1.  Alcohol
2. Phenol and phenolic compounds
3. Halogen compounds
4. Heavy metals and their compounds
5. Aldehydes
6. Gaseous agents
7. Detergents
8. Antibiotics

1. Alcohol:

• Alcohols are antimicrobial agents. Germicidal action of alcohol increases with increase in molecular weight of alcohol.
• Ethanol is the most commonly used alcohol for controlling microorganisms.
• Ethanol between concentration of 50-90% are effective against vegetative cell. for practical purposes 70% ethanol is used.
• Alcohol causes death of organism by denaturing the cellular proteins.
• Alcohol is a lipid solvent that damages the lipid bilayer of cell membrane and cell wall. It is also a dehydrating agent and causes loss of water from cell.

Uses:

• Alcohol is commonly used as sanitizer on skin, disinfectant to clinical instruments, thermometers and surgical instruments.
• Concentration above 60% is effective in killing viruses.

Mode of action:

• When alcohol is used as disinfectant, it solubilizes the lipid bilayer of cell wall and membrane and creates pores. The remaining alcohol enters into the cytoplasm through the pore and denature the cellular proteins killing the bacteria.
• 70% alcohol is more effective than absolute (100%) alcohol because absolute alcohol only brings bacteriostasis.
• With increase in concentration of alcohol, both denaturing and lipid solubilizing power and dehydration power increases and are counter to each other. Absolute alcohol causes extreme dehydration resulting in shrinking of cell, hence further alcohol cannot enter the cell. Therefore it only brings bacteriostasis.
• However, 70% alcohol is very effective in dehydration and denaturation, almost equilibrium manner causing bacteriocidal effects.

2. Phenol and phenolic compounds

• Phenol have wide spectrum of antimicrobial action.
• Vegetative cell are more and rapidly killed by concentrated aqueous solution of phenol whereas bacterial spore are resistant.
• Usually 2-5% aqueous solution of phenol is used as disinfectant.
• Phenol has limited application because it is absorbed by skin and mucus membrane and causes toxicity.

Some of the derivatives of phenolic compound and their application are:

i) Cresol (methyl phenol): it is used in lysol in solution form to sterilize glasswares and to mop the floor of hospital rooms.

ii) Chloroxylenol (dimethyl phenol): it is an active ingredient of dettol and commonly used as antiseptic.

iii) Chlorohedidane (Hibitane): it is one of the component of Savlon, which is used as antiseptic in burns, wounds and preoperative antisepsis of skin.

iv) Hexachlorophane: it is insoluble in water. It is used in soap.

Mode of action:

• Phenol and phenolic compounds kills the microorganisms by varities of effects such as disruption of cell, precipitation of cellular protein, inactivation of enzymes and leakage of cellular materials.

3. Halogen compounds:

i. Iodine:

• Iodine is used in many forms such as aqueous solution, tincture of iodine and iodophor.
• Aqueous iodine and tincture of iodine have some side effects such as staining and irritation. So, now a days Iodophore is as replacement because it has less side effects.

Uses:

• Iodine is effective against all kind of bacteria. it also possess sporicidal activity.
• Iodine is highly fungicidal and to some extent virucidal.
• Iodophore are widely used for antisepsis of skin, mucus membrane and wound.
• Iodine preparation can also be used for other purposes such as disinfection of water, air and sanitization of food utensils.

Mode of action:

• Iodine is powerful oxidizing agents and irreversibly oxidises the cellular materials.
• Iodine also brings halogenation of tyrosine residue of protein and enzymes and inactivates it.

ii. Chlorine:

• Chlorine in the form such as Hypochlorite and chloramine is used as disinfectant. Free gaseous chlorine is difficult to handle, as it is corrosive and toxic.
• Calcium hypochlorite and sodium hypochlorite are commonly used.
• Aqueous solution of sodium hypochlorite (5.25%) is called house hold bleach.
• Chloramine is more stable than hypochlorite, so it is more effective germicidal than hypochlorites.

uses:

• Cholrine is one of the commonly used water disinfectant.
• Calcium hypochlorite is used as sanitizer for cooking utensils.
• 1% bleach is used for personal hygiene eg. bathing water
• Higher concentration (5-12%) bleach is used in swimming pool, house hold purposes.

Mode of action:

• when hypochlorite or chloramine is added in water, free chlorine is releases which form hypochlorous acid (HClO).
• Hypochlorous acid decomposes to release nascent oxygen which is powerful oxidizing agent and kills the microorganisms by oxidizing the cellular components.
• chlorine and chlorine compounds also inactivates the proteins and enzymes by direct chlorination.

4. Heavy metals and their compounds:

• Most of the heavy metals have antimicrobial action.
• Most effective and commonly use are Mercucry (Hg), Silver (Ag) and Cupper (Cu).

Mode of action:

• Heavy metals and their compounds combines directly with cellular proteins and enzymes and inactivates them.
• High concentration of heavy metals salts also coagulates and precipitates the cellular proteins and kills the microorganisms.

Some commonly used metal compounds are:

i) HgCl, HgCl2: used in ointments as antiseptic.

ii) AgNO3: it is bacteriostatic as well as bactericidal. it is used in eye-drops to prevent Ophthalmia neonatarum in children.

iii) Cupper suphate: it is widely used against algae and mold in swimming pool.

5. Aldehydes:

• Formaldehyde and Gluteraldehyde are commonly used aldehydes. Both are highly microbicidal including sporicidal.

i. Formaldehyde:

• Formaldehyde is stable only in higher concentration and higher temperature. At room temperature, it polymerized to form para-formaldehyde.
• Formaldehyde is used in two form- gaseous formaldehyde and formalin (40%solution of formaldehyde).

Uses:

• Formaldehyde vapour either from formalin or paraformaldehdye is used for disinfection and sterilization of closed room, such as Operation theater.
• Formaldehyde vapour is also used to disinfect woolen blanket, wools and footwares of fungal infected person.
• Formalin is used for preservation of biological specimens.

ii. Gluteraldehyde:

• Gluteraldehye is used in 2% solution.
• like formaldehyde it is effective against bacteria, fungi, spores and viruses.
• Gluterladehyde is used to sterilize Urological instruments and respiratory therapy instruments.

6. Gaseous agents:

• Ethylene oxide, β-propiolactone and formaldehyde are commonly used gaseous sterilizing agents.

i. Ethylene oxide:

• It is gaseous above 10.8°C.
• Ethylene oxide have high antimicrobial activity, it kills even endospores.
• It is used for sterilization of heat sensitive materials such as spices, oils, plastics etc.
• Ethylene oxide is used in formulation with CO2 as Freon (CClFe).

ii. β-Propiolactone:

• It is gas above 15.5°C.
• Penetration power of β-propiolactone is less than ethylene oxide but it is more active in killing microorganisms.
• Due to its carcinogenic effects, it is not commonly used.

7. Detergents:

• Detergents are used primarily for leaning purposes but it has also antimicrobial properties.
• There are three types of detergents- Cationic detergent, anionic detergent and non-ionic detergent.
• Cationic detergents is more significant germicidal agent than other two.
• For example: Quaternary ammonium compound is a cationic detergent having germicidal action. It is more effective against Gram positive bacteria.
• Detergents are used as disinfectants, sanitizers and antiseptic. They are also used to disinfect hospital floor.

Mode of action:

• Detergents kills the microorganisms by denaturing proteins and enzymes and interfering with glycolysis
• Detergents also damages cell wall and cell membrane.

8. Antibiotics:

• Antibiotics are secondary metabolites produced by certain microorganisms which inhibits the growth of other microorganisms.
• Different groups of antibiotics have different mode of actions

Mode of action:

• Inhibits cell wall peptidoglycan synthesis: eg, penicillin, cephalosporin
• Inhibit cell membrane biosynthesis: eg, Polymyxin, Polyenes
• Inhibits protein synthesis: eg, Tetracycline, Chloramphenical
• Reacts with nucleic acids: Rifampin, Quinolone
• Inhibits folic acid synthesis: eg, Sulfonamide, trimethoprim

Chemical agents to control microorganisms

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Some commonly used physical agents in controlling microbial growth are;

1. Heat or temperature
2. low temperature
3. Radiation
4. Desiccation
5. Lyophilization (freeze drying)
6. Osmotic pressure
7. Filtration

 1. Heat:

• 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.

3. Radiation:

• 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

4. Desiccation:

• 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

7. Filtration:

• 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

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