What is the fundamental difference in the mechanism of action between penicillin and streptomycin?

The fundamental difference lies in their cellular targets. Penicillin, a beta-lactam antibiotic, primarily inhibits the synthesis of the bacterial cell wall. It does this by binding to and inactivating transpeptidase enzymes (penicillin-binding proteins) responsible for cross-linking peptidoglycan strands, leading to a weakened cell wall and subsequent bacterial lysis. In contrast, streptomycin, an aminoglycoside, inhibits bacterial protein synthesis. It binds irreversibly to the 30S ribosomal subunit, causing misreading of mRNA and premature termination of protein synthesis, ultimately leading to the production of non-functional proteins and bacterial death.

Why is penicillin considered a 'narrow-spectrum' antibiotic, while streptomycin is 'broad-spectrum'?

The terms 'narrow-spectrum' and 'broad-spectrum' refer to the range of bacterial species an antibiotic is effective against. Natural penicillins (like Penicillin G) are primarily effective against Gram-positive bacteria and some Gram-negative cocci, hence their narrow spectrum. This is partly due to the outer membrane of Gram-negative bacteria acting as a barrier. Streptomycin, on the other hand, is effective against a wider range of bacteria, including many Gram-negative aerobic bacteria and *Mycobacterium tuberculosis*, making it a broad-spectrum antibiotic. The development of semi-synthetic penicillins aimed to broaden their spectrum.

What is the significance of the beta-lactam ring in penicillin's activity, and how does bacterial resistance often target it?

The beta-lactam ring is the pharmacologically active core of penicillin. Its strained, four-membered cyclic amide structure is crucial for its ability to irreversibly bind to and inhibit bacterial transpeptidase enzymes. Bacteria often develop resistance by producing beta-lactamase enzymes (also known as penicillinase). These enzymes hydrolyze the amide bond within the beta-lactam ring, opening it up and rendering the penicillin molecule inactive, thus preventing it from binding to its target PBPs.

What are the major side effects associated with streptomycin, and why are they a concern?

Streptomycin, like other aminoglycosides, is associated with significant dose-related toxicities. The two major concerns are ototoxicity and nephrotoxicity. Ototoxicity involves damage to the eighth cranial nerve, leading to irreversible hearing loss (cochlear damage) or balance problems (vestibular damage). Nephrotoxicity refers to kidney damage, which is usually reversible upon discontinuation of the drug. These severe side effects necessitate careful monitoring of drug levels and patient renal function during streptomycin therapy, limiting its use to serious infections where other less toxic alternatives are not suitable.

How did the discovery of streptomycin impact the treatment of tuberculosis?

The discovery of streptomycin in 1943 was a monumental breakthrough for tuberculosis (TB) treatment. Before streptomycin, TB was a chronic, often fatal disease with limited effective therapies. Streptomycin was the first antibiotic found to be highly effective against *Mycobacterium tuberculosis*, the causative agent of TB. It transformed TB from an incurable disease into a treatable one, significantly reducing mortality and morbidity rates. While it is now used in combination with other drugs to prevent resistance, its initial impact was revolutionary, saving countless lives and laying the foundation for modern TB chemotherapy.

Penicillin, Streptomycin

Chemistry

NEET UG

Version 1Updated 22 Mar 2026

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Antibiotics are chemical substances, produced wholly or partly by microorganisms, that in low concentrations inhibit the growth of or kill other microorganisms without significant harm to the host. Penicillin, discovered by Alexander Fleming, was the first true antibiotic, revolutionizing medicine by effectively treating bacterial infections. Streptomycin, an aminoglycoside antibiotic, was later d…

Quick Summary

Antibiotics are chemical agents that selectively kill or inhibit the growth of bacteria. Penicillin, discovered by Alexander Fleming, was the first antibiotic, belonging to the beta-lactam class. Its core structure features a beta-lactam ring, which is crucial for its action.

Penicillin works by inhibiting bacterial cell wall synthesis, specifically by inactivating transpeptidase enzymes (penicillin-binding proteins) that cross-link peptidoglycan. It is primarily bactericidal and effective against Gram-positive bacteria.

Bacterial resistance often arises from beta-lactamase enzymes that hydrolyze the beta-lactam ring.

Streptomycin, discovered by Selman Waksman, is an aminoglycoside antibiotic. Its structure contains amino sugars linked to an aminocyclitol ring. Streptomycin is bactericidal and inhibits bacterial protein synthesis by binding irreversibly to the 30S ribosomal subunit, causing mRNA misreading and production of non-functional proteins.

It has a broad spectrum, notably effective against Gram-negative bacteria and *Mycobacterium tuberculosis*. Key side effects include ototoxicity (hearing/balance loss) and nephrotoxicity (kidney damage).

Both antibiotics exemplify selective toxicity, targeting bacterial processes absent or different in human cells.

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Key Concepts

Beta-Lactam Ring and its Significance

The beta-lactam ring is a four-membered cyclic amide that forms the core of penicillin's structure. Its…

Inhibition of Protein Synthesis by Streptomycin

Streptomycin, an aminoglycoside, exerts its bactericidal effect by targeting the bacterial ribosome,…

Selective Toxicity and Drug Targets

Selective toxicity is the cornerstone of effective antimicrobial therapy, meaning a drug can kill or inhibit…

Penicillin: — Beta-lactam antibiotic. Inhibits bacterial cell wall synthesis (peptidoglycan cross-linking) by binding to PBPs. Bactericidal. Natural: Penicillin G (acid-labile, parenteral), Penicillin V (acid-stable, oral). Resistance: Beta-lactamase enzymes. Side effect: Hypersensitivity.

Streptomycin: — Aminoglycoside antibiotic. Inhibits bacterial protein synthesis by binding to 30S ribosomal subunit. Bactericidal. Broad-spectrum. Used for TB. Side effects: Ototoxicity, Nephrotoxicity. Resistance: Ribosomal mutations, enzymatic inactivation.

Penicillin Cracks Walls, Streptomycin Prevents Proteins.

Penicillin: Cell Wall synthesis inhibitor.

Streptomycin: Protein Production inhibitor (30S ribosome).

Streptomycin's Outstanding Nuisances: Ototoxicity & Nephrotoxicity.