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DrX Whiz Niraj
Antimicrobials: Mechanisms & Compounds
Comprehensive exam notes on Inorganic Antimicrobials. Understand Mechanisms of Action, Iodine Preparations, and Assay of Hydrogen Peroxide & Chlorinated Lime.
Antimicrobials are chemical agents used to destroy or inhibit the growth of pathogenic microorganisms. Inorganic antimicrobials generally act on microbes via three primary mechanisms.
| Mechanism | How it works? | Examples |
|---|---|---|
| 1. Oxidation | Non-metals and certain highly oxidized compounds release active oxygen or reactive oxygen species (ROS) which oxidize the active functional groups in microbial proteins. | Hydrogen Peroxide (H₂O₂), Potassium Permanganate (KMnO₄), Halogens. |
| 2. Halogenation | Halogens act by modifying the peptide linkages of the protein molecule. The halogen replaces the hydrogen in the -NH group of peptide bonds, altering the protein structure. | Iodine, Chlorinated Lime (Bleaching powder). |
| 3. Protein Precipitation | Many metal ions (especially heavy metals) bind to the vital sulfhydryl (-SH) groups of microbial enzymes, causing them to precipitate and denature. | Boric acid, Silver nitrate, Mild silver protein. |
Potassium Permanganate (KMnO₄)
Dark purple crystals. It acts via the Oxidation mechanism.
- Used as an antiseptic for wounds and ulcers (0.1% to 1% solution).
- Known as Condy's Lotion in very dilute solutions.
- Used for Stomach wash (gastric lavage) in cases of alkaloid poisoning (e.g., Strychnine, Morphine) because it oxidizes and destroys the poison.
Boric Acid (H₃BO₃)
A very weak acid that acts as a mild, non-irritating local anti-infective.
- Mainly used in Ophthalmic preparations (Eye drops & eye washes) because it is non-irritating to delicate tissues.
- Used in dusting powders (with talc) for diaper rash.
Compounds marked with an asterisk (*) in the syllabus require you to memorize their Assay procedure.
A. Hydrogen Peroxide (H₂O₂)*
H₂O₂ acts as an antimicrobial via oxidation. It effervesces (releases oxygen gas) when applied to tissue due to the enzyme Catalase.
Assay of Hydrogen Peroxide:
It is assayed by a Redox Titration against a standard solution of Potassium Permanganate (KMnO₄) in an acidic medium (using dilute H₂SO₄).
Indicator: KMnO₄ acts as a Self-Indicator. The end point is a faint permanent pink color.
B. Chlorinated Lime (Bleaching Powder)*
Formula: CaOCl₂. It contains approximately 30-35% of available chlorine. It acts via Halogenation and Oxidation.
Assay of Chlorinated Lime:
It is assayed by an Iodometric Titration.
The sample is treated with Acetic acid and excess Potassium Iodide (KI). The available chlorine displaces an equivalent amount of Iodine from KI. The liberated Iodine is then titrated against standard Sodium Thiosulphate (Na₂S₂O₃).
Indicator: Starch solution (added near the end point). End point is disappearance of the blue color.
Iodine is one of the oldest and most effective topical microbicides. It is poorly soluble in water but dissolves readily in the presence of Potassium Iodide (KI) due to the formation of the polyiodide ion (I₃⁻). Important official preparations include:
| Preparation Name | Composition | Uses / Characteristics |
|---|---|---|
| 1. Aqueous Iodine Solution (Lugol's Solution) |
5% w/v Iodine + 10% w/v Potassium Iodide in water. | Used internally for the treatment of Thyrotoxicosis (hyperthyroidism). |
| 2. Weak Iodine Solution (Tincture of Iodine) |
2% w/v Iodine + 2.4% w/v Sodium Iodide in 50% Alcohol. | A powerful topical antiseptic for wounds and cuts. |
| 3. Mandl's Paint (Compound Iodine Paint) |
Iodine + KI + Peppermint oil + Glycerin. | Used for treating throat infections (Pharyngitis, Tonsillitis). Glycerin keeps it adhered to the throat. |
| 4. Povidone-Iodine (Betadine) |
A complex of Iodine with the polymer Polyvinylpyrrolidone (PVP). | An Iodophor. It slowly releases iodine. It is non-staining and much less irritating than the tincture. |
Clinical Alert: Povidone-Iodine vs Tincture
Traditional Iodine Tincture causes intense burning when applied to open wounds because of its high alcohol content, and it leaves permanent yellow-brown stains on clothes.
Povidone-Iodine (Betadine) solved this problem. The polymer (PVP) acts as a carrier, slowly releasing iodine to the wound without causing burning, and it washes off easily with water (non-staining).