Fundamentals of microbial cotrol

Fundamentals of
microbial control
By
Hasnain akmal

 Control: Control refers to the reduction in number or activity of microbes.
 Why we need to control microorgamisns:
1. To prevent transmission of disease and infection.
2. To prevent contamination.
3. To prevent spoilage and deterioration.
 Microorganism can be removed, inhibited or killed by various physical and chemical agents.

Control of microorganism
by physical agent

Important terms
 Disinfection: Reducing the number of pathogenic microorganisms to the point
where they no longer cause diseases. Usually involves the removal of vegetative
or non-endospore forming pathogens.
 Sterilization: Killing or removing all forms of microbial life (including
endospores) in a material or an object. Heating is the most commonly used
method of sterilization.
 Cidal: suffix meaning agent that kills.
 Static: suffix meaning agent that inhibit growth.

Physical agents: High temperature

Physical agent: High temperature

Physical agent
5.Radiation:
Three types of radiation kill microbes:
1. Ionizing Radiation: Gamma rays, X rays, electron beams, or higher
energy rays.
 Have short wavelengths (less than 1 nanometer).
 Dislodge electrons from atoms and form ions. Cause mutations in DNA and
produce peroxides.
 Used to sterilize pharmaceuticals and disposable medical supplies. Food
industry is interested in using ionizing radiation.
 Disadvantages: Penetrates human tissues. May cause genetic mutations in
humans.

Physical agent
2. Ultraviolet light (NonionizingRadiation):
 Wavelength is longer than 1 nanometer.
 Damages DNA by producing thymine dimers, which cause mutations.
 Used to disinfect operating rooms, nurseries, cafeterias.
 Disadvantages: Damages skin, eyes. Doesn’t penetrate paper, glass, and cloth.
3. Microwave Radiation:
 Wavelength ranges from 1 millimeter to 1 meter.
 Heat is absorbed by water molecules. May kill vegetative cells in moist foods.
 Bacterial endospores, which do not contain water, are not damaged by microwave radiation.
 Solid foods are unevenly penetrated by microwaves.
 Trichinosis outbreaks have been associated with pork cooked in microwaves.

Control of microorganism
by chemical agent

Ideal Antimicrobial Chemical Agent
1. Antimicrobial activity: the capacity of the substance to kill or inhibit microorganisms.
2. Solubility: the substance must be soluble in water or other solvents to the extent
necessary for effective use.
3. Stability: there should be minimum changes in the substance upon standing and should
not result in significant loss of germicidal action.
4. Nontoxicity to humans and other animals: the compound should be lethal to
microorganisms and non-injurious to humans.
5. Homogeneity: the preparation must be uniform in composition so that active ingredients
are present in each application. Pure chemicals are uniform but mixtures of materials may
lack uniformity.
6. Toxicity to microorganisms at room or body temperatures.

7. Non-contamination with extraneous organic material: some disinfectants have affinity
for proteins and organic material. When such disinfectants are used where there is organic
material besides microbial cells , a little disinfectant will be available for action against
microorganisms.
8. Capacity to penetrate: unless the substance can penetrate through surfaces, the
germicidal action is limited to the site of application only.
9. Non-corroding and non-staining: it should not rust nor stain or damage fabrics.
10. Deodorizing ability: Ideally, it should be either odorless or have a pleasant smell.
11. Detergent capacities: a disinfectant which is also a detergent (cleaning agent) gives
better action. The cleansing action increases the effectiveness of the disinfectant.
12. Availability: the compound must be available in large quantities at a reasonable price.

Important terms
 Germicide (microbicide): an agent that kills the growing forms but not necessarily the
resistant spore forms of germs. A germicide is almost the same as disinfectant but germicides
are commonly used for all kinds of germ(microbes) for any application.
 Bactericide: An agent that kills bacteria (bactericidal). Similarly the terms fungicide,
virucide and sporicide refer to agents that kill fungi, viruses and spores respectively.
 Bacteriostasis: a condition in which the growth of bacteria is prevented (bacteriostatic).
Similarly, fungistatic describes an agent that inhibit the growth of fungi. Agents that have the
common ability to inhibit growth of microorganisms are collectively called microbistatic
agents.
 Antimicrobial agent: one that interferes with the growth and metabolism of microbes. Some
antimicrobial agents are used to treat infections and they are called chemotherapeutic agents.

Types of chemical agent:
1. Phenol
2. Alcohols
3. Halogens
4. Heavy metals and their compounds
5. Dyes
6. Detergents
7. Quaternary ammonium compounds
8. Aldehydes
9. Gaseous agents.

1. Phenols and Phenolics:
 Phenol (carbolic acid) was first used by Lister as a disinfectant.
 Rarely used today because it is a skin irritant and has strong odor.
 Used in some throat sprays and lozenges. Acts as local anesthetic.
 Phenolic are chemical derivatives of phenol.
 Cresols: Derived from coal tar (Lysol).
 Biphenols (pHisoHex): Effective against gram-positive staphylococci and
streptococci. Used in nurseries. Excessive use in infants may cause neurological
damage.
 Destroy plasma membranes and denature proteins.
 Advantages: Stable, persist for long times after applied, and remain active in the
presence of organic compounds.

2. Alcohols:
 Mode of action: Alcohols are protein denaturants and this accounts for their antimicrobial
activity.
 Alcohols are also solvents for lipids and thus they may damage lipid complexes in the cell
membrane.
 They may also act as dehydrating agents.
 Alcohol is effective in reducing microbial flora on the skin.
 Examples: Methanol, ethanol.
3. Halogens:
 Mode of action: These act as oxidizing agents and oxidize proteins which accounts for the
antimicrobial action.
 Chlorine compounds: Used in disinfecting municiple water supplies (as sodium hypochlorite,
calcium hypochlorite, or chlorine gas)

 Sodium Hypochlorite (Chlorine Bleach) used at 10 - 20% dilution as benchtop
disinfectant .
 Halazone tablets (parasulfone dichloroamidobenzoic acid) used by campers to
disinfect water for drinking.
 Iodine compounds: Tincture of iodine (iodine solution in alcohol).
 Potassium iodide in aqueous solution.
 Iodophors: Iodine complexed to an organic carrier; e.g. Wescodyne, Betadyne.
 Used as antiseptics for cleansing skin surfaces and wounds.

4. Heavy Metals:
 Mode of action: they combine with cellular proteins and inactivate them.
 Oligodynamic action: Very tiny amounts are effective.
 Silver: 1% silver nitrate used to protect infants against gonorrheal eye infections until
recently.
 Mercury: Organic mercury compounds like merthiolate and mercurochrome are used to
disinfect skin wounds.
 Copper: Copper sulfate is used to kill algae in pools and fish tanks.
 Selenium: Kills fungi and their spores. Used for fungal infections. Also used in dandruff
shampoos.
 Zinc chloride: is used in mouthwashes. Zinc oxide is used as antifungal agent in paints.

5. Quaternary Ammonium Compounds (Quats):
 Widely used surface active agents.
 Cationic (positively charge) detergents.
 Effective against gram positive bacteria, less effective against gram-negative bacteria.
 Also destroy fungi, amoebas, and enveloped viruses.
 Zephiran, Cepacol, also found in our lab spray bottles.
 Pseudomonas strains that are resistant and can grow in presence of Quats are a big concern in
hospitals.
 Advantages: Strong antimicrobial action, colorless, odorless, tasteless, stable, and nontoxic.
 Disadvantages: Form foam. Organic matter interferes with effectiveness. Neutralized by
soaps and anionic detergents.

6. Dyes:
 There are two classes of dyes- triphenylmethane (crystal violet, brilliant green) dyes and
acridine (acriflavine, tryptoflavin) dyes.
 Triphenylmethane Dyes: Gram positive organisms are more susceptible to lower
concentrations of dyes than gram negative ones. Susceptibility to various dyes can be
used for identification of bacteria.
 Crystal violet can be used as a fungicide.
 Acridine dyes: they are used for the treatment of burns and wounds and bladder
irrigation.

7. Synthetic Detergents:
 These are surface-tension depressants which are used for cleaning surfaces.
 They are extensively used in laundry and dish-washing powders, shampoos and other
washing preparations.
 Some are highly bactericidal.
 There are three types of detergents: Anionic detergent (sodium lauryl sulphate), Cationic
detergent (cetylpyridinium chloride) and Nonionic detergents (these do not possess any
antimicrobial activity).
 Cationis detergents are more germicidal than anioinis compounds.

8. Aldehydes:
 Include some of the most effective antimicrobials.
 Inactivate proteins by forming covalent crosslinks with several functional groups.
 A. Formaldehyde gas: Excellent disinfectant.
 Commonly used as formalin, a 37% aqueous solution.
 Formalin was used extensively to preserve biological specimens and inactivate viruses and
bacteria in vaccines.
 Irritates mucous membranes, strong odour.
 Also used in mortuaries for embalming.
 B. Glutaraldehyde: Less irritating and more effective than formaldehyde.
 One of the few chemical disinfectants that is a sterilizing agent.

 A 2% solution of glutaraldehyde (Cidex) is:
 Bactericidal, tuberculocidal, and viricidal in 10 minutes.
 Sporicidal in 3 to 10 hours.
 Commonly used to disinfect hospital instruments.
 Also used in mortuaries for embalming.

9. Gaseous Agents:
 Certain medical devices that need to be available in sterile condition are damaged by heat,
e.g. plastic syringes, blood transfusion apparatus. Also some laboratory equipments like
plastic pipettes are also heat sensitive.
 Sterilization by means of gaseous agents is an effective method for such needs.
 The main agents used are- formaldehyde, ethylene oxide and β-propiolactone.
 Mode of action of ethylene oxide: it causes alkylation of enzymes and other proteins.
 Ethylene oxide is a unique and powerful sterilizing agent. It can pass through and sterilize
large packages of materials, bundles of cloth and certain plastics.
 β-propiolactone is sporicidal, fungicidal and virucidal. But it has low penetration power as
compared to ethylene oxide.

10. Antibiotics and Other Chemotherapeutic Agents:
 They destroy or prevent the activity of a parasite without injuring the cells of the host or
with only minor injury to cells.
 Be able to come in contact with the parasite by penetrating the cells and tissues of the host
in effective concentration.
 Leave unaltered the host’s natural mechanisms such as phagocytosis and the production of
antibodies.
 Examples: penilcillins, cephalosporins, bacitracin, vancomycin.

Fundamentals of microbial cotrol

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