Transcript Control of Microbial Growth

Control of Microbial Growth
We will talk about antifungals,
antibiotics, antivirals, etc. when we
begin to talk about specific diseases.
Definitions
• Sterilization: destruction of all forms of
microbial life (including endospores).
• Disinfectant: destruction of vegetative
pathogens (does not include endospores).
• Antiseptic: destruction of vegetative
pathogens on living tissue (not endospores).
• -cide: this suffix means, kills
– Ie. Germicide: kills germs
• biocide: kills life
• -stasis or static: inhibits bacterial growth.
– Ie. Bacteriostatic: inhibits growth of bacteria.
• Main targets that are used to control
microbial growth are bacterial cell wall,
plasma membrane, DNA, RNA, and protein
synthesis.
Methods of Sterilization
• Osmotic Pressure: beef jerky (addition of salt or sugar).
• Radiation (UV light): UV light causes lethal mutations in
DNA when exposed for long periods of time. It is
frequently used in virology labs to sterilize rooms where
viruses are transferred from old media to fresh media. It is
also used to sterilize equipment such as goggles.
• Filtration: air filters are used in hospitals to keep patients
from contracting disease while there. This is especially
important in the burn unit where patients are extremely
susceptible to infection. Death among burn patients is
usually due to infection.
– Some liquids are heat sensitive and can be damaged if exposed to
the temperatures needed to sterilize it. Filters are used to sterilize
those too.
• Dry heat: means using an open flame to kill
organisms, for example.
– In lab flaming loops with the bunsen burner is an
example of dry heat.
– Incineration (or burning) of carcasses is also an
example of dry heat.
• Moist heat: The best example of moist heat is an
autoclave. An autoclave is a machine that uses
water and heat. It creates hot steam by increasing
the pressure inside of it. As the pressure increases,
the temperature increases. It is able to reach very
hot temperatures and is very effective in killing
many organisms and endospores.
– Autoclaves are found in many places. They are used to
sterilize surgical equipment, gowns, towels, media, test
tubes, etc.
• Some Chemicals: Formaldehyde,
glutaraldehyde
– Most chemicals are not considered sterilizing
agents.
– Formaldehyde and glutaraldehyde are so
effective in killing organisms that they are
included as sterilizing agents.
– They work by inhibiting enzyme function.
– Generally these chemicals are used to disinfect
instruments or materials that may be damaged
by heat. They are also used to preserve tissue
so that it can be studied.
Types of Disinfectants and
Antiseptics
• Most chemicals have a particular concentration and length
of exposure to microbes, to be most effective.
– For example, isopropyl alcohol is frequently used in the clinic
when administering injections or drawing blood. It is most
effective when the solution is 65-75% alcohol. Some stores sell
95% alcohol and consumers buy it because they think that the
greater the concentration, the better.
– The other challenge with alcohol is that it evaporates very quickly.
So as an antiseptic, it is not very effective because it evaporates off
the skin so quickly and there is not in contact with the bacteria
long enough to kill it.
– The other challenge with alcohol is that it denatures (or inactivates)
protein but does not remove it very well. So if a surface is
particularly dirty the alcohol just denatures the protein and leaves it
on the surface.
• Triclosan: common antibacterial agent added to
soaps, cutting boards and other products.
– Triclosan is a pretty effective antibacterial agent
(antiseptic) but it is being overused. A decreased
sensitivity is being seen in several microorganisms
including Staphylococcus.
– Triclosan is what is added to antibacterial hand soaps.
It kills the organisms but the soap leaves a residue of
triclosan in the sink. As the organisms are continually
exposed to the triclosan it gives them opportunities to
develop resistance.
– In addition, there is no need, particularly in a home, to
use antibacterial soap. Ordinary hand soap removes the
organisms just as effectively and does not needlessly
use an extra chemical to which microbes are now
growing resistant.
• Chlorhexadine: used for surgical hand scrubs and
pre-operative skin preparation as well as other
uses.
– This chemical damages the plasma membrane and
causes protein denaturation.
– In medium to high concentrations it is very effective
against vegetative pathogens but not against
endospores.
– An advantage to using chlorhexadine is that it has a
very low toxicity. That means that it can be used in
large or amounts as an antiseptic without being toxic.
• Halogens: iodine, chlorine
– Effective against all kinds of bacteria and endospores,
various fungi, and viruses.
– This group interferes with protein synthesis and
folding.
– Chlorine can be used as an antiseptic or a disinfectant
depending upon the form in which it is used.
• It is used to treat drinking water and swimming pools for
example.
• In higher concentrations it is used to disinfect equipment,
bedding, etc.
– Iodine is used as a disinfectant to sterilize cutting
blades, plastic, and rubber items. (It’s exact method of
killing microorganisms is not really understood yet.)
• It is also used as an antiseptic for wound care.
• Tincture of iodine: combination of iodine and alcohol
• An iodophore is a combination of iodine and an organic
molecule that allows iodine to be released slowly.
– The best example I can think of is those tablets they sell in the
pet stores for feeding fish while your on vacation. The longer
the tablet sits in the water, the more the outer coating is exposed
to the water and wears it away. As it is worn away, the food is
gradually released.
• Betadine, and Isodine are examples of iodophores.
• Hydrogen Peroxide: decomposes in the
presence of light, metals, or catalase into
water and oxygen gas.
– That is why it is stored in dark bottles, to
prevent its decomposition.
– It is more stable now than used to be.
– It is highly toxic to cells that don’t have the
enzyme, catalase.
– Bactericidal, virucidal, fungicidal, and can be
sporicidal
• Surface-active agents (surfactants): soaps and
detergents
– Surfactants disrupt cell membranes, more useful in
killing microbes.
– Not an antiseptic but does bind to org. to remove from
skin.