CONTROL OF MICROORGANISMS..

A) PHYSICAL METHOD FOR CONTROLLING MOS: 

1) HEAT:  i) High Temperature:  Growth of microorganisms depends upon metabolic activities which depends upon different chemical reactions. These reactions are influence by temperature. Each organism has their cardinal temperature for their growth ie minimum,optimum, & maximum temperature. Heat is employed either at moist heat or dry heat.
Moist Heat: Moist  heat kills mos by denaturation of proteins. It is more effective than dry heat because moist heat penetrates materials more rapidly than dry heat. Therefore, moist heat requires low temperature & short exposure because water molecules conducts heat better than air. Eg Clostridium botulinum are killed in 4- 20 minutes by moist heat at  120 c whereas these spores are killed in 2 hours by dry heat at same temperature.
  Moist heat can be used or classified as;
  • At temperature above 100c  
  • At temperature below 100c 
  • At temperature at 100c 
Moist Heat Above 100C: Using moist heat above 100c is known as steam under pressure which is most practicable & dependable agent for sterilization. The temperature in steam under pressure is higher than that of boiling point,& it has advantages of rapid heating, penetration, & moisture which favors for denaturation of protein.When the pressure of steam is increased the temperature of gas increases proportionally. So, it is the temperature that kills mos .

                                   The lab. apparatus designed to use steam under pressure is called Autoclave is shown in above figure. It is used to sterilize discarded cultures, solution media, media, contaminated materials. Generally it is operated at 15 lbs pressure at 121.5 c for 15 minutes.   

Moist Heat at 100C : It is simply boiling in water. However, boiling in water isn't considered as sterilization rather it is disinfection .Though boiling water kills vegetative cells within few minutes but most bacterial spores, fungal spores, protozoan cyst, some viruses withstand this condition for many hours.   



TYNDALLIZATION( FRACTIONAL STERILIZATION): It is another method that uses moist heat at 100c for three successive days with incubation period in between. It is used for certain microbial media, chemicals, & biological materials which cannot be heated above 100c without being damage.
        
Fig:TYNDALLIZATION 

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