Difference Between Polymyxin B And Colistin

Antibiotics are essential tools in the fight against bacterial infections, with Polymyxin B and Colistin being pivotal in treating multidrug-resistant bacteria. Despite their similar therapeutic uses, these two antibiotics have distinct characteristics and applications that merit close examination. This article will explore these differences to provide a clear understanding of their unique profiles.

Polymyxin B and Colistin are both used primarily to target severe infections caused by Gram-negative bacteria, including Pseudomonas aeruginosa and certain strains of E. coli. While Polymyxin B is often used in less severe cases due to its lower toxicity, Colistin is reserved for more critical conditions, reflecting its potent efficacy and higher risk of nephrotoxicity. This differentiation is crucial for healthcare professionals when selecting the most appropriate treatment.

These antibiotics, although powerful, come with significant risks and side effects, necessitating careful consideration and understanding of their mechanisms. The distinction between Polymyxin B and Colistin lies not only in their chemical structures but also in their specific uses, side effect profiles, and the bacterial resistance they encounter.

What is Polymyxin B?

Origins and History

Polymyxin B, first isolated in 1947, originates from the bacterium Bacillus polymyxa. It was part of a group of antibiotics that offered hope against Gram-negative bacteria. Developed during an era of rapid antibiotic discovery, Polymyxin B soon became recognized for its effectiveness against pathogens that were resistant to penicillin.

Common Uses

Polymyxin B is primarily used to treat infections caused by Gram-negative bacteria, such as Pseudomonas aeruginosa, Acinetobacter, and certain forms of Escherichia coli. It is commonly administered in cases where other antibiotics are ineffective due to resistance. Its usage spans from eye infections to severe systemic infections.

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Key Applications:

  • Eye infections
  • Meningitis
  • Urinary tract infections
  • Septicemia

Mechanism of Action

Polymyxin B operates by binding to the lipopolysaccharides and phospholipids in the outer membrane of Gram-negative bacteria. This binding disrupts the bacterial cell membrane, increasing its permeability, which leads to cell death. This mechanism is crucial in its ability to combat bacteria that are resistant to other antibiotics.

What is Colistin?

Background Information

Colistin, also known as polymyxin E, was discovered shortly after polymyxin B. It is produced by the same Bacillus polymyxa strain but has slightly different chemical properties. Over the years, Colistin has been a critical antibiotic for treating life-threatening infections due to its potent activity.

Primary Applications

The primary use of Colistin is in treating infections by multi-drug resistant Gram-negative bacteria, including those resistant to nearly all other available antibiotics. It is often considered a “last-resort” antibiotic for severe infections such as pneumonia, bloodstream infections, and infections in cystic fibrosis patients.

Key Applications:

  • Pneumonia
  • Bloodstream infections
  • Infections in cystic fibrosis
  • Critical care settings

Action Mechanism

Like Polymyxin B, Colistin disrupts the outer membrane of Gram-negative bacteria. It binds to and displaces magnesium and calcium ions from the membrane, which destabilizes and increases its permeability, leading to rapid bacterial death. This action is vital for treating severe infections where other treatments have failed.

Key Differences

Chemical Structure

Although both antibiotics are derived from the same bacterium, their chemical structures differ slightly, affecting their toxicity and efficacy. Polymyxin B has a direct peptide chain, while Colistin has a cyclic peptide with a fatty acid chain, influencing their binding properties and toxicity profiles.

Spectrum of Activity

Both antibiotics target similar types of bacteria, primarily Gram-negative. However, slight differences in their spectrum of activity exist, with Polymyxin B being less potent but safer for certain patient groups, and Colistin being more effective but associated with higher risks.

Side Effects Comparison

The main concern with these antibiotics is their nephrotoxicity and neurotoxicity. Polymyxin B is generally associated with fewer and less severe nephrotoxic effects compared to Colistin. However, both can cause significant kidney damage and require careful monitoring.

Clinical Uses

Polymyxin B Scenarios

  • Topical applications: Due to its lower systemic toxicity, Polymyxin B is favorable in topical formulations like creams and eye drops.
  • Less severe infections: It’s used in less critical conditions where bacteria show resistance to other antibiotics.
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Colistin Scenarios

  • Severe, life-threatening infections: Colistin is reserved for severe cases where other treatments are ineffective.
  • Hospital settings: Often used in intensive care units to treat critically ill patients.

Comparative Effectiveness

While both antibiotics are effective against a similar range of bacteria, the choice between Polymyxin B and Colistin often depends on the severity of the infection and the patient’s overall health. Polymyxin B’s lower toxicity makes it suitable for less severe infections, whereas Colistin’s potent efficacy is reserved for more critical conditions.

Resistance Concerns

Resistance Mechanisms

Both Polymyxin B and Colistin face challenges with bacterial resistance, which can greatly diminish their effectiveness. The resistance mechanisms include modifications to the target sites on the bacterial membrane, enzymatic degradation, and efflux pump activation. These mechanisms prevent the antibiotic from binding effectively, reducing their ability to kill the bacteria.

Recent Studies and Findings

Recent research has highlighted an increase in resistance to these antibiotics, particularly among carbapenem-resistant bacteria. Studies have shown that repeated and improper use contributes to this resistance, prompting a reevaluation of how these antibiotics are prescribed.

Impact on Antibiotic Choice

The emergence of resistance impacts clinical decisions regarding antibiotic use. It necessitates a more judicious use of Polymyxin B and Colistin, reserving them for cases where alternative treatments have failed. This strategy helps preserve their effectiveness and reduces the development of further resistance.

Administration and Dosage

Forms of Administration

Polymyxin B and Colistin are administered in several forms:

  • Intravenous solutions for severe systemic infections
  • Inhalation solutions for lung infections in cystic fibrosis patients
  • Topical creams and eye drops for local infections

Dosage Variations

Dosage of these antibiotics varies based on the infection severity, patient’s kidney function, and body weight. For instance, dosages must be adjusted for patients with renal impairment to avoid toxicity.

Recommendations for Use

Key recommendations include:

  • Monitoring kidney function before and during treatment
  • Adjusting dosages based on renal and hepatic health
  • Using the minimal effective dose for the shortest period necessary to reduce risk of resistance

Side Effects and Warnings

Common Side Effects of Polymyxin B

  • Kidney toxicity (nephrotoxicity)
  • Neurotoxic effects (tingling, vertigo)
  • Allergic reactions (skin rash, itching)
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Common Side Effects of Colistin

  • Higher incidence of kidney damage
  • Muscle weakness
  • Respiratory difficulties if aerosolized

Managing Side Effects

Effective management includes:

  • Regular monitoring of kidney and neurological functions
  • Adjusting the dose immediately if side effects occur
  • Providing supportive treatments to mitigate symptoms

Global Usage

Usage Trends in Polymyxin B

Polymyxin B has seen a stable use in many regions, particularly in topical and ophthalmic formulations, due to its lower systemic toxicity.

Usage Trends in Colistin

Colistin usage has increased, especially in hospital settings to treat drug-resistant infections. However, concerns over its side effects and rising resistance are influencing its use patterns.

Geographic Variations in Use

There is significant variation in the use of these antibiotics across different countries. Developing nations use them more frequently due to their cost-effectiveness, whereas developed countries are more cautious, using them as last-resort options.

Future Prospects

Ongoing Research and Developments

Current research focuses on developing derivatives of Polymyxin B and Colistin that maintain their effectiveness but have reduced toxicity. Efforts are also aimed at enhancing their efficacy against resistant strains.

Potential New Formulations

Researchers are exploring formulations that can deliver these antibiotics more effectively to infection sites while minimizing systemic exposure and side effects.

Predictions for Future Use

The future use of Polymyxin B and Colistin will likely involve more targeted applications, advanced formulations, and combination therapies to manage resistance. Predictions suggest a careful balance between effective use and resistance management will shape their roles in modern medicine.

Frequently Asked Questions

What are Polymyxin B and Colistin?

Polymyxin B and Colistin are antibiotics that combat infections caused by Gram-negative bacteria. They are particularly valuable in treating infections resistant to typical antibiotics, making them critical in certain clinical situations.

How do Polymyxin B and Colistin work?

Both antibiotics target the bacterial cell membrane but do so in slightly different ways. They bind to the lipid A component of lipopolysaccharides, disrupting the cell membrane and leading to cell death. This action makes them effective against bacteria that are resistant to other antibiotics.

When should Polymyxin B versus Colistin be used?

Polymyxin B is typically used for less severe infections or as a topical treatment due to its lower toxicity. Colistin, on the other hand, is used for more severe infections where other treatments have failed, despite its potential for causing kidney damage.

What are the main side effects of Colistin and Polymyxin B?

The most significant side effect of both antibiotics is nephrotoxicity, although Colistin is more likely to cause kidney damage. Other side effects include neurotoxicity and, in some cases, respiratory failure if administered improperly.


Understanding the differences between Polymyxin B and Colistin is crucial for effectively treating severe bacterial infections while minimizing potential risks. These antibiotics serve as vital options in the arsenal against antibiotic-resistant bacteria, each playing a unique role depending on the severity and specifics of the infection.

The careful distinction between these two agents ensures that healthcare providers can tailor treatments to achieve the best outcomes while managing the associated risks. As antibiotic resistance continues to evolve, the roles of Polymyxin B and Colistin will likely become even more critical in clinical settings, emphasizing the need for ongoing research and judicious use.

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