Control of microorganisms (Bacteria) by physical and chemical agents

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Control of microorganism is a very important process.
Microorganisms can be controlled by physical as well as chemical methods.
Physical methods are Temperature, Radiation, Dessication, Surface tension and interfacial tension, etc.
Chemical method implies various chemicals to destroy microorganisms (MO now on.)

The bacterial growth curve

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Bacterial Growth curve

Bacteria, when transferred from one medium to another, shows characterstic growth curve.
It shows four distinct phase of growth.
Lag phase
Log or exponential phase
Stationary phase
Decline or death phase

Lag phase :- When the bacteria are transferred from one medium to another they do not show sudden growth as they may be depleted of energy. The nutrients may be different in two media. Organisms synthesize new enzymes to utilize new nutrients. Organisms increase in size but show no change in number.
Log phase :- In this phase bacteria divide exponentially. The graph shows curve which means not all the bacteria multiply simultaneously. After some time period the no. of division fall down.

Stationary phase :- In this phase bacterial population does not change with time - No. of bacteria born are same as no. of bacteria died.
Decline phase :- In this phase bacteria die exponentially. Bacterial population fall down as inverse of log phase. This phase come due to accumulation of waste products and depletion of nutrients.

Bacteriological media

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Bacteriological media contain nutritional requirements for bacteria.
It contains peptones, meat extract and yeast extract.
Agar is used as a solidifying agent and not as nutritive.
Medium may be liquid ( Nutrient broth medium) or solid (Nutrient agar medium) or even semisolid.
Special kind of media are also there like Selective media, Differential media, Maintenance media, Enumeration media, Characterization media, Assey media. etc.

Nutritional requirements of Bacteria

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Like the all other form of life, bacteria also have some nutritional requirements.
They need energy source - It may be Chemical compound,
It may be Radient energy (Light).
They need carbon source - It may be Organic compound,
It may be Carbon dioxide.
They need nitrogen source - It may be Atmospheric nitrogen,
It may be Inorganic nitrogen compounds - NItrates, Nitrites, ammonium salts,
It may be Organic nitrogen compounds - Amino acids.
They need electorn donor - It may be inorganic compound,
It may be organic compound.
They need Oxygen source - Water, Various nutrients or molecular oxygen.
They need phosphorus source - Phosphate, Nucleotides, Nucleic acids, Phospholipids, Teichoic acids and other compounds.
They need sulfur source - Inorganic/organic sulfur compounds or even elemental sulfur.
They also require metal ions for their normal growth and metabolism, Vitamins which may act as coenzymes for various enzymes.
And they also require WATER for the growth.
There are some physical requirements for microorganisms. Like temperature, pH, Gaseous requirements, ect.

Serial dilution method

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Serial dilution method :-
The original culture suspension is diluted more one time - Serial dilution, in tubes or in appropriate medium. Because of the reduction in the no. of the bacteria due to dilution isolation is obtained. When greatly diluted the speciman contains only few organisms of only one species. The culture obtained is conserved by the spread plate or streak plate method.

Pour plate method

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Pour plate method :-
The speciman is serially diuted - More than one times as the density of the original speciman is unknown. The agar medium is maintained at the temperature of 45 C and the diluted suspension is inoculated and mixed well. The liquid state permits the through out distribution within the medium. The inoculated medium is poured in the sterile empty petriplate and allowed to solidify & then incubated. Next day the surface colonies as well as submerged colonies are observed.

Spread plate method

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Spread plate method :-
A serially diluted inoculum is transferred over the surface of the solid medium and then spreaded uniformly with a sterile bent glass spreader. But this technique does not ensure a nice isolation when dense suspension is used.