Principle Microbiology

The simple filtration apparatus is illustrated which consists of a support covered with a porous filter cloth. A filter cake gradually builds up as filtrate passes through the filter cloth. As the filter cake increases in thickness the resistance to flow will gradually increase. Thus, if the pressure applied to the surface of slurry is kept constant the rate of flow will gradually diminish. Alternatively, if the flow rate is to be kept constant the pressure will gradually have to be increased. The flow rate may also be reduced by blocking of holes in the filter cloth and closure of voids between particles, if the particles are soft and compressible. When particles are compressible it may not feasible to apply increased pressure.

Low temperature : -
The low temperature method is not used as the method of sterilization as it does not assure sterilization. Temperature below optimum depress the rate of metabolism and if the temperature is sufficiently low the growth and metabolism cease.
Low temperature can be used for preservation of culture.
Agar-slant culture of bacteria, yeasts and molds are stored for long period of time at 4 to 7 degree C.
Many bacteria and viruses are stored in deep-freeze unit at temperature of -20 to -70 degree C.
Many viruses and microorganisms, and stock of mammalian tissues are stored at -196 degree C in liquid nitrogen.

Hot air oven : -
This kind of dry heat sterilization is recomended when it is undesirable that steam make contact with the material to be sterilized. This is true for certain glasswares - Petriplates, Pipettes as well as for substances like oil, powder, etc. The apparatus employed is an electric/gas oven. Even the kitchen oven can be used. For laboratory glasswares 2 hours exposure to a temperature 160 degree C if enough for sterilization.

Incineration : -
Killing microorganisms by burning is one of the nice way to control them. It is the routine practice in microbiology lab when a transfer needle / loop is placed into the flame of the Bunsen burner.
A note of caution should be added here - When the transfer needle / loop is sterilized, care should be taken to prevent the droplets flying off from needle / loop, as they may contain viable microorganisms which may be pathogenic.
The danger can be avoided by using electric heat coil with a tube surrounding it into which transfer needle / loop can be inserted.
This method is used for carcasses, infected laboratory animals and other infected material to be disposed off. Care should be taken that exhaust fumes do not contain viable microorganisms into atmosphere.

Boiling water : -
Boiling water destroyes all the vegetative cells. But this method does not assure complete sterility of contaminated material or the object because the resistant bacterial spores can withstand this condition for many hours.


Pasteurization : -
Milk and some other beverages are subjected to a controlled heat treatment called pasteurization. In which even all vegetative cells are not killed. But only some of them which may be pathogenic are killed. The pasteurized milk in not the sterilized milk.

Fractional sterilization : -
Some mocrobiological materials can not be heated above 100 degree C. But if still they can withstand the temperature of free flowing steam (i.e. 100 degree C) they can be sterilized by fractional sterilization / Tyndalization. In this method the material is heated with steam of 100 degree C on three successive days with incubation period in between. The vegetative cell will die on the 1st exposure with heat and the spores will germinate in incubation period. This germinated spores can be killed on second exposure with heat. If the spore will germinate it will become vegetative cell which can be easily destroyed, but if it doed not germinate sterilization will not be obtained. After three exposures sterilization is obtained. For this method one can use steam arnold. Even an autoclave with free flowing steam can be used for this purpose.