Quinolones and Fluoroquinolones: An Overview


Quinolones and fluoroquinolones are groups of antibiotics that have been used to treat various bacterial infections since the 1960s. This article discusses the importance of fluoroquinolones, their mechanism of action, classification, and spectrum of activity.

The Significance of Fluoroquinolones

Fluoroquinolones have gained significant importance due to their excellent oral absorption and perfect distribution in the body, leading to deep tissue penetration. This makes them highly efficient at killing intracellular organisms like chlamydia, TB, Legionella, and Brucella. Additionally, newer fluoroquinolones are broad-spectrum antibiotics with high potency and well-tolerated, making them safer than other antibiotic medicines.

Mechanism of Action of Quinolones

Quinolones and fluoroquinolones work as bactericidal agents that inhibit bacterial replication by blocking the DNA replication pathway. During protein synthesis and DNA replication, enzymes in the bacteria called DNA gyrase or DNA topoisomerase are responsible for unwinding double-stranded DNA into a single-stranded structure. DNA gyrase, a topoisomerase II-type enzyme, unwinds DNA by introducing negative supercoils and relaxing positive supercoils. Quinolones and fluoroquinolones inhibit this enzyme by binding to the A-subunit of the enzyme. This prevents bacteria from replicating or synthesizing proteins, leading to cell death. [1]

Classification of Quinolones and Fluoroquinolones

Quinolones and fluoroquinolones are classified into four generations. The first-generation quinolone is Nalidix acid, and the other three are fluoroquinolones, which are derivatives of nalidixic acid. The following are the classes of quinolones and fluoroquinolones:

First Generation

The only quinolone, Nalidixic acid, is moderately effective against gram-negative bacteria but is not commonly used due to poor distribution in the body. It is only used in rare uncomplicated urinary tract infections.

Second Generation

They are more effective against gram-negative bacteria than Nalidixic acid and have some gram-positive coverage. However, they do not work on Streptococcus pneumonia. Ciprofloxacin (Ciproxin) is the most widely used drug in this class and is effective against atypical organisms like Chlamydia and Mycoplasma, which cause respiratory tract infections.

Third Generation

Levofloxacin is a third-generation fluoroquinolone with more potent gram-positive coverage and helps treat respiratory tract infections.

Fourth Generation

Moxifloxacin is a fourth-generation fluoroquinolone effective against many gram-positive and gram-negative bacteria. It is also used to treat respiratory tract infections. [2]

Spectrum of Activity

The spectrum of activity of quinolones and fluoroquinolones varies depending on their generation. The first-generation quinolone, Nalidixic acid, is moderately effective against gram-negative bacteria but is not commonly used due to poor bodily distribution. Second-generation quinolones are more efficient against gram-negative bacteria than Nalidixic acid but have limited activity against gram-positive bacteria. The third and fourth-generation quinolones have full gram-positive coverage, making them more effective against respiratory tract infections. [3]

Newer agents and resistance:

Although fluoroquinolones are considered broad-spectrum antibiotics, the emergence of resistant strains of bacteria has become a major concern. The widespread use of these antibiotics has led to an increase in bacterial resistance. Resistance occurs through chromosomal mutations, transfer of resistance genes, and overuse of antibiotics.

The most common mechanism of resistance to fluoroquinolones is mutations in genes encoding DNA gyrase and topoisomerase IV enzymes. These mutations can cause a decrease in fluoroquinolone binding to the enzymes, making the drugs less effective. In addition, bacteria can also develop efflux pumps that actively remove the drugs from the bacterial cell, further reducing their effectiveness.

Despite these concerns, fluoroquinolones remain an essential tool in treating bacterial infections. When used appropriately, these drugs can effectively treat many infections.

Side effects and precautions:

While fluoroquinolones are generally safe and well-tolerated, they have potential side effects. Common side effects include gastrointestinal upset, headache, dizziness, and photosensitivity. More severe side effects include tendonitis and tendon rupture, particularly in older patients and those on long-term therapy.

Fluoroquinolones should be used cautiously in patients with pre-existing conditions such as epilepsy or kidney disease and not in pregnant or breastfeeding women. They may also interact with other medications, so it is important to discuss all medications with your healthcare provider before starting treatment with fluoroquinolones.


In conclusion, quinolones and fluoroquinolones are an important class of antibiotics that have revolutionized the treatment of bacterial infections. These drugs have a unique mechanism of action that targets bacterial DNA replication and are highly effective against a broad range of gram-negative and some gram-positive bacteria. While the emergence of resistant strains is a concern, these drugs remain an essential tool in treating bacterial infections when used appropriately.


  1. Can fluoroquinolones be used to treat viral infections? No, fluoroquinolones are only effective against bacterial infections and should not be used to treat viral infections.
  2. Are fluoroquinolones safe for everyone to use? While fluoroquinolones are generally safe and well-tolerated, they should be used cautiously in specific populations, such as older adults and those with pre-existing conditions.
  3. How do fluoroquinolones differ from other types of antibiotics? Fluoroquinolones have a unique mechanism of action that targets bacterial DNA replication, making them highly effective against a broad range of bacteria.
  4. Can I drink alcohol while taking fluoroquinolones? It is generally recommended to avoid alcohol while taking antibiotics, including fluoroquinolones.
  5. Are fluoroquinolones available over-the-counter? No, fluoroquinolones are prescription-only drugs and should only be taken under the supervision of a healthcare provider.

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4 thoughts on “Quinolones and Fluoroquinolones: An Overview”

  1. Pingback: Whole Pharmacology Drugs Classification - DrugsBank

  2. Pingback: Important Information About Moxifloxacin - DrugsBank

  3. Jane Crosby

    Please be very responsible with the fluoroquinolone class of antibiotics. No where in this article does it mention these drugs are so dangerous they carry a black box warning. The fluoroquinolones are still being prescribed for simple infections without warnings or caution. Thousands are being maimed and many are left with permanent damage. The responsibility should lie with the one who made the oath to do no harm. Please help to minimize the horrendous harm they are causing to countless victims who get the bullet instead of healing. This is not rare I assure you. Thank you.

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