Usually we mean negative control and the rest of this discussion relates to the destruction or inhibition of microbes:
PHYSICAL AND CHEMICAL METHODS - see Tortora et. al. for terms.
When disinfecting, one must be concerned with the most resistant life forms present. The following list of organisms is arranged according to ease (or difficulty) of destruction:Most resistant to destruction (least sensitive to physical and chemical agents)
Small Non-enveloped viruses (polio, rotavirus, rabies)
Vegetative Fungal Cells
Enveloped Viruses (Herpes, Hepatitis B, Hepatitis C, HIV)
Most Sensitive to destruction (least resistant to physical and chemical agents)
When disinfecting, one must be concerned with INTERFERRING MATTER, such as organic matter (Pus, Blood, Feces, etc.) which inactivates certain disinfectants (halogens). Soap inactivates Quaternary Ammonium Compounds.
When disinfecting, one must remember that "Things Take Time." No disinfecting treatment is instantaneous.
HEAT - Denatures proteins; Moist heat is more effective than Dry heat.
1. Boiling - vegetative cells are killed within minutes. Spores can survive. What is Tyndallization?
2. Steam under pressure - AUTOCLAVE. Standard load = 121oC (250oF), 15 lbs. steam pressure for 15 min. This kills spores, therefore this is sterilizing. Things can go wrong therefore it is important to have quality control.: SPORE TEST also there are color change indicators.
3. Pasteurize - kills vegetative bacteria. Thermoduric bacteria will survive.
4. Dry Heat - Baking - 160oC (320oF) for 2 hours, or 170oC (338oF) are common sterilizing treatments.
FILTRATION - See the discussion and figure in Tortora et. al. on this. Membrane filters (MilliporeTM) are used to sterilize heat sensitive liquids. HEPA filters are used to sterilize air in biohazard hoods.
ULTRASONIC CLEANERS - Good for removing organic contaminants - "presterilize." Not bacteriocidal.
Ultraviolet Light (See Tortora et.al., for a description of what UV light does to cells).
CHEMICALS - many of these are called antiseptics or disinfectants or both.
Sterilizing Chemicals:Ethylene Oxide
PHENOL - (Carbolic Acid) Works well in organic materials therefore good for blood and body fluid decontamination. Neurotoxic. Has residual effect (lasts a long time). Tuberculocidal, Not Sporocidal.
ALCOHOLS - (Ethyl and Isopropyl) Tuberculocidal but not sporocidal. Kills enveloped viruses but not naked viruses. 70% is commonly used concentration. Must make surfaces REALLY WET in order to achieve optimum killing.
HALOGENSCHLORINE - Excellent disinfectant, sporocidal, however easily inactivated by organic material. Dilutions of household bleach between 1:10 - 1:100 are sporocidal after 10 minutes of treatment. 2-4 drops of bleach per liter of water can be used to treat drinking water (let stand 30 min.)
CATIONIC DETERGENTS - (Quaternary Ammonium Compounds) Benzalkonium chloride and Cetylpyridium chloride are some common QUATS found in mouth washes and sore throat remedies. These are weak antiseptics which are active against a wide variety of vegetative bacteria. They are not tuberculocidal. They are not sporocidal. They do not kill Pseudomonas. Many QUATS are inactivated by soaps and other detergents.
ALKYLATING AGENTS - These are very powerful disinfectants. They are sporocidal and therefore sterilizing. Formaldehyde and glutaraldehyde are used to fix and preserve infectious tissues. These are also known to be carcinogens. Ethylene oxide is used in gas sterilizers. This is a very noxious, explosive gas.
HEAVY METALS - Compounds made with mercury, silver, copper and tin have long been used for their antiseptic properties. In ancient times copper and silver containers were used to preserve water. Merthiolate, mercurochrome, silver nitrate and copper sulfate are all used as antiseptics. Silver nitrate has long been used to prevent neonatal gonorrheal opthalmia and in the treatment of burns.
CHLORHEXIDINE - active against vegetative bacteria and commonly used as a surgical scrub. This disinfectant is not sporocidal nor tuberculocidal.
CHEMOTHERAPEUTIC AGENTS - THE ANTIBIOTICS
Most are derived from compounds which are biosynthesized by other microorganisms. Streptomyces and Penicillium are two organisms which have given us antibiotics. The famous immunologist Paul Ehrlich devoted much of his career looking for the "Magic Bullet" or the chemical compound with SELECTIVE TOXICITY. That is, such a compound would be toxic for the infecting microbe but not for the human host. This dream was achieved by Flemming and Florey with the discovery and mass production of penicillin.
Antibiotics achieve selective toxicity by exploiting the differences between eucaryotes and procaryotes. The most profound differences are at the cell wall and at the ribosome level. It follows then that most of our antibiotics either inhibit bacterial cell wall synthesis or inhibit the synthesis of bacterial proteins. For instance: ampicillin and penicillin interfere with cell wall synthesis; chloramphenicol, gentamicin, streptomycin and tetracycline interfere with protein synthesis. How does triple sulfa work?
Antibiotic resistance is a growing problem which has at its source the indiscriminate and inappropriate use of antibiotics. Some of the current "Super Bugs" include methicillin resistant Staphylococcus aureus (MRSA), vancomycin resistant enterococcus (VRE) and multidrug resistant Mycobacterium tuberculosis. It is important that physicians order sensitivities on bacteria isolated from infection sites so that appropriate antibiotics can be prescribed. Also, it is important that the infection be hit hard with high, optimum doses of the antibiotic and that tissue levels of the antibiotic remain high throughout therapy. Therapy must continue until all of the pathogens are dead - this means that the patient must continue to take the antibiotic even after the symptoms are gone.
What is the most effective method of controlling microbial growth?
Sterilization and decontamination through heat is the most effective way to control microbial growth. There are ways to selectively inhibit the growth of only certain microorganisms, like the use of antibiotics.
What are the methods of microbial control?
Microorganisms are controlled by means of physical agents and chemical agents. Physical agents include such methods of control as high or low temperature, desiccation, osmotic pressure, radiation, and filtration.
What are the 2 methods of controlling microbial growth?
Chemical control refers to the use of disinfectants, antiseptics, antibiotics, and chemotherapeutic antimicrobial chemicals. Sterilization is the process of destroying all living organisms and viruses. Disinfection is the elimination of microorganisms, but not necessarily endospores, from inanimate objects or surfaces.
What is a microbial control?
Control of microbial growth means to inhibit or prevent growth of microorganisms. This control is affected in two basic ways: (1) by killing microorganisms or (2) by inhibiting the growth of microorganisms.