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Synthetic Antimicrobial Agents

For medical students2 min readUpdated 2026-10-10

Synthetic antimicrobial agents are chemotherapeutic drugs obtained exclusively through chemical synthesis. They do not occur naturally in living organisms, but by their mechanism, type, and spectrum of activity, they successfully mimic classic antibiotics by selectively inhibiting pathogens of infectious diseases.

Core ConceptThe "magic bullet" concept: to destroy the microbe without harming the host organism.
Historical MilestoneIn 1908, P. Ehrlich synthesized an arsenic-based compound, arsphenamine (Salvarsan), for the treatment of syphilis.
Sulfonamide TargetThese drugs are structural analogs of para-aminobenzoic acid (PABA).
NitroimidazolesConverted into their active form exclusively inside the cells of anaerobic microorganisms.

History and Classification

Unlike true antibiotics, synthetic agents are created entirely in chemical laboratories. Their history begins in the early 20th century when Paul Ehrlich developed Salvarsan. Later, in 1935, Gerhard Domagk discovered the therapeutic properties of Prontosil (a red azo dye). Inside the human body, this substance broke down to release its active component—sulfanilamide.

Modern synthetic drugs are classified by their clinical use into antibacterials, antifungals, and antiprotozoals (antivirals are traditionally classified in a separate, independent group). Basic chemical classes include:

Sulfonamides: The Mechanism of Dual Blockade

The structural basis of sulfonamides is the para-amino group. Their mechanism of action is based on structural similarity to para-aminobenzoic acid (PABA). They act as competitive antagonists: the bacterium mistakenly takes up the drug instead of PABA, thereby completely blocking the synthesis of folic (tetrahydrofolic) acid. Because this acid is a precursor for purines and pyrimidines, microorganisms lose their ability to replicate (exhibiting a bacteriostatic effect).

Today, single-agent therapies from this group are rarely used due to high microbial resistance, toxicity, and lower efficacy compared to modern antibiotics. However, the combination drug co-trimoxazole (sulfamethoxazole paired with trimethoprim) is widely used. Trimethoprim blocks a different enzyme in the same metabolic pathway. This dual blockade produces a powerful synergistic effect, shifting the type of action to bactericidal. The drug is actively prescribed for urinary tract infections caused by Gram-negative flora.

Evolution of Quinolones and Fluoroquinolones

The first generation of this group (classic quinolones, a prominent example being nalidixic acid, used since 1962) had a narrow spectrum of activity and was intended exclusively for treating urinary tract infections. A major drawback was the rapid development of bacterial resistance.

Modern fluoroquinolones (e.g., ciprofloxacin, norfloxacin) contain fluorine atoms in their molecule, which dramatically improves their pharmacological properties:

Special-Purpose and Reserve Drugs

Nitroimidazoles (e.g., metronidazole) possess unique selectivity: they are reduced and converted into their bactericidal form only inside anaerobic cells. Their targets are obligate anaerobes and protozoa (Entamoeba histolytica, Giardia lamblia, Trichomonas vaginalis).

Nitrofurans (e.g., furazolidone) are broad-spectrum bactericidal agents. Their most important feature is that they accumulate in urine in high concentrations, making them effective urinary antiseptics.

Oxazolidinones (e.g., linezolid) were designed as "heavy artillery" to combat severe infections. They are active against Gram-positive flora (bacteriostatic against staphylococci and bactericidal against a number of other microbes). The main clinical niche for this group is treating infections caused by multidrug-resistant pathogens: methicillin-resistant Staphylococcus aureus (MRSA), as well as pneumococcal and enterococcal strains that have acquired resistance to penicillin and vancomycin.

Mnemonic

To remember the targets of nitroimidazoles (metronidazole), use the association "A-P": they are strictly active against Anaerobes and Protozoa.

Frequently asked questions

What is the molecular mechanism of action of fluoroquinolones?

The molecular mechanism of fluoroquinolones involves inhibiting bacterial topoisomerases during the DNA strand-breaking step. The drugs stabilize the DNA-enzyme complex, leading to the destruction of the DNA polymer.

  • In Gram-negative bacteria, the primary target is DNA gyrase (type II topoisomerase).
  • In Gram-positive bacteria, the primary target is topoisomerase IV.

Fluoroquinolones inhibit these enzymes before the second segment of DNA can pass through the break. As a result, double-stranded DNA breaks are formed, making replication and transcription impossible. The subsequent cellular response to these breaks (the SOS response) leads to cell death, producing a bactericidal effect.

What side effects are characteristic of sulfonamides?

Sulfonamides are associated with allergic skin reactions, organ toxicity, and systemic disorders. Key side effects include:

  • Allergic and dermatologic reactions — fever, pruritus, rash, photosensitivity.
  • Organ toxicity — gastrointestinal disturbances and hepatotoxicity.
  • Hematologic abnormalities — anemia, leukopenia, thrombocytopenia.
  • Renal toxicity — crystalluria, occurring in acidic urine.
  • Drug-induced pathologies — polyarteritis nodosa.
  • Specific complication in newborns — kernicterus, caused by the displacement of bilirubin from its binding to albumin, leading to severe toxic brain damage.
Which drugs belong to the first-generation quinolones?

First-generation quinolones include classic non-fluorinated agents:

  • Nalidixic acid
  • Oxolinic acid
  • Pipemidic acid

Classic first-generation quinolones are characterized by a narrow spectrum of activity, rapid development of resistance, and use in urinary tract infections caused predominantly by Gram-negative bacteria. Oxolinic and pipemidic acids are analogs of nalidixic acid with similar pharmacological action and higher potency.

What is the exact mechanism of the bactericidal action of nitroimidazoles after their intracellular reduction?

The exact mechanism of the bactericidal action of nitroimidazoles following the reduction of their nitro group is mediated through two primary pathways of cellular damage:

  • Disruption of respiration — the reduced drug integrates into the electron transport chain and blocks flavoproteins (electron carriers), causing respiratory failure and cell death.
  • DNA damage — reduction of the nitro group generates free radicals that exert direct toxic effects on DNA.

Additionally, the reduced forms of the drug bind to pathogen proteins, membranes, and DNA, causing severe damage and resulting in marked cytotoxic and bactericidal effects.

How do synthetic antimicrobial agents differ from antibiotics?

Synthetic agents lack natural counterparts and are created exclusively via chemical synthesis in the laboratory, whereas true antibiotics are of natural or semi-synthetic origin. However, their mechanisms of bacterial destruction are similar.

Why is co-trimoxazole more effective than conventional sulfonamides?

It contains two components (sulfamethoxazole and trimethoprim) that sequentially block bacterial folic acid synthesis at different stages. This provides mutual enhancement (synergism) and a bactericidal effect.

What is the main advantage of fluoroquinolones over early quinolones?

The introduction of a fluorine atom into the molecular structure broadened their spectrum of action, made the drugs bactericidal, provided excellent tissue barrier penetration, and sharply reduced the frequency of microbial resistance.

What is the clinical indication for oxazolidinones?

Linezolid is used in severe cases to treat serious infections caused by multidrug-resistant Gram-positive flora that are unresponsive to other drugs (e.g., MRSA infections or vancomycin-resistant enterococci).

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