Antimicrobial drugs Classification

Antimicrobial agents are graded based on the type of organism they are effective against.
Antibacterial drugs
Antiviral drugs
Antifungal drugs
Antiprotozoal drugs
Anthelminthic drugs


A few antimicrobials are effective against several types of bacteria. Metronidazole, for example, inhibits anaerobic bacteria (such as Clostridium perfringens) and protozoa that use anaerobic metabolic pathways (such as Trichomonas vaginalis)

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Antimicrobial drugs are further divided into the following categories:
• bacteriostatici.e., Sulphonamides, tetracyclines, and chloramphenicol are examples of antibiotics that work by stopping bacteria from multiplying.

  • bactericidal,i.e., Penicillins, cephalosporins, aminoglycosides, isoniazid, and rifampicin are examples of antibiotics that destroy bacteria.

The classification is somewhat subjective in current clinical practice since most bacteriostatic drugs can be seen to be bactericidal at high concentrations, under certain incubation conditions.  conditions in vitro and against some bacteria.

Bactericidal drugs are most effective against species that divide quickly. By reducing multiplication, a bacteriostatic drug can protect the organism from the killing effects of a bactericidal drug. Antimicrobial mutual antagonism can be clinically significant, but the issue is complicated by the numerous and evolving factors that decide each drug’s effectiveness at the site of infection.

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How antimicrobials Act

It’s important to note that medications are rarely used alone to treat illnesses; instead, they work in tandem with the body’s natural defences.. Antimicrobials act at different sites in the target organism as follows:

cell wall.

This gives the bacterium its characteristic shape and provides protection against the much lower osmotic pressure of the environment. The breakdown and extension of the bacterial wall are needed for multiplication; interference with these processes prevents the organism from resisting osmotic pressures, causing it to burst. As the cells of higher, e.g. human, organisms do not possess this type of wall, drugs that act here may be especially selective; obviously, the drugs are effective only against growing cells.

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Examples : penicillins, cephalosporins, vancomycin, bacitracin, cycloserine.

cytoplasmic membrane:  

inside the cell wall is the site of most of the microbial cell’s biochemical activity. Drugs that interfere with its function include: polyenes (nystatin, amphotericin), azoles (fluconazole, itraconazole, miconazole), polymyxins (colistin, polymyxin B)

Protein synthesis.

Drugs that interfere with the formation of peptide chains on the ribosomes of an organism at different points


tetracyclines, aminoglycosides, quinupristin/dalfopristin, linezolid, chloramphenicol, erythromycin, fusidic acid.

Nucleic acid metabolism.

 Drugs may interfere
• directly with microbial DNA, replication, or repair, for example quinolones, metronidazole, or with RNA

Example: rifampicin
• indirectly on nucleic acid synthesis

Example: sulphonamides, trimethoprim.


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