Antimicrobial drugs are classified in a variety of ways. They are: 1. Classification Based on Cidal or Static Action 2. Classification Based on Mechanism of Action of the Drugs 3. Classification Based on Spectrum of Activity.
1. Classification Based on Cidal or Static Action:
e.g. bactericidal or bacteriostatic (Table 25.1) and fungicidal or fungi static drugs. In most of the drugs, activity however, depends upon the concentration of the drug used.
2. Classification Based on Mechanism of Action of the Drugs:
Drugs act at one or the other step of biochemical reactions involved in the synthesis of microbial compounds. In addition, some drugs are effective against formed structures e.g. cell membrane and microtubules of the parasites. Synthesis of bacterial macromolecules involves three sequential biochemical reactions.
Glucose and other carbon sources are used in class I reaction to generate ATP and intermediates of Krebs’ cycle. ATP and precursors are utilized to produce basic units of macromolecules in class II reaction e.g. amino acids, nucleotides etc.
In class III reaction, these basic units are converted into macromolecules e.g. protein, nucleic acid, peptidoglycans and polysaccharides. Class II and III reactions are good targets for antimicrobial drugs due to their being specific to parasites.
The example for classification based on mechanism of action is listed below:
A. Inhibition of bacterial cell wall synthesis: e.g. penicillins, cephalosporins, vancomycin, bacitracin etc.
B. Impairment of cell membrane functions by increasing permeability: e.g. polymyxins, colistin, miconazole, amphotericin, nystatin etc.
C. Interference with microtubules and microfilaments: e.g. griseofulvin, colchicine etc.
D. Inhibition of protein synthesis:
(a) By binding with 50s ribosome sub unit e.g. chloramphenicol, macrolides and lincosamides.
(b) By binding with 30s ribosome sub units e.g. aminoglycosides, tetracyclines and spectinomycin.
(c) By inhibiting transcription : e.g. rifampin and rifamycins.
(d) By inhibiting binding of aminoacyl tRNA to the ribosomal A site, e.g. fusidic acid and erythromycin.
(e) Premature termination of peptide chain : e.g. puromycin.
E. Inhibition of nucleic acid synthesis:
(a) Interference in the synthesis of nucleotides: e.g., fluorouracil, mercaptopurine, flucytosine etc.
(b) Alteration of base pairing property of template: e.g., acridines (acriflavine, proflavine), chloroquine, antiviral drugs.
(c) Inhibition of DNA and RNA polymerases: RNA polymerases are inhibited by rifampin and rifamycins and DNA polymerases by acyclovir and cytarabine
(d) Direct effect on DNA: e.g. alkylating agents.
F. Inhibition of bacterial folic acid synthesis by competitive inhibition mechanism:
e.g., sulfa drugs, sulfones and par-amino salicylic acid or by sequential blockade e.g. trimethoprim and sulfa drug combination.
3. Classification Based on Spectrum of Activity:
(a) Narrow Spectrum:
Penicillins, bacitracin, vancomycin and macrolides are only effective against Gram-positive bacteria. Similarly polymyxins and streptomycin are also narrow spectrum drugs effective against Gram-negative bacteria.
(b) Broad-Spectrum:
These are active against Gram-positive and Gram-negative bacteria, mycoplasmas and rickettsia. Tetracyclines, ampicillin, chloramphenicol, gentamicin, neomycin, sulfa drugs, cehalosporins, nitro-furans and fluoroquinolones are broad spectrum drugs.
