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.
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