--- title: "Amphotericin B | Microbiology" description: "Amphotericin B is a broad-spectrum antifungal that binds ergosterol and disrupts fungal membranes, making it a major drug in Microbiology." canonical: "https://fiveable.me/microbio/key-terms/amphotericin" type: "key-term" subject: "Microbiology" unit: "Unit 14" --- # Amphotericin B | Microbiology ## Definition Amphotericin B is a polyene antifungal drug in Microbiology that binds ergosterol in fungal cell membranes, making them leaky and killing the cell. ## What It Is Amphotericin B is a powerful antifungal drug in Microbiology, best known for treating serious fungal infections that can spread beyond the lungs or bloodstream. It is usually reserved for severe cases because it is effective, but also harsh on the body. Its main target is ergosterol, a sterol found in fungal cell membranes. When amphotericin B binds to ergosterol, it disrupts membrane structure and can form pores or otherwise make the membrane leaky. Once the membrane loses control over what enters and leaves the cell, the fungus cannot maintain normal balance and dies. That membrane target is what makes the drug so useful. Human cells use cholesterol instead of ergosterol, so amphotericin B is selective for fungi, even though that selectivity is not perfect. The overlap is why the drug can damage human tissues too, especially the kidneys. In microbiology classes, amphotericin B usually shows up when you are comparing antifungal mechanisms. Some antifungals block ergosterol synthesis, while amphotericin B binds the finished sterol directly. That difference matters because it changes how fast the drug works, where resistance can show up, and why the side effects are so strong. You will also see it tied to invasive infections such as cryptococcosis and invasive aspergillosis, especially in immunocompromised patients. In those cases, the goal is not just to slow growth, but to rapidly reduce a life-threatening fungal burden. Because it is so toxic, amphotericin B is often used when the infection is severe enough that the benefit outweighs the risk. In lab or lecture questions, if you see a drug that binds ergosterol and disrupts fungal membranes, amphotericin B is usually the answer. ## Why It Matters Amphotericin B matters because it is a clean example of how antimicrobial drugs exploit differences between microbial cells and human cells. In Microbiology, that makes it a useful bridge between cell structure, drug action, and clinical treatment choices. It also helps you compare antifungal classes. If a question asks whether a drug blocks ergosterol synthesis, binds ergosterol, or attacks a fungal wall component, amphotericin B fits the direct membrane-binding pattern. That distinction shows up in mechanism tables, matching questions, and case studies about invasive fungal disease. The drug is also a good reminder that effectiveness and toxicity usually travel together in antimicrobial therapy. Amphotericin B can save a patient with a dangerous fungal infection, but the kidney toxicity and infusion reactions mean clinicians have to monitor closely and sometimes choose other drugs when the infection is less severe. In respiratory mycoses, this term often signals a serious infection rather than a mild one. If amphotericin B is mentioned in a case, you should think about an immunocompromised patient, a deep fungal infection, and why a broad-spectrum antifungal is needed quickly. ## Connections ### Polyene Macrolides Amphotericin B belongs to the polyene macrolide group, so this term tells you the drug’s chemical family as well as its action. If you remember that polyenes bind sterols in fungal membranes, you can connect amphotericin B to other drugs in the same class and predict a similar membrane-targeting mechanism. ### Ergosterol Ergosterol is the fungal membrane sterol that amphotericin B binds. This connection is central because the drug does not target just any membrane, it targets a fungal-specific lipid. If you understand ergosterol, you can explain both the drug’s selectivity and why fungal cells are damaged while human cells are relatively spared. ### Fungal Cell Membrane The fungal cell membrane is where amphotericin B does its damage. Instead of blocking a metabolic pathway, it destabilizes the membrane itself, which quickly leads to leakage and cell death. That makes it a good term to compare with drugs that act on cell walls or enzyme pathways. ### [Antifungal Resistance](/microbio/key-terms/antifungal-resistance) Antifungal resistance matters here because changes in membrane composition can reduce how well amphotericin B works. Even though resistance is less common than with some other antifungals, it can still show up in difficult infections. This is the term to think about when a treatment stops working as expected. ## On the AP Exam A quiz question may ask you to match amphotericin B with its mechanism, side effects, or clinical use. You should identify it as a polyene antifungal that binds ergosterol, disrupts fungal membranes, and is used for severe invasive infections. If the question gives a patient case, look for an immunocompromised person, a respiratory or bloodstream fungal infection, and a drug choice that is strong enough to be reserved for serious disease. In a short answer or discussion prompt, you might explain why the drug is effective but toxic, then connect that to membrane selectivity. In lab or image-based work, you may not see the drug itself, but you may be asked to interpret why a particular antifungal was chosen or why kidney monitoring is part of treatment. ## Key Takeaways - Amphotericin B is a broad-spectrum antifungal used for serious fungal infections, especially invasive ones. - It works by binding ergosterol in the fungal cell membrane and making the membrane leaky. - Because human cells use cholesterol instead of ergosterol, the drug is selective, but not harmless. - Kidney toxicity and infusion reactions are common reasons this drug is reserved for severe cases. - If a Microbiology question mentions a drug that attacks fungal membranes directly, amphotericin B is the one to think about. ## FAQs ### What is Amphotericin B in Microbiology? Amphotericin B is a polyene antifungal drug that kills fungi by binding ergosterol in their cell membranes. In Microbiology, it comes up as a treatment for severe invasive fungal infections, especially when rapid and broad antifungal action is needed. ### How does Amphotericin B work? It binds ergosterol in the fungal cell membrane and disrupts membrane integrity. That makes the membrane leaky, so the cell loses control over ions and other molecules and eventually dies. ### Why is Amphotericin B toxic? The drug is selective for ergosterol, but it can still affect human tissues enough to cause side effects. Kidney toxicity is the big one to remember, and infusion-related reactions can also happen during treatment. ### Is Amphotericin B an antifungal or antibiotic? It is an antifungal drug, though it is often described as a polyene macrolide antibiotic in older terminology. For Microbiology, the important part is that it targets fungi, not bacteria. ## Related Study Guides - [14.4 Mechanisms of Other Antimicrobial Drugs](/microbio/unit-14/4-mechanisms-antimicrobial-drugs/study-guide/7T11XM2vS6zHCitw) - [25.4 Parasitic Infections of the Circulatory and Lymphatic Systems](/microbio/unit-25/4-parasitic-infections-circulatory-lymphatic-systems/study-guide/Gfr4tyMMW7bIpAjf) - [22.4 Respiratory Mycoses](/microbio/unit-22/4-respiratory-mycoses/study-guide/PCs821KkDXNVT2qY) ## About This Document Canonical Fiveable pages are available as Markdown at the same path plus `.md`. - [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns - [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing - [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`) - [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/microbio/key-terms/amphotericin#resource","name":"Amphotericin B | Microbiology","url":"https://fiveable.me/microbio/key-terms/amphotericin","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/microbio/key-terms/amphotericin#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:23:45.862Z","isPartOf":{"@type":"Collection","name":"Microbiology Key Terms","url":"https://fiveable.me/microbio/key-terms"},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/microbio/key-terms/amphotericin#term","name":"Amphotericin B","description":"Amphotericin B is a polyene antifungal drug in Microbiology that binds ergosterol in fungal cell membranes, making them leaky and killing the cell.","url":"https://fiveable.me/microbio/key-terms/amphotericin","inDefinedTermSet":{"@type":"DefinedTermSet","name":"Microbiology Key Terms","url":"https://fiveable.me/microbio/key-terms"}},{"@type":"FAQPage","mainEntity":[{"@type":"Question","name":"What is Amphotericin B in Microbiology?","acceptedAnswer":{"@type":"Answer","text":"Amphotericin B is a polyene antifungal drug that kills fungi by binding ergosterol in their cell membranes. 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