Colistin

Colistin is a polymyxin antibiotic used against multidrug-resistant Gram-negative bacteria. In Microbiology, you study it as a membrane-targeting drug that binds LPS and can kill bacteria that resist many other antibiotics.

What is colistin?

Colistin is a last-resort antibiotic in Microbiology that attacks the outer membrane of Gram-negative bacteria. It belongs to the polymyxin group and is usually brought up when a pathogen resists safer, more common drugs.

Its target is the lipopolysaccharide, or LPS, in the outer membrane. Gram-negative bacteria have this extra outer layer, and colistin sticks to it, destabilizes the membrane, and makes it leaky. Once the membrane loses integrity, the cell cannot control what enters and leaves, so it dies.

That mechanism is very different from antibiotics that block the cell wall, ribosomes, or DNA replication. Colistin does not need to enter the cell first to do its job. Instead, it damages the bacterial surface from the outside, which is why it can work against organisms that have picked up resistance to other antibiotic classes.

You also need to know why it is reserved for serious cases. Colistin can injure human kidneys and nervous tissue, so it is not a casual choice for routine infections. In a microbiology course, that makes it a good example of selective toxicity with a tradeoff: the drug hits a bacterial structure, but the side effects can still be harsh.

Resistance to colistin often comes from changes in the bacterial LPS, which reduce how well the drug can bind. A well-known example is the mcr-1 gene, which can help bacteria alter their surface and survive exposure. That is why colistin shows up in discussions of antimicrobial resistance, especially when instructors connect drug mechanism to treatment failures.

When labs test colistin, they usually use broth microdilution rather than simpler diffusion methods. That detail matters because colistin does not behave like many other antibiotics in a disk test, so the testing method has to match the drug's pharmacodynamics.

Why colistin matters in MICROBIO

Colistin shows how Microbiology links bacterial structure, drug action, and resistance in one example. If you know that it targets LPS in the outer membrane, you can explain why it works only on Gram-negative bacteria and why it is saved for difficult infections.

It also gives you a concrete way to compare antibiotic classes. Some drugs block peptidoglycan synthesis, some stop protein synthesis, and colistin disrupts membrane integrity. That comparison shows up when you sort antibiotics by mechanism or explain why one class may fail against a particular pathogen.

This term also comes up in the antibiotic resistance unit because resistance can arise from changing the LPS target itself. That makes colistin a useful case study for how bacteria escape treatment, how treatment choices narrow over time, and why labs need accurate susceptibility testing before a clinician commits to therapy.

How colistin connects across the course

Antimicrobial Stewardship

Colistin is a good example of why stewardship matters. Because it is toxic and often reserved for resistant infections, you do not want it used when a safer, narrower drug would work. Stewardship thinking asks you to match the drug to the organism, use susceptibility data, and avoid unnecessary exposure that can speed up resistance.

mcr-1 gene

The mcr-1 gene is tied to colistin resistance. It can help bacteria modify their LPS so colistin binds less effectively, which means the drug loses its membrane-disrupting power. When you see mcr-1 in a case study, think about target modification and how a resistance gene can change a treatment outcome.

Nephrotoxicity

Nephrotoxicity is one of the main reasons colistin is used carefully. In microbiology and clinical discussions, the drug is not just evaluated by whether it kills bacteria, but also by whether the benefit outweighs the risk of kidney injury. That tradeoff is a classic part of antibiotic selection.

Antimicrobial Gradient Method

Colistin is often discussed alongside susceptibility testing methods, but not every method works well for every drug. The antimicrobial gradient method is useful to know as a comparison point because it measures a concentration range on agar, while colistin is usually tested with broth microdilution. That difference shows how method choice affects results.

Is colistin on the MICROBIO exam?

A quiz or lab question may ask you to identify colistin from a mechanism description, then explain why it works on Gram-negative bacteria but not Gram-positive bacteria. You might also see a case study with a multidrug-resistant isolate and need to connect colistin to last-resort therapy, nephrotoxicity, or LPS binding.

In susceptibility-testing questions, the move is to choose the correct method and justify it. If the prompt mentions colistin, think about broth microdilution and why a disk diffusion-style result could be unreliable. In resistance questions, link a failed treatment to LPS modification or the mcr-1 gene. If you can trace drug target, bacterial structure, and resistance mechanism in that order, you have the right answer path.

Colistin vs Polymyxin B

Polymyxin B and colistin are closely related polymyxin antibiotics, so they are easy to mix up. Both attack Gram-negative outer membranes and both can be toxic, but colistin is the one most often described as a last-resort option for certain resistant infections. If a question asks about membrane disruption through LPS binding, either may sound similar, so pay attention to the exact drug named in the prompt.

Key things to remember about colistin

• Colistin is a polymyxin antibiotic used against multidrug-resistant Gram-negative bacteria.

• Its main target is LPS in the outer membrane, where it disrupts membrane integrity and kills the cell.

• Because it can cause nephrotoxicity and neurotoxicity, it is usually reserved for serious infections.

• Resistance can happen when bacteria modify LPS, including changes linked to the mcr-1 gene.

• In lab work, colistin is commonly tested with broth microdilution because its behavior does not fit every standard susceptibility method.

Frequently asked questions about colistin

What is colistin in Microbiology?

Colistin is a polymyxin antibiotic used as a last-resort treatment for some multidrug-resistant Gram-negative infections. It works by binding to LPS in the outer membrane and damaging the bacterial cell envelope.

How does colistin kill bacteria?

Colistin binds to the lipopolysaccharide layer on Gram-negative bacteria and destabilizes the outer membrane. Once the membrane becomes leaky, the cell can no longer maintain homeostasis and dies.

Why is colistin reserved for resistant infections?

It is effective against some bacteria that resist other drugs, but it can also harm human kidneys and nerves. That toxicity makes it a drug you save for situations where safer antibiotics will not work.

How is colistin resistance detected or explained?

A common explanation is altered LPS that reduces colistin binding, including resistance linked to the mcr-1 gene. In the lab, colistin is often tested with broth microdilution because that method gives more reliable susceptibility data for this drug.