Antimicrobial Agents

Introduction

Antimicrobial is an agent which kills microorganisms (microbicide) or stops their growth (bacteriostatic agent). Use of antimicrobials is a common practice for at least 2000 years. Ancient Egyptians and ancient Greeks used specific molds and plant extracts to treat infection. Antimicrobial is a broad terminology describing activity against microbes. There are various specific terminologies to describe exact mechanism of action. In this article we will see antimicrobials, mechanism, classification and some antimicrobial agents.

Mechanism of antimicrobials

Various chemical agents are used as antimicrobials for their systemic and topical applications. Most inorganic compounds posses some degree of germicidal action yet few find use in actual medical practice. Compounds acting as effective antimicrobial should at least have some specific property to exhibit action. Inorganic antimicrobials generally exhibit antimicrobial action by any one of the three mechanisms, oxidation, halogenation or protein binding (precipitation).

• Oxidation: They act on proteins containing sulphadryl group and oxidizes free sulphadryl to disulphide bridge and inactivate its function. Compounds acting by this mechanism belong to class of peroxide peroxiacids, oxygen liberating like permanganate and certain oxo-halogen anions.
• Halogenation: Agents falling under this category act on peptide linkage and alter its potential and property. The destruction of specific function of protein results in death of microorganisms. Compounds which liberate chlorine or hypochlorite or iodine act by this mechanism.
• Protein precipitation: many metals, when are in cationic form, exhibit protein binding or protein precipitation. The nature of interactionwith protein occurs through polar group of protein which acts as ligands and metal ion as Lewis’s acid. The action is general and non-specific. Germicidal action results when the concentration of ion is such that reaction is restricted largely to the parasite cell. Depending upon the concentration used and the extent of reaction, astringent, irritant, corrosive or even caustic action is observed on the host.

Classification of antimicrobials

Antimicrobials is a broad terminology describing activity against microbes. Specific terminology describes the class of antimicrobial agent. This classification is given below.

• Antiseptics: These are the substances which kill or prevent the growth of microorganisms. This term is specific for preparations intended to be applied to living tissues.
• Disinfectants: These are the agents which prevents infection by the destruction of pathogenic microorganisms. It is generally used with reference to the substances applied to inanimate objects.
• Germicides: These are the agents which kills microorganisms. This is a broad category. More specific terminology like bactericide (against bacteria), fungicide (against fungi), virucide (against virus), etc. denotes the exact action of the agent.
• Bacteriostatic: These are the agents which primarily functions by inhibiting the growth of bacteria. Thus, bacteriostatic drugs or agents do not kill but arrest (inhibit) the growth of bacteria.

Ideal properties of antimicrobial agents

• It should have antiseptic or germicide activity and not bacteriostatic activity. If the microorganisms are not killed, they may resume growth and cause infections.
• It should have rapid onset and sustained activity. This will reduce incidence of drug resistance.
• It should have good therapeutic index indicating usefulness in the concentration employed.
• It should not produce local cellular damage or should not interfere with body defenses.
• It should show no systemic toxicity from topical application.
• In general, it should have broad spectrum activity against bacteria, fungi, protozoa, virus, etc.
• The topical antimicrobial agent should posses favorable lipid-water distribution coefficient so that its effectiveness is best.

Some inorganic antimicrobial agents

Potassium permanganate

Molecular formula: KMnO4

Molecular weight: 158.03

Preparation

Commercially it is prepared by mixing solutions of KOH and powered manganese oxide. The mixture is boiled, evaporated and the residue is heated in iron pans until it has acquired a pasty consistency.

6KOH + 3MnO2 + 6KClO3 — > 3K2MnO9 + 6 KCl + 3H2O

The green colored potassium manganate formed is boiled with large quantity of water and current of chlorine, CO2 and ozonized air is passed into the liquid until it is converted into permanganate.

6K2MnO4 + 3Cl2 — > 6KMnO4 + 6KCl

The crystals are centrifuged and then dried.

Properties

• Dark purple colour monoclinic prisms
• Opaque with blue metallic luster
• Odourless
• Aqueous solution has sweetish astringent taste
• Decomposes at high temperature
• Powerful oxidizing agent
• Oxidizes alcohol to aldehyde, sulphide to free Sulphur, nitrites to nitrate and iodide to free iodine under acidic conditions

Uses

• Antiseptic and antibacterial
• Solution is used to clean wounds and ulcers
• Antidote for barbiturates, chloral hydrate and alkaloidal poisoning

Assay

Assay of potassium permanganate is based upon oxidation reaction.

0.8 gm of KMnO4 is dissolved in water and volume made to 250 ml. 5 ml of sulphuric acid is added into the 25 ml of 0.1N oxalic acid, contents heated to 70⁰C and titrated with prepared solution of KMnO4.

5(COOH)2 + 2KMnO4 + 4H2SO4 — > 2KSO4 + 2MnSO4 + 10CO2 + 8H2O

Storage: It is kept in tightly closed container.

Boric acid

Molecular formula: H3BO3

Molecular weight: 61.83

Preparation

In laboratory it is prepared by using borax. When dilute sulfuric acid is added to boiling solution of borax, it produces boric acid.

Na2B4O7 + H2SO4 + 5H2O — > Na2SO4 + 4H3BO3 + HBO

For medicinal use, high grade of boric acid required. It is prepared by treating borax with hydrochloric acid. As HCl is volatile residual traces of HCl do not remain in the crystals of boric acid.

Properties

• It is available in three forms i) colourless, odourless, pearly scales ii) six-sided triclinic crystal and iii) white odourless powder
• Stable in the air
• Sparingly soluble in water
• More soluble in boiling water and alcohol
• Weakly acidic in nature

Uses

• Preparation of buffer solution
• Bacteriostatic in action
• Ingredient of dusting powder
• Antiseptic in ointments
• Anti-infective in eye and mouth wash

Assay

Boric acid is dissolved in glycerin water mixture and titrated with standard alkali using phenolphthalein as an indicator. It contains not less than 99.5% and not more than 100.5% H3BO3, calculated with reference to the substance dried over sulfuric acid for five hours.

Storage: It is stored in well closed container

Hydrogen Peroxide

Molecular formula: H2O2

Molecular weight: 34

Preparation

It is prepared by adding paste of barium peroxide in ice cold water to a calculated quantity of ice cold dilute sulfuric acid. The insoluble barium sulphate is filtered off.

BaO2 + H2SO4 — > BaSO4 + H2O2

It is an aqueous solution of hydrogen peroxide containing not less than 5% and not more than 7% of H2O2

Properties

• Colourless liquid
• Slightly acidic in taste
• Decomposes in contact with oxidizable matter
• Act as oxidizing or reducing agent depending upon the chemical environment

Uses: Antiseptic or germicidal. It is an effective antidote for phosphorous and cyanide poisoning.

Assay

The sample of hydrogen peroxide is diluted and 5N sulfuric acid is added and the contents titrated with potassium permanganate solution till faint pink colour is obtained.

Storage

It is stored in containers protected from light, in bottles with glass stoppers. It is kept in cool place.

Chlorinated Lime

Molecular formula: CaOCl2

Molecular weight: 126.98

Preparation

It is also known as calcium hypochlorite or bleaching powder. It is prepared by the action of chlorine on calcium hydroxide. This is a complex process in which intermediate product basic chloride is formed. It is further treated with chlorine and calcium hypochlorite to obtain chlorinated lime.

Ca(OH)2 + Cl2 — > CaOCl2 + H2O

Properties

• Dull white powder
• Characteristics odour
• Sparingly soluble in water
• Insoluble in organic solvents
• Decomposes when comes in contact with moisture liberating chlorine

Uses: It is mainly used for its disinfecting and bleaching properties. It is commonly used in the chlorination of water and in treatment of swimming tank. The action is due to liberated chlorine.

Assay

Chlorinated lime is assayed by oxidation-reduction reaction.

Storage

It should be stored in well closed container and kept in a cool place. It is affected by moisture and heat.

Iodine

Molecular formula: I₂

Molecular weight: 253.8

Preparation

Iodine is manufactured by extracting seaweed ash (kelp) with water and solution is concentrated.

2NaI + 3H2SO4 + MnO2 — > MnSO4 + 2 NaHSO4 + I2 + H2O

Impurities like ICl, IBr and ICN are removed by heating crude iodine with potassium iodide.

Properties

• Heavy, bluish-black rhombic prism or plates with metallic luster.
• Peculiar odor
• Volatile at ordinary temperature
• Practically insoluble in water
• Soluble in organic solvents

Uses

• Germicide and fungicide
• In treatment of thyrotoxicosis
• Purification of drinking water
• Chemical antidote for alkaloidal poisoning
• Locating agent in TLC

Assay

Iodine is analyzed by oxidation-reduction titration method, using starch as an indicator.

Incompatibilities

Being an oxidizing agent, it oxidizes hypophosphite, sulphites, some metals and reducing agents and gets itself reduced to iodide. It reacts with ammonia to form explosive iodide of nitrogen. It reacts with alkali hydroxide and carbonates to form iodide or iodates.

Storage: It is stored in amber-colored bottles with tight glass stopper or earthenware containers and kept in cool place.

Iodine preparations

Iodine acts as antimicrobial agent by the halogenation mechanism. Some of its important official preparations are given below.

Aqueous iodine solution

It is also known as Lugol’s solution. It contains 5% w/v of iodine and 10% w/v potassium iodide in water. It is a transparent, brown liquid with the smell of iodine.

Composition

Preparation: Potassium iodide and iodine are dissolved in 100ml of water with trituration and shaking. Finally, volume is made up to 1000ml.

Weak iodine solution

Commonly known as tincture of iodine. The solution contains 2% w/v of iodine and 2.5% w/v of potassium iodide in 50% alcohol. It is a transparent, brown liquid with the strong smell of iodine and alcohol.

Composition

Preparation

Potassium iodide and iodine are dissolved in sufficient 50% alcohol by shaking and volume is made up to 1000ml.

Strong iodine solution

It contains 10% w/v of iodine and 6% w/v of potassium iodide in water alcohol mixture. It is a transparent liquid having reddish brown color with the odor of iodine and alcohol.

Composition

Preparation

Potassium iodide and iodine are dissolved by trituration in purified water and sufficient alcohol (90%) is added to produce 1000ml.

Povidone-iodine solution

Povidone-iodine is an aqueous solution of povidone-iodine complex produced by the interaction between iodine and povidone (polyvinyl pyrrolidine). The complex contains 10% w/v of available iodine.

Description: The complex is yellowish brown, amorphous powder and has characteristics odor. It is soluble in water and alcohol. The solution is transparent in nature and has reddish brown color and faint smell of iodine. Aqueous solution is acidic in nature.

Uses: General uses of povidone-iodine solution are disinfection of skin, mouth or wounds. It is also used in the management of burns and cuts.

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