat-sensitive materials
(antibiotics, vaccines, enzymes etc.).
5. Osmotic Pressure
- Cells in hypertonic solution (concentrated
salt or sugar) will lose water and
therefore inhibits cellular metabolism.
6. Radiation.
- Particulate radiation : consists highspeed subatomic particles that have
been freed from their atoms.
- Electromagnetic radiation: energy without
mass traveling in waves in the speed of
lights.
i.
-
Ionizing Radiation
Wavelengths shorter than 1 nm.
Electron beams, gamma rays, and X
rays.
ii. Nonionizing Radiation
- Wavelength greater than 1 nm.
- UV light, visible light, infrared radiation
and radio waves.
CHEMICAL METHODS OF
MICROBIAL CONTROL
i.
-
-
-
Phenol and phenolics.
Phenolics: compounds derived from phenol molecules
that have been chemically modified by the addition of
halogens or organic functional groups.
Commonly used in health care settings, laboratories
and households.
+ve; effective even in the presence of contaminating
organic material such as vomit, pus, saliva and feces.
They remain active on surfaces for a prolonged time.
-ve: disagreeable odor and possible side effect (skin
irritation, brain damage in infants).
ii. Alcohols
- Commonly used are isopropanol and
ethanol.
- Denature proteins and disrupt cytoplasmic
membranes.
- -ve: not effective against fungal spores or
bacteria endospores.
- Pure alcohol is not an effective as 70%
and 90% because no water.
• however, water is needed for the
coagulation reactions of proteins.
• this also called denaturation of proteins.
• 100% ethanol contained no water to carry
on the coagulation process and therefore
not effective in microbial control.
• moreover, 70% alcohol-water mixture
penetrates more deeply than pure alcohol
into most materials to be disinfected.
iii. Halogens (I-, Cl-, Br-, Fr-)
- Used both alone and combined with other
elements in organic and inorganic compounds
- Iodine: well-known antiseptic for water.
- -ve: cannot destroy protozoan cysts.
- Iodophor (iodine-containing organic compound):
used in medical institution.
- Chlorine: treat drinking water, swimming pools
and waste water.
- -ve: by-products are trihalomethanes (THMs),
increased risk of cancer.
iv. Oxidizing Agents
- peroxides, ozone and peracetic acid.
- +ve: effective against anaerobic microorganisms
contaminating deep wounds.
- Used by health care workers to kill anaerobes in deep
puncture wounds.
- -ve: ozone is expensive.
v. Surfactants (soaps, detergents)
- To reduce the surface tension of solvents (water) by
decreasing the attraction among molecules that the
solvent becomes more effective at dissolving solute
molecules (exp. oils) and any bacteria they harbor – are
more easily wash away.
- -ve: not effective against nonenveloped viruses,
mycobacteria and endospores.
vi. Heavy metals
- Such as arsenic, zinc, mercury, silver and
copper.
- Can combine with sulfur atoms in molecules of
cystein, an amino acid.
- Low-level bacteriostatic and fungistatic agents.
- -ve: mercury is a metabolic poison, silver nitrate
is irratating.
vii. Aldehydes
- Compounds containing terminal –CHO groups.
- Usually used by hospital personnel for
disinfecting medical and dental equipments.
- -ve: formadehyde irritates mucous membranes
and is carcinogenic.
viii. Gaseous Agents
- Suitable for large or bulky items.
- Gases such as ethylene oxide, propylene
oxide, and beta-propiolactone.
- -ve: can be extremely hazardous
(explosive and poisonous) to the people
using them.
ix. Antimicrobials Drugs
- antibiotics, semisynthetics and synthetics.
- Antibiotics: chemicals produced naturally
by microorganisms.
i. from fungi
ii from bacteria
- Semisynthetic: antibiotic undergo
modification.
- Typically used for treatment of disease.