A friend of mine was studying for her pharmacology exam and stared at a practice question that asked, “which of the following is not true of polymyxins.” She laughed, said she’d never even heard the name before, and wondered why anyone would care about an old‑school antibiotic. I told her the story behind these drugs is actually a pretty wild ride — from forgotten last‑resort agents to modern lifesavers for multidrug‑resistant bugs. If you’ve ever wondered why some antibiotics seem to disappear from textbooks only to reappear in ICU corridors, keep reading.
What Is Polymyxins
Polymyxins are a small family of cyclic peptide antibiotics produced by certain strains of Paenibacillus* (formerly Bacillus*) bacteria. The two members you’ll see most often in clinical settings are polymyxin B and polymyxin E, better known as colistin. They’re not the kind of drug you’d reach for a simple strep throat; instead, they’re reserved for infections caused by Gram‑negative pathogens that have become resistant to almost everything else.
Chemically, polymyxins consist of a fatty acid tail attached to a cyclic peptide ring. That tail is what lets them insert into bacterial membranes, and it’s also the piece that causes most of their toxicity in humans. Because they’re large, hydrophilic molecules, they don’t absorb well from the gut, which is why colistin is usually given intravenously or inhaled for lung infections, while polymyxin B is primarily used topically or systemically in a similar IV fashion.
Why It Matters
You might ask why we still bother with a drug that was first discovered in the 1940s. Practically speaking, the answer is simple: resistance. Over the past two decades, carbapenem‑resistant Enterobacteriaceae* (CRE), Pseudomonas aeruginosa*, and Acinetobacter baumannii* have spread worldwide. When standard antibiotics fail, clinicians sometimes have no other option but to pull polymyxins out of the shelf.
Their importance goes beyond the bedside, too. Studying polymyxin resistance mechanisms has revealed new pathways — like modifications to lipid A — that bacteria use to evade not just polymyxins but also host antimicrobial peptides. Because polymyxins act on the bacterial outer membrane, they’ve become a valuable tool for researchers trying to understand how Gram‑negative bacteria protect themselves. In short, these old drugs are teaching us how superbugs evolve, and that knowledge helps shape the next generation of antibiotics.
How It Works
Membrane Disruption
Polymyxins are cationic, meaning they carry a positive charge at physiological pH. Worth adding: the outer membrane of Gram‑negative bacteria is rich in negatively charged molecules like lipopolysaccharide (LPS). The drug’s positive groups bind to the phosphate groups of LPS, displacing essential divalent cations (calcium and magnesium) that normally stabilize the membrane.
Once those bridges are broken, the polymyxin molecule inserts its fatty‑acid tail into the lipid bilayer, creating a sort of detergent‑like effect. The membrane becomes leaky, allowing intracellular contents to spill out and the proton gradient to collapse. The bacterium can’t maintain its internal environment, and it dies quickly.
Spectrum of Activity
Because the target is the outer membrane, polymyxins are effective against most Gram‑negative organisms, including Pseudomonas*, Acinetobacter*, Klebsiella*, Enterobacter*, and many Proteus* strains. They have little to no activity against Gram‑positive bacteria, anaerobes, or atypical pathogens like Mycoplasma* or Chlamydia*.
Pharmacokinetics and Dosing
Colistin is often administered as colistin methanesulfonate (CMS), an inactive prodrug that gets converted to active colistin in the body. The conversion is variable, which makes therapeutic drug monitoring important — especially in critically ill patients where kidney function can change rapidly. Here's the thing — typical dosing ranges from 2. On top of that, 5 to 5 mg/kg of colistin base activity per day, divided into two or three IV doses. Inhaled forms for ventilator‑associated pneumonia use roughly 1‑2 million units nebulized two to three times daily.
Polymyxin B doesn’t require a prodrug step; it’s given directly as the active sulfate salt. Dosing is similar, usually 2.Day to day, 5 mg/kg per day in divided doses. Both drugs are primarily cleared by the kidneys, so dose adjustments are necessary in renal impairment.
Resistance Mechanisms
Bacteria can thwart polymyxins by altering the lipid A component of LPS. Think about it: common modifications include addition of phosphoethanolamine or lysine residues, which reduce the net negative charge and thus lower drug binding. These changes are often mediated by genes such as mcr‑1* (mobile colistin resistance) or chromosomal regulators like pmrAB* and phoPQ*. The spread of plasmid‑borne mcr genes has raised alarms because it can transfer resistance between species quickly.
Common Mistakes / What Most People Get Wrong
Assuming Polymyxins Are Broad‑Spectrum
One frequent error is thinking polymyxins work like a typical “broad‑spectrum” antibiotic that covers both Gram‑positives and Gram‑negatives. In reality, their spectrum is narrow — limited to Gram‑negative organisms. Using them for a Gram‑positive infection (say, MRSA) would be ineffective and unnecessarily expose the patient to toxicity.
Want to learn more? We recommend for rna is the t a u and the journal of physical chemistry c impact factor for further reading.
Overlooking Neuro‑ and Nephrotoxicity
Because polymyxins were shelved for decades, many clinicians forget how toxic they can be. Neurotoxicity (tingling, numbness, weakness) and nephrotoxicity (acute
nephrotoxicity (acute tubular necrosis) and neurotoxicity (paresthesia, ataxia, or even seizures). Even so, these adverse effects are dose‑related and tend to accumulate with prolonged therapy or in patients with pre‑existing renal disease. Monitoring serum creatinine, urine output, and, when available, plasma colistin concentrations helps to strike a balance between efficacy and safety.
Practical Guidance for Clinicians
| Situation | Preferred Polymyxin | Key Points |
|---|---|---|
| Ventilator‑associated pneumonia (VAP) due to MDR Pseudomonas or Acinetobacter** | Polymyxin B or inhaled colistin | Inhaled therapy is often combined with IV dosing; aim for a total daily dose of 2. |
| Patients with chronic kidney disease (CrCl < 30 mL/min) | Colistin | Reduce the maintenance dose by 50 % and extend the dosing interval; consider therapeutic drug monitoring. Consider this: , meningitis) with MDR organisms** |
| Severe sepsis from carbapenem‑resistant Enterobacterales | Colistin (CMS) | Initiate with a loading dose of 2 mg/kg (colistin base activity) to quickly achieve therapeutic levels; adjust for CrCl. 5 mg/kg. |
| **CNS infection (e.On top of that, 5 mg/kg IV every 12 h. g. | ||
| Patients with hepatic impairment | Either | Polymyxin B is not renally cleared; colistin dose should be unchanged, but monitor for neurotoxicity. |
When to Avoid Polymyxins
- Non‑Gram‑negative infections (e.g., MRSA, Enterococcus*).
- Infections where a safer, narrower‑spectrum agent is available (e.g., carbapenems for Klebsiella* if susceptible).
- Patients with severe pre‑existing neuropathy or renal failure who cannot tolerate dose adjustments.
Emerging Trends and Future Directions
- Novel Formulations – Liposomal colistin and nanoparticle‑based delivery systems aim to reduce systemic toxicity while maintaining high local concentrations.
- Combination Therapies – Studies show synergistic activity when polymyxins are paired with fosfomycin, aminoglycosides, or tigecycline, potentially lowering the required dose of each agent.
- Rapid Diagnostics – Point‑of‑care PCR for mcr genes can inform early de‑escalation or escalation, minimizing unnecessary exposure.
- Pharmacokinetic/Pharmacodynamic (PK/PD) Modeling – Adaptive dosing algorithms that incorporate real‑time creatinine clearance and plasma levels are becoming available in some ICUs.
Key Take‑Home Messages
| ✅ | Insight |
|---|---|
| Polymyxins are not a pan‑panacea; they target Gram‑negative bacteria with a compromised outer membrane. | |
| Resistance is rising – plasmid‑borne mcr genes threaten the last line of defense; stewardship is essential. | |
| **CMS vs. On top of that, | |
| Toxicity is real – monitor renal function and neurologic status closely, especially with prolonged therapy. On the flip side, polymyxin B** – CMS requires careful conversion monitoring; polymyxin B offers a more predictable PK profile. | |
| Use only when necessary – reserve polymyxins for MDR Gram‑negative infections where alternatives are lacking or have failed. |
Conclusion
Polymyxins, long considered a “last‑resort” weapon, have re‑entered mainstream practice as the global rise of carbapenem‑resistant Gram‑negative pathogens forces clinicians to look beyond first‑line agents. Their potent activity against difficult‑to‑treat organisms is tempered by a narrow spectrum and a steep toxicity profile that demands vigilant dosing, monitoring, and stewardship. When used judiciously—guided by susceptibility data, patient renal function, and emerging PK/PD tools—polymyxins can bridge the therapeutic gap and save lives in the era of antimicrobial resistance. The future will likely see refined formulations, smarter dosing algorithms, and, critically, a renewed commitment to preserving their efficacy through responsible use.
