Microbiology
Chapter 7
Control of
Microbial Growth
1
Levels & Types of Control
Sterilization - completely destroys all forms of microbial life
Commercial Sterilization - limited heat treatment, destroys
pathogens but not all bacteria
Disinfection - destroys vegetative cells on a surface
reduces # of viable organisms in the material
disinfectant - chemical treatment used to disinfect inanimate
objects
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Controls continued
Antisepsis - treatment of living skin or tissue to kill
microorganisms
Degerming - physical removal of microbes (alcohol swab, soap)
Sanitize - systematic cleansing of inanimate objects to reduce the
microbial count to a safe level (for public health, used in
restrooms, kitchens)
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Effects of Agents
Bacteriocidal (-cidal, to kill) - kill all bacteria
Bacteriostatic (-static, to stop) halt (inhibit) the growth of
bacteria for as long as the inhibitory substance is present
if inhibitor is removed, growth begins again
Aseptic – prevention of (or absence of) sepsis
sepsis – bacterial contamination (decay, putrefaction)
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Microbial Death
Killing in a population is not instantaneous
death, like growth, occurs exponentially (if the log of the number
of cells is plotted on the Y axis, then a straight line results, (see
fig. 7.1)
5
In this example, 90% of survivors
are dying each minute
This figure indicates shorter time
required to kill a smaller population
Action of Antimicrobials
1. Alter membrane permeability
damage to lipids or proteins causes leaks and interferes with
growth
2. Damage proteins or nucleic acids
Proteins - enzymes which cell needs for growth are damaged
damage to DNA or RNA prevents information from being transferred
for protein synthesis
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Physical Methods of Control
Heat (moist & dry)
Filtration
Cold
Desiccation
Osmotic Pressure
Radiation
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each will be discussed in some detail in the following minutes…
Heat
How it works - denatures (unwinds) proteins
Thermal Death Time (TDT) the time at a given temperature in
which all the microorganisms in a liquid culture will be killed
Thermal Death Point (TDP) the lowest temperature at which all
microorganisms in a liquid suspension are killed in 10 minutes
Heat sterilization high heat and high humidity together are the
most effective in killing microorganisms, but dry heat can also be
used
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Moist Heat
Boiling - kills all vegetative cells, method called Tyndallization
can be used with endospore-forming bacteria
Steam alone can reach 100oC, but under pressure (15 p.s.i.) can
reach 121oC = Autoclave
to kill all microorganisms, all contents must be available for
steam to reach on surface
kills endospores in 15 minutes at this temperature
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More Moist Heat Techniques
Pasteurization uses temperature below boiling to kill pathogens
and reduces total microorganism count (doesn’t kill all, some
harmless microorganisms survive), does not alter food taste
Classical Pasteurization - 63oC for 30 minute
Flash Pasteurization - now use high-temperature short-time
72oC for 15 seconds. Sometimes called High Temperature Short
Time (HTST) Pasteurization
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Dry Heat
Examples include:
Empty glassware can be sterilized in oven in 1-3 hours at 180oC
Aseptic techniques including flaming mouth of tubes, inoculating
loops/needles
Incineration – (burning) a convenient method for destroying
some disposable wastes
13
Cold
low temperatures are bacteriostatic
home refrigerator is at 4oC, freezer at -10oC (commercial is 20oC)
some bacteria can grow at refrigerator temperatures, but most
pathogens do not
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Desiccation
Desiccation = the absence of water
prevents growth (also used for bacterial storage, lypholization)
Lypholization = ‘freeze-drying’
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Osmotic Pressure
hypertonic environments prevent growth of most organisms
(dehydrates them)
Fungi are more resistant to osmotic pressure extremes than are
bacteria
16
Ultraviolet Radiation
most lethal at wavelength of 260 nm
absorbed by DNA, leads to formation of thymine (pyrimidine)
dimers
DNA has several methods of repair from this damage
UV used to sterilize air & surfaces (but cannot expose skin)
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Ionizing Radiation
example Gamma radiation from Cobalt 60
Advantage - penetrates deep into objects
can be used to sterilize plastic objects, even foods
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Filtration
• physically remove bacteria from liquid or
air
• most bacterial filtration by .45 or .2 micron
pore size
Review table 7.5 for a good summary
of Physical control of microbes
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Chemical Microbial Controls
21
Evaluating Effectiveness of
Chemical Agents
Use-Dilution Testing – standardized metal rings dipped in
bacterial cultures of Salmonella choleraesuis, Staphylococcus
aureus & Pseudomonas aeruginosa, dried briefly then placed into
disinfectants for 10 minutes at 20oC. These cultures are then
allowed to grow and are later evaluated.
Disk-Diffusion Method – Filter paper soaked with chemical
agent placed on inoculated agar plate containing a known
organism, usually Staphylococcus aureus or Pseudomonas
aeruginosa. The clear ‘death zone’ around the filter paper is
measured 24-48 hours later.
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Phenol & Phenolics
phenol and phenol derivatives
Phenol was the first chemical used to control microorganisms by
Joseph Lister in 1867; used as disinfectants and antiseptics
(example, Lysol)
Action of chemical: denature proteins and disrupt lipid
containing cell membranes
Advantage: Stable & persist for long periods, good surface
disinfectants
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Bisphenols
bis = 2
A phenol derivative with TWO phenols linked by a bridge.
pHisoHex© used in surgical scrubbing & for hospital control of
microbes.
particularly effective against G+ staphylococci & streptococci
which affect newborns
Triclosan – another bisphenol that can be incorporated into
plastics probably disrupts membranes – broad spectrum but
particularly effective against G+ & fungi
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Phenol
Bisphenols
Biguanide
similar to phenolics, disrupts membranes
Used in some hospital surgical scrubs
Advantage: binds to skin & mucous membranes but has low
toxicity to humans
Effective against vegetative bacterial cells (not spores), & fungi
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Halogens
halogens are elements in the same group of the periodic table
(fluorine, chlorine, bromine, iodine, etc.).
Iodine and chlorine are most important as antimicrobial agents
iodine often used in a tincture (alcohol solution)
chlorine may be used as Cl2 gas or in solution with other
chemicals
Action: damage cellular materials (through oxidation)
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Alcohols
both disinfectants and antiseptics
ethanol and isopropanol used most (at 70-80% concentrations)
Actions: denature proteins, possibly dissolve membrane lipids
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Heavy metals (fig. 7.8)
examples: mercury, silver, arsenic, zinc, copper
silver nitrate prevents neonatal ophthalmic gonorrhea
Actions: heavy metals inactivate cell proteins
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Surfactants
surface acting agents
Soaps & detergents.
Action: mechanical removal of microbes.
Some contain antibacterial agents such as triclocarban (effective
against G+)
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Acid-Anionic Surfactants
Important surface agents in dairy industry.
Action: Negative portion (anion) of molecule disrupts plasma
membranes
Benefit: Acid-anionic sanitizers effective against broad spectrum
of organisms including those resistant to heat (thermoduric).
These agents are nontoxic, noncorrosive & quick acting
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Quaternary Ammonium Compounds
Quats are composed of charged quaternary nitrogen and a long
hydrophobic chain. A type of surfactant
The positively charged end is a version of the ammonium ion
(NH4+) – this give the name (quat) and is the effective portion of
the molecule.
Quats probably disrupt membranes.
stable and non-toxic, can be used on food utensils and as skin
antiseptics.
Some bacteria actively grow in these compounds.
32
an example of a quat
Aldehydes
examples: formaldehyde, glutaraldehyde
Action: cross-link proteins rendering them inactive
Benefit: sporicidal
aldehydes are highly reactive molecules
34
Chemical Food Preservatives
examples: sodium benzoate, sorbic acid, calcium propionate
Usually organic acids which are metabolized,
Action: keep food pH low to inhibit growth
these are biostatic and merely halt growth, they don’t kill
organisms
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Gases
useful for sterilizing heat-sensitive objects (example: the
disposible Petri dishes in the lab)
Ethylene oxide, reacts with cell proteins
bactericidal and sporicidal
36
Peroxygens
oxidizing agents – these oxidize cellular components.
Examples: ozone, hydrogen peroxide, peracetic acid
Ozone often used to treat water
hydrogen peroxide often used to treat open wounds. Broken
down to water & oxygen by the enzyme catalase
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Chemical Methods & Microbial Resistance
Most Gram-positive organisms are
generally more sensitive to chemical
agents than are Gram-negatives
Endospores are less susceptible
Mycobacterium also resistant
(See Table 7.7 for examples)
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