Replication Mechanisms and Target Enzymes
During the duplication of bidirectional bacterial DNA, excessive twisting—supercoils—develops ahead of the replication fork, creating mechanical stress. Furthermore, upon completion of replication, the circular chromosomes form intertwined rings called catenanes.
To resolve these topological barriers, bacteria require topoisomerases:
- In Gram-negative bacteria, the primary target is DNA gyrase (a type II topoisomerase), which relieves positive supercoils via double-strand breaks.
- In Gram-positive bacteria, topoisomerase IV plays the dominant role, facilitating decatenation of daughter DNA rings to ensure proper segregation into dividing cells.
Molecular Mechanism of Action
Fluoroquinolones interfere with enzyme function during the DNA cleavage stage:
- The drugs stabilize the "DNA–enzyme" cleavage complex, arresting it before passage of the second DNA segment.
- Low concentrations cause reversible inhibition (bacteriostatic effect).
- High (therapeutic) concentrations convert topoisomerases into DNA-damaging agents: the trapped enzymes generate double-stranded DNA breaks, triggering cell death (bactericidal effect).
These agents are safe for the host organism because human topoisomerase II is inhibited only at ultra-high concentrations.
Spectrum of Activity and Pharmacokinetics
Early-generation fluoroquinolones possess a broad antibacterial spectrum with predominant activity against Gram-negative bacilli (Escherichia coli, Salmonella, Klebsiella, Pseudomonas aeruginosa, Haemophilus influenzae, Neisseria meningitidis, Neisseria gonorrhoeae).
- Expanded coverage: They also cover Gram-positive cocci and intracellular pathogens (Mycoplasma, Chlamydia, Legionella).
- Tuberculosis: Ciprofloxacin, ofloxacin, and lomefloxacin show activity against Mycobacterium tuberculosis.
- Limitations: The drugs are inactive against anaerobes.
- Pharmacokinetics: When administered orally, they achieve high concentrations in blood and tissues regardless of food intake and penetrate efficiently into cells.
Mechanisms of Resistance
Bacterial resistance develops relatively slowly via the following mechanisms:
- Chromosomal mutations in topoisomerase genes (altering the target structure).
- Reduced permeability of the outer cell membrane due to decreased porin expression.
- Upregulation of efflux pumps that actively extrude the drug from the cell.