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Lesson Fifteen: Methods of Controlling Pathogenic Micro-organisms
Teaching Strategy: - Lecture method, group activity and presentations
Introducing the topic by posing focus questions
Let the students suggest what they think about the control of micro-organisms.
The students should be organized into small groups and list down the methods of controlling micro-organisms. The teacher is expected to supervise each group while the group work is in progress. After listing the methods of control of infectious agents, each group is required to explain how each method is employed to control microbial infections.
Sterilization: is the complete destruction of micro organisms from medical and laboratory rooms.
Sterilization involves:
Sterilization is an extreme level of cleanliness that is usually not required outside of a medical, industrial, or laboratory environment.
Basically, there are two types of methods for the control of infectious agents. These are physical and chemical methods.
Physical Methods: The physical agents used to kill microorganisms may involve heat, radiation, freezing and drying.
Chemical Methods: The chemical agents involve chemicals to kill micro-organisms.
The chemical method of the controlling of microbial growth is generally involving highly toxic agents. Therefore, they should be used in low concentration, but these are generally not suitable for medical use.
Disinfection is the use of a chemical agent (a disinfectant) to kill microbes found on or in inanimate objects. This reduces the numbers of pathogenic organisms. Most disinfectants are not safe for use on human skin.
An antiseptic is a chemical agent that can be used safely on tissue surfaces (skin usually) to kill microbes or inhibit their growth, but cannot be safely taken internally. Antiseptics include many creams and lotions, some of them with a phenol compound that might be safe on the skin but toxic in the body. Antiseptics are generally less toxic than disinfectants because they do not cause too much damage to the host living tissue. Examples of antiseptics include iodine, 70% ethanol and 3% hydrogen peroxide, and the term is also used for the process of washing the hands with soap and water.
A sanitizer is a chemical agent usually used on food handling equipment to reduce bacterial population.
Heat- is one method of sterilization. There are dry and moist heat methods.
Dry heat - is used to sterilize surfaces, and materials which are not likely to break down on high heating and which do not contain any liquids. For example, glasses, Petri dishes, culture vessels, and metal surgical instruments can be sterilized using dry heat method. Dry heat requires higher temperatures typically 160-170 0C and longer period of heating for several hours, depending on the distribution and the load of materials in the oven.
Moist heat (steam): The classic autoclave is the prime example here, which is a sophisticated version of the pressure cooker used in home canning and bottling of foods. Moist heat penetrates more quickly than dry heat, and is used to sterilize culture solutions and agar preparations, and to sterilize surgical instruments etc. A prime reason for pressurized steam heat is that it is needed to kill bacterial endospores, which can withstand boiling. Typically a pressure of 15 psi (pounds per square inch) is needed to create steam at a high enough temperature (1210C) to kill endospores.
Pasteurization is NOT sterilization. Solutions (milk is the best example) are heated at much gentler temperatures than in an autoclave for shorter periods of time. This reduces the microbial load and kills many pathogens but does not kill all bacterial pathogens and does not kill endospores. Pasteurization was originally developed by Louis Pasteur to improve the shelf life of wine (he was under contract to a large group of French winemakers), but it does also make milk products safer to drink.
Other methods of controlling micro-organisms include freezing, freeze drying and salting.
Freezing- keeping the medium under lower temperature retards the growth of microorganisms by slowing their metabolism, but it does not always kill them. Some bacteria (like Listeria) and fungi may resist freezing temperatures. Refrigeration at 50C retards the growth of many bacteria and fungi, freezing at - 10 0C to -20 0C (typical home freezer) is also an effective but not perfect means to retard microbial growth.
Drying- preserves foods because microbes need water to grow and cannot get it on dried foods. Drying often just prevents growth of microorganisms, and does not always kill them. Freeze drying (lyophilization) is a very effective method of retarding the growth of microbes.
Give a brief summary of the lesson (the major methods to be employed in controlling disease casing micro organisms).
Assessment Questions