Autoclave and Control of Microbial Growth     

“Control of microbial growth”, is referred to as inhibition or prevention of the growth of microorganisms. This control can be achieved:

(1)  By killing microorganisms or
(2)  By inhibiting the growth (metabolism) of microorganisms.

Control of growth usually involves the use of physical or chemical agents which either kill or prevent the growth of microorganisms.

Agent that kills the microbial cells is called Cidal agent and the agent that inhibits the growth or the metabolism of cells (without killing them) is referred to as Static agent i.e Stasis of Metabolism. Thus, the term Bactericidal refers to killing bacteria, and Bacteriostatic refers to inhibiting the growth by inhibiting the metabolism of bacterial cells. A bactericide kills bacteria; a fungicide kills fungi, and so on.

Sterilization refers to the complete destruction or elimination of all viable organisms in a substance to be sterilized. There is no degree of sterilization: an object or substance is either sterilized or not. Sterilization procedures involve the use of heat, radiation or chemicals, or physical removal of cells.

Heat is one of the physical methods for the control of microbial growth.

•  Moist heat sterilization
•  Dry heat sterilization

The control of microbial growth is necessary in many practical situations, and significant advances in agriculture, medicine, and food science have been made through study of this area of microbiology.

Autoclave (Steam under pressure)    

An autoclave is a device that uses saturated steam to sterilize equipment and other objects by subjecting them to high pressure saturated steam at 121 °C and 15 pounds of pressure per square inch depending  for around 15–20 minutes on the size of the load and the contents.. This means that all bacteria, viruses, fungi, and spores are destroyed.

Autoclaves work by allowing steam to enter and maintaining extremely high pressure for at least 15 minutes. Because damp heat is used, heat-labile products (such as some plastics) cannot be sterilized or they will melt. Autoclaves are filled with water and work by creating steam within an enclosed environment, which builds up pressure. The air within the autoclave is gradually replaced with steam, which can reach higher temperatures than the air.

High-temperature steam can surround and infiltrate the items, even reaching within the crevices in stainless steel instruments. This process kills all bacteria, viruses and bacterial spores. The autoclave comes in several types. One of the simplest autoclaves looks a great deal like a pressure cooker. It is a large pot with a gauge on top and bolts that fasten the top to the pot. The idea behind this is that water inside a pressurized container can be heated above the boiling point. It will only reach 212°F (100° C) in an open container. However, in a pressurized autoclave, the water will reach much higher temperatures.

Autoclave is commonly used for Moist Heat Sterilization Moist heat is thought to kill microorganisms by causing denaturation or the coagulation of essential proteins. Autoclaving 121ºC/15 psi for 15 minutes exceeds the thermal death time for most organisms except some extraordinary spore formers .The time required to kill a known population of microorganisms in a specific suspension at a particular temperature is referred to as thermal death time (TDT). Temperature is inversely proportional to TDT. Processes conducted under high temperatures for short periods of time are preferred over lower temperatures for longer times.

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