Mechanism of action of ciprofloxacin in gram negative bacteria:-
**Core Concept**
Ciprofloxacin is a fluoroquinolone antibiotic that targets bacterial DNA replication by inhibiting bacterial type II topoisomerases, specifically DNA gyrase and topoisomerase IV. DNA gyrase is essential for introducing negative supercoils into DNA, enabling replication and transcription in Gram-negative bacteria.
**Why the Correct Answer is Right**
Ciprofloxacin binds to the DNA gyrase enzyme (a type II topoisomerase) in Gram-negative bacteria, inhibiting its ability to introduce negative supercoils into DNA. This leads to double-strand DNA breaks and disruption of DNA replication, ultimately causing bacterial cell death. DNA gyrase is particularly critical in Gram-negative bacteria due to their outer membrane and high metabolic activity.
**Why Each Wrong Option is Incorrect**
Option A: Inhibition of mycolic acid is the mechanism of isoniazid, used in tuberculosis, not ciprofloxacin. Mycolic acids are found in mycobacteria, not Gram-negative bacteria.
Option B: While topoisomerase IV is also a target of fluoroquinolones, especially in Gram-positive bacteria, DNA gyrase is the primary target in Gram-negative bacteria. This makes option B less accurate in this context.
Option D: Helicase is not a direct target of ciprofloxacin; helicase activity is mediated by proteins like DnaB and is unrelated to fluoroquinolone action.
**Clinical Pearl / High-Yield Fact**
Fluoroquinolones like ciprofloxacin primarily inhibit DNA gyrase (in Gram-negative bacteria) and topoisomerase IV (in Gram-positive bacteria), with DNA gyrase being the main target in Gram-negative pathogens. This makes ciprofloxacin especially effective against Enterobacteriaceae, such as *E. coli* and *Salmonella*.
✓ Correct Answer: C. Inhibition of DNA gyrase