--- title: "Facultative Anaerobes | Microbiology" description: "Facultative anaerobes are microbes that grow with or without oxygen, switching between aerobic respiration and fermentation in Microbiology." canonical: "https://fiveable.me/microbio/key-terms/facultative-anaerobes" type: "key-term" subject: "Microbiology" unit: "Unit 9" --- # Facultative Anaerobes | Microbiology ## Definition Facultative anaerobes are microorganisms that can grow with oxygen or without it. In Microbiology, they switch to aerobic respiration when oxygen is available and use anaerobic pathways like fermentation when it is not. ## What It Is Facultative anaerobes are microbes in Microbiology that can live and grow in both oxygen-rich and oxygen-poor environments. If oxygen is present, they usually use aerobic respiration because it makes more ATP. If oxygen disappears, they can keep going by switching to anaerobic respiration or fermentation. That switch is the whole idea behind the term. These organisms are not tied to oxygen the way obligate aerobes are, and they are not harmed by oxygen the way obligate anaerobes can be. Instead, they adjust their metabolism based on what is available in the environment. The reason this works is that facultative anaerobes have the enzymes and pathways needed for both conditions. With oxygen, they can run glycolysis, the Krebs cycle, and an electron transport chain. Without oxygen, they cannot rely on oxygen as the final electron acceptor, so they shift to pathways that regenerate NAD+ in other ways, often through fermentation. In a lab, this flexibility shows up in growth patterns. A facultative anaerobe can grow throughout a tube or plate, but it often grows better where oxygen is available because aerobic respiration yields more energy. That extra ATP means faster growth when oxygen is around. A classic example is Escherichia coli, which lives in the gut, where oxygen levels are low and change from place to place. Staphylococcus aureus is another common example. These organisms can survive in body sites, soil, and water because they are not limited to one oxygen condition. A common misconception is that facultative anaerobes "prefer no oxygen." That is not quite right. They can survive without oxygen, but many of them actually grow more efficiently with oxygen present. "Facultative" means flexible, not indifferent. ## Why It Matters Facultative anaerobes come up again and again in Microbiology because they connect oxygen requirements to metabolism, growth, and habitat. Once you know a microbe is facultative, you can predict where it might live, how it will behave in culture, and why it can persist in changing environments. This term also helps you make sense of lab results. On an oxygen gradient or in a culture tube, a facultative anaerobe often grows strongest near the top but still shows growth lower down. That pattern tells you the organism can use oxygen when it is available, but it does not depend on it. The concept also connects directly to fermentation. When oxygen is absent, facultative anaerobes need another way to keep glycolysis running, so they regenerate NAD+ through anaerobic pathways. That is the same basic survival strategy you see in fermentation questions, from lactic acid production to alcoholic fermentation in microbes. In medicine and environmental microbiology, the term helps explain why some bacteria spread so successfully across different sites in the body or different habitats in nature. If an organism can keep growing through oxygen changes, it has more chances to colonize, persist, and cause disease or influence nutrient cycles. ## Connections ### Obligate Aerobes Obligate aerobes need oxygen to grow because they depend on aerobic respiration for energy. Facultative anaerobes are the opposite in flexibility, since they can use oxygen but do not require it. Comparing the two helps you identify an organism’s oxygen preference from a lab pattern or growth description. ### Obligate Anaerobes Obligate anaerobes cannot tolerate oxygen well and grow only when oxygen is absent. Facultative anaerobes differ because oxygen does not stop their growth, and often improves it. This comparison is useful when you are looking at oxygen requirements in a tube or trying to classify an unknown microbe. ### Fermentation Fermentation is one of the main ways facultative anaerobes survive when oxygen is unavailable. It lets them keep glycolysis running by regenerating NAD+. In Microbiology, this connection shows up when you trace how a microbe makes ATP under low-oxygen conditions. ### [ATP (Adenosine Triphosphate)](/microbio/key-terms/atp-adenosine-triphosphate) ATP production explains why facultative anaerobes often grow better with oxygen. Aerobic respiration produces more ATP than fermentation, so the same organism can have very different growth rates depending on the environment. This makes ATP yield a useful way to compare metabolic pathways. ## On the AP Exam A quiz question might ask you to identify an organism from its oxygen-growth pattern, or to explain why one microbe grows better near the top of a tube but still grows lower down. In a lab report, you may need to connect that pattern to aerobic respiration and fermentation. If you are given a case study about a bacterium living in the gut, soil, or a wound, facultative anaerobe is often the classification you check first. You may also need to compare it with obligate aerobes and obligate anaerobes. The clean move is to tie the oxygen condition to the energy pathway the microbe is using, then explain whether oxygen is required, tolerated, or avoided. ## Facultative Anaerobes vs Obligate Anaerobes These two are easy to mix up because both can grow without oxygen, but they do not behave the same way in oxygen-rich environments. Facultative anaerobes can still grow when oxygen is present and usually grow better with it. Obligate anaerobes are harmed by oxygen and must live where oxygen is absent. ## Key Takeaways - Facultative anaerobes are microbes that can grow with oxygen or without it. - They usually prefer aerobic respiration when oxygen is available because it produces more ATP. - When oxygen is scarce, they switch to anaerobic respiration or fermentation to keep making energy. - Their oxygen flexibility helps them survive in changing habitats like the gut, soil, and water. - In lab work, they often show the strongest growth where oxygen is present, but they are not limited to that area. ## FAQs ### What is a facultative anaerobe in Microbiology? A facultative anaerobe is a microbe that can grow either with oxygen or without oxygen. In Microbiology, that usually means it uses aerobic respiration when oxygen is present and switches to fermentation or another anaerobic pathway when it is not. ### How is a facultative anaerobe different from an obligate anaerobe? A facultative anaerobe can tolerate oxygen and often grows better with it, while an obligate anaerobe cannot handle oxygen well. That difference shows up in oxygen-growth tests, where facultative anaerobes grow in both conditions and obligate anaerobes only grow where oxygen is absent. ### Why do facultative anaerobes grow better with oxygen? Oxygen lets them use aerobic respiration, which makes more ATP than fermentation or other anaerobic pathways. More ATP usually means faster growth, so even though they can survive without oxygen, they often thrive when oxygen is available. ### What is an example of a facultative anaerobe? Escherichia coli is a classic example, and Staphylococcus aureus is another common one. Both can survive in environments where oxygen levels change, which is why facultative metabolism is such a useful adaptation. ## Related Study Guides - [9.2 Oxygen Requirements for Microbial Growth](/microbio/unit-9/2-oxygen-requirements-microbial-growth/study-guide/0d3nmhcM3ewVHDg8) - [8.4 Fermentation](/microbio/unit-8/4-fermentation/study-guide/ur4ulWfFaG3QOfHE) ## 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/facultative-anaerobes#resource","name":"Facultative Anaerobes | Microbiology","url":"https://fiveable.me/microbio/key-terms/facultative-anaerobes","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/microbio/key-terms/facultative-anaerobes#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:23:49.053Z","isPartOf":{"@type":"Collection","name":"Microbiology Key Terms","url":"https://fiveable.me/microbio/key-terms"},"author":{"@type":"Organization","@id":"https://fiveable.me/editorial#content-team","name":"Fiveable Content Team","url":"https://fiveable.me/editorial","parentOrganization":{"@id":"https://fiveable.me/#organization"},"member":[{"@type":"Person","name":"Amanda DoAmaral","sameAs":["https://www.linkedin.com/in/adoamaral"]},{"@type":"Person","name":"Becky Bahr","sameAs":["https://www.linkedin.com/in/becky-bahr-b52743132"]}]},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/microbio/key-terms/facultative-anaerobes#term","name":"Facultative Anaerobes","description":"Facultative anaerobes are microorganisms that can grow with oxygen or without it. 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