Physical agents to control microorganisms
Some commonly used physical agents in controlling microbial growth are;
- Heat or temperature
- low temperature
- Lyophilization (freeze drying)
- Osmotic pressure
- Different organisms shows different degree of resistance or susceptibility to heat or temperature.
- Some organisms are more resistant such as bacterial endospores while vegetative cell are less resistant and are killed at lower temperature.
- two types of heat are used for sterilization or killing of microbes-a) moist heat and b) dry heat
- moist heat is one of the most effective method of sterilization.
- moist heat kills the microorganisms by coagulating their enzymes and protein. Also the water vapour present in moist heat facilitates the penetration of heat in to the materials.
- moist heat is more effective in killing microorganism than dry heat
Some of the techniques for sterilization which utilize heat are-
i. Steam under pressure: eg. Autoclave
ii. Boiling: it only kills vegetative cells but not endospores
iii. Pasteurization: it only kills harmful bacteria present in milk and beverages.
iv. Fractional sterilization or tyndallization: it is effective even for bacterial endospore
v) Incineration: it kills microorganism by burning
vi) Hot air sterilization: it is dry heat method for sterilization of materials which may be damaged by moist heat such as powder, oils, pipette etc
2. low temperature:
- low temperature do not bring sterilization, however at lower temperature metabolism of bacteria decreases. and if the temperature is sufficiently lower then growth and metabolism ceases.
- low temperature treatment usually bring stasis rather than sterilization.
- Mainly electromagnetic radiation are used for sterilization.
- Two types of radiations are used- Non ionizing radiation and ionizing radiation
i. Non ionizing radiation:
- Ultra violet light (UV rays) includes all the light having wave length between 1500-3900 A°.
- however wave length around 2600A° has highest bactericidal effects.
- UV light is non ionic radiation and it forms thymine dimer which causes mutation and kills microorganisms.
- Examples: germicidal lamps, UV lamp etc are available which emits light ray in the range of 2600-2700A° .
ii. Ionizing radiation:
- Ionizing radiation are high energy radiation and causes ionization of cellular materials and water and kills the microorganisms.
- Ionization of water and cellular materials generates large amount of toxic oxygen metabolites such as hydroxyl free radicals, super-oxide ion, hydrogen peroxide. these metabolites are powerful oxidizing agents, therefor oxidizes the cellular components of bacteria and kills them.
- sterilization by ionizing radiation is known as cold sterilization.
- Examples: x-rays, gamma rays, cathode rays etc
- It is the method of removing available water to microorganisms. desiccation causes cessation of metabolic activities and ultimately kill the organisms.
- Some organisms such as Neisseria spp are killed rapidly by drying while Mycobacterium tuberculosis is highly resistant to desiccation.
5. Lyophilization ( freeze drying):
- lyophilization is freexe drying of microorganism.
- It is uused for preservation of organisms.
- In lyophilization, the suspension of bacteria is frozen upto -60 to -80° C. freezing preserves the organisms for many years.
6. Osmotic pressure:
- Cell membrane of bacteria is semi permeable. water moves from the solution of lower solute concentration to solution of higher solute concentration across cell membrane of bacteria.
- Approximate solute concentration of bacterial cytoplasm is 0.85% NaCl. which gives normal osmotic pressure.
- when the bacteria is placed in hyper tonic solution, it causes plasmolysis or cell shrinking, similarly in hypotonic solution, bacteria undergoes plasmotysis or turgid state.
- This plasmolysis and plasmotysis kills bacteria because it causes change in osmotic pressure.
- Cell wall lacking microorganism are rapidly killed. eg. Mycoplasma, parasites
- Filtration is used to sterilize liquid materials such as culture broth, body fluidds, vaccines, antibiotics, etc.
- Different types of filters are available which are made up of different materials such as Seitz filter is made up of asbesters pad, Bafeld filter is made up of diatomaceous earth, Chamberland filter is made up of porcelain.
- Filters are selective and the pore size of filtering materials is very minute such that bacterial cell, endospore cannot pass through it.
- Examples: membrane filter is made up of cellulose ester having pore size of 0.01-10 µm diameter. the thickess of membrane is 150 µm, membrane filters are used extensively to sterilize heat labile fluids such as body fluids, enzymes, antibiotics etc.
Physical agents to control microorganisms