Agricultural microbiology course Practical lessons of Agricultural microbiology course

By Dr. Rasha Mohamed El-Meihy lecturer of Agric. Microbiology rashaelmehy83@gmail.com http://www.bu.edu.eg/staff/elmehyrasha6

What the meaning of microorganisms What the meaning of microorganisms? What’s the importance of microorganisms? What’s the groups of microorganisms?

Lesson (1)

Sterilization is the killing or removal of all microorganisms, including bacterial spores which are highly resistant. Sterile meaning completely free of microbes. Disinfection is the killing of many, but not all microorganisms. It is a reduction of number of contaminating organisms to a level that cannot cause infection, i.e. pathogens must be killed. Some organisms and bacterial spores may survive. Disinfectants are chemicals that are used for disinfection. Disinfectants should be used only on inanimate objects. Antiseptics are moderate forms of disinfectants that are used externally on living tissues to kill microorganisms, e.g. on the surface of skin and mucous membranes.

Sterilization

Methods of sterilization and disinfection Physical heat dry heat flaming hot air oven incineration infera red moist heat below 100°c at 100°c above 100°c radiation non-ionizing ionizing Chemical liquid alchohols aldehydes phenolics halogens heavy metals surface active agents dyes gaseous formaldehyde ethylene oxide plasma Mechanical filtration membrane sintered glass asbestos

Physical methods

Heat Types of heat sterilization Sterilization by dry heat Is the most effective and rapid method of sterilization and disinfection Excessive (extreme) heat acts by coagulation of cell proteins. Less heat cause defect of metabolic reactions (Imbalance in the biological processes within the cell). Types of heat sterilization Sterilization by dry heat Sterilization by moist heat

Sterilization by Dry Heat

Temperature is not high to cause sterilization. (1) Flaming The article is passed through flame (Bunsen burner), allowing it to become red hot, e.g. inoculating loops. Temperature is not high to cause sterilization. (2) Sterilization by Hot Air Hot Air Oven (Sterilizer): All glasses are sterilized in hot air oven. For sterilization, two temperature degrees were used: 180°C for 30 min. in case of glasses without cotton caps. 160°C for 60 min. in case of glasses with cotton caps. Because cotton carbonized at a temperature higher than 160°C. Spores are killed at these temperature degrees, it leads to sterilization.

Sterilization by Moist Heat

Sterilization at 100°C Steaming Steam (100°C) is more effective than dry heat at the same temperature as: Bacteria are more susceptible to moist heat. Steam has more penetrating power. Steam has more sterilizing power as more heat is given up during condensation.

It works at 100°C under normal atmospheric pressure Steam Sterilizer It works at 100°C under normal atmospheric pressure i.e. without extra pressure. It is ideally suitable for sterilizing media which may be damaged at a temperature higher than 100°C like those containing sugar, milk, and gelatin.

That is 'steaming' is done for 3 successive days. Sterilization in steam sterilizer is done by two methods: Single Exposure for 1.5 hours. It leads to disinfection. Tyndallization (Fractional Sterilization). Steaming at 100°C is done in steam sterilizer for 20 min. followed by incubation at room temperature overnight. This procedure is repeated for another 2 successive days. That is 'steaming' is done for 3 successive days. Spores, if any, germinate to vegetative bacteria during incubation and are destroyed during steaming on second and third day. It leads to sterilization.

Sterilization above 100°C (Autoclaving) In this method sterilization is done by steam under pressure. Principle: When the autoclave is closed and made air-tight, and water starts boiling, the inside pressures increases and now the water boils above 100°C. At 1.5 atmospheric pressure, 121°C temperatures is obtained. This is kept for 15 minutes for sterilization to kill spores. It works like a pressure cooker.

Chemical Methods

Ethyl alcohol The effectivity of ethanol as e.g. disinfectant or antiseptic agent depends on the concentration of ethanol-water-mixture. An ethanol percentage of 50-70% are the most effective percentages depends on two mechanisms: Destroys the cell wall/membrane of bacteria by denaturing their proteins and dissolving their lipids (effective against most bacteria, fungi and some viruses; ineffective against bacterial spores). Therefore, the ethanol has to pass the bacterial membrane/wall to get into the bacteria. Another mechanism is the high osmotic pressure of ethanol/water-mixtures; and the 70% has the highest one.

If you use 100% ethanol instead, Pure Alcohol is water free so it works to pull out all the water from inside the cell and become a closed cell, but alive.

If you use 100% ethanol instead, the bacteria get 'sealed' and they will survive, pure alcohol coagulates protein in contact. Suppose the pure alcohol is poured over a single celled organism: The alcohol will go through the cell wall of the organism in all direction, coagulating the protein just inside the cell wall. The ring of the coagulated protein would then stop the alcohol from penetrating farther from the cell, and no more coagulation would take place. At this time the cell would become inactive but not dead. Under the favorable conditions the cell would then begin to function.

(2) Moist heat prefer for sterilization than dry heat? Why (1) Using two temperature degrees in hot air oven? (2) Moist heat prefer for sterilization than dry heat? (3) Fractional Sterilization is done for 3 successive days? (4) Ethanol at 50-70% is more effective than pure alcohol ?

I hope you Committed the instructions well Than you I hope you Committed the instructions well