Concept 03 / 05Chemistry in Everyday Life
Antimicrobials: Antibiotics, Antiseptics and Disinfectants; Antifertility Drugs
Antimicrobial drugs including the history of salvarsan, prontosil and sulfanilamide, penicillin, bactericidal and bacteriostatic antibiotics, narrow- and broad-spectrum antibiotics, antiseptics and disinfectants with examples and concentrations, and antifertility drugs (norethindrone, ethynylestradiol).
Antimicrobials destroy or prevent the development of microorganisms (bacteria, fungi, viruses, other parasites), or inhibit their pathogenic action. Examples are antibacterials, antifungals, antivirals and antiparasitic drugs. The main types are antibiotics, antiseptics and disinfectants.
Antibiotics. Originally defined as chemicals produced by microorganisms that inhibit the growth of or destroy other microorganisms. Now: a substance produced wholly or partly by chemical synthesis which, in low concentrations, inhibits the growth of or destroys microorganisms by intervening in their metabolic processes.
History. Paul Ehrlich developed salvarsan (arsphenamine), an arsenic compound, for syphilis in 1909. He noticed the resemblance between salvarsan and azo dyes: the link resembles . In 1932 the azo dye prontosil was found to be effective; in the body it is converted to sulfanilamide, the real active compound, which led to many sulfonamides (sulfapyridine and others). Alexander Fleming discovered penicillin in 1929 from the mould Penicillium.
Bactericidal antibiotics kill bacteria (penicillin, aminoglycosides, ofloxacin); bacteriostatic antibiotics inhibit their growth (erythromycin, tetracycline, chloramphenicol).
Spectrum of action. Broad-spectrum antibiotics kill or inhibit a wide range of Gram-positive and Gram-negative bacteria (chloramphenicol, used for typhoid, dysentery, acute fever, some urinary infections, meningitis and pneumonia; vancomycin; ofloxacin). Narrow-spectrum antibiotics act mainly on Gram-positive or Gram-negative bacteria (penicillin G). Limited-spectrum antibiotics act on a single organism or disease. Ampicillin and amoxycillin are semi-synthetic modifications of penicillin with a broader spectrum. Patients are tested for penicillin sensitivity (allergy) before it is given.
Antiseptics and disinfectants.
Antiseptics are applied to living tissue: wounds, cuts, ulcers and diseased skin. Examples: furacine and soframicine; dettol (a mixture of chloroxylenol and terpineol); bithionol (added to soaps as a deodorant and antiseptic); tincture of iodine (2-3% iodine in alcohol-water); iodoform (on wounds); dilute aqueous boric acid (weak antiseptic for eyes).
Disinfectants are applied to inanimate objects: floors, drains, instruments. Examples: 1% phenol; chlorine at 0.2 to 0.4 ppm in drinking water; sulfur dioxide in very low concentration.
The same substance can serve both roles depending on concentration: 0.2% phenol is an antiseptic, while 1% phenol is a disinfectant.
Antifertility drugs (birth control pills) are mixtures of synthetic estrogen and progesterone derivatives, which are more potent than the natural hormones. Progesterone suppresses ovulation. Norethindrone is a synthetic progesterone derivative; ethynylestradiol (novestrol) is an estrogen derivative used in combination with progestogens.
Key Takeaways:
- Salvarsan (Ehrlich, syphilis); prontosil → sulfanilamide; penicillin (Fleming).
- Bactericidal: penicillin, ofloxacin, aminoglycosides; bacteriostatic: erythromycin, tetracycline, chloramphenicol.
- Antiseptics are for living tissue; disinfectants for non-living objects.
- 0.2% phenol is antiseptic, 1% phenol is disinfectant; chlorine at 0.2-0.4 ppm sterilises water.
- Pitfall: chloramphenicol is bacteriostatic but broad-spectrum; the two classifications are independent.
Brain-Teaser 1: Why are disinfectants not applied to wounds?
Solution: Disinfectants are used at concentrations that kill microorganisms but are also toxic to living cells, so they would damage tissue. Antiseptics are chosen or diluted so that they act on microbes without significantly harming human tissue (for example phenol at 0.2% rather than 1%).
Brain-Teaser 2: How many grams of iodine are needed to prepare of a 2% (w/v) tincture of iodine?
Solution: 2% (w/v) means per . For : of iodine, dissolved in alcohol-water.
Brain-Teaser 3: Prontosil kills bacteria in the body but has no effect on bacteria in a test tube. Explain.
Solution: Prontosil is a prodrug. It is inactive by itself; enzymes in the body reduce its azo link () to release sulfanilamide, the actual antibacterial agent. In a test tube there are no such enzymes, so no sulfanilamide forms.
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