---
title: "Auxotroph — AP Biology Definition & Exam Guide"
description: "An auxotroph is an organism that lost the ability to make a needed compound, so it must get that compound from its environment. Learn how mutations cause it for AP Bio."
canonical: "https://fiveable.me/ap-bio/key-terms/auxotroph"
type: "key-term"
subject: "AP Biology"
unit: "Unit 6"
---

# Auxotroph — AP Biology Definition & Exam Guide

## Definition

In AP Biology, an auxotroph is an organism (usually a bacterium) that carries a mutation disabling its ability to synthesize a specific compound, such as an amino acid, so it can only grow when that compound is supplied in its medium.

## What It Is

An **auxotroph** is an organism that can't make some molecule it needs to live, like a particular [amino acid](/ap-bio/key-terms/amino-acid "fv-autolink") or vitamin. The original ("wild-type") strain could build that molecule from scratch. A mutation broke the gene for one of the [enzymes](/ap-bio/unit-3/enzyme-structure/study-guide/jsjBfuk2jmYAZVrmVwtF "fv-autolink") in that pathway, and now the organism has to get the finished product handed to it in the growth medium.

This ties straight into **[Topic 6.7](/ap-bio/unit-6/mutations/study-guide/WIuGA11Yy2RsVq8JpSnt "fv-autolink") (Mutations)**. A change in the DNA sequence alters the protein produced, which changes the phenotype (EK 6.7.A.1). For an auxotroph, the broken protein is an enzyme. A point mutation, frameshift, or nonsense mutation can knock out that enzyme so it no longer works, and the organism loses a metabolic ability it used to have. The flip side, a **prototroph**, makes everything it needs on a minimal medium with no extras.

## Why It Matters

Auxotrophs live in **[Unit 6](/ap-bio/unit-6 "fv-autolink"): Gene Expression and Regulation**, specifically Topic 6.7. They're a clean, concrete example of the chain the CED wants you to trace: a DNA mutation changes a protein, and the changed protein changes the phenotype (EK 6.7.A.1, learning objective [AP Bio](/ap-bio "fv-autolink") 6.7.A). You can also link them to AP Bio 6.7.B, since whether losing that pathway hurts the organism depends entirely on the environment. On a plate full of the missing amino acid, an auxotroph is perfectly fine. On a minimal plate, it can't grow. That environment-dependent cost is exactly the idea behind "beneficial, detrimental, or neutral depends on the environmental context" (EK 6.7.B.1).

## Connections

### [Auxotrophic Mutant (Unit 6)](/ap-bio/key-terms/auxotrophic-mutant)

These are basically the same idea named two ways. The [mutation](/ap-bio/key-terms/mutation "fv-autolink") is the cause; the auxotroph is the organism that results. If a strain has an auxotrophic mutation in its tryptophan pathway, that strain IS a tryptophan auxotroph.

### Nonsense and Frameshift Mutations (Unit 6)

Auxotrophy is what those mutation types look like in real life. A nonsense or [frameshift mutation](/ap-bio/key-terms/frameshift-mutation "fv-autolink") in an enzyme gene can shut down that enzyme entirely, which is exactly how a working biosynthesis pathway breaks and turns a prototroph into an auxotroph.

### Natural Selection and Environmental Context (Unit 7)

Losing a pathway isn't automatically bad. If the missing nutrient is always available, the auxotroph saves [energy](/ap-bio/unit-3/environmental-impacts-on-enzyme-function/study-guide/Q8PevM3BI76060aoWtit "fv-autolink") not building it and may even outcompete the wild type. The same mutation that's harmful in one environment can be favored in another (EK 6.7.B.1).

## On the AP Exam

Auxotroph is a vocabulary anchor for cause-and-effect reasoning about mutations. Expect multiple-choice stems describing a bacterium that grows on "complete" medium but not "minimal" medium, then asking you to identify that the strain has a mutation in a biosynthesis gene. The skill is connecting genotype to phenotype: a broken gene means a broken enzyme means an organism that can't make a needed compound. No released FRQ uses the word "auxotroph" verbatim, but the underlying logic, that a DNA change alters a protein and therefore the phenotype, is exactly what Topic 6.7 free-response prompts reward. If asked, be ready to say whether the mutation is harmful, neutral, or beneficial and explain that it depends on the environment.

## auxotroph vs prototroph

A prototroph is the wild-type that can synthesize everything it needs and grows on bare minimal medium. An auxotroph lost one of those abilities to a mutation and needs the missing compound supplied. Quick test: minimal medium only feeds prototrophs; auxotrophs need the extra ingredient.

## Key Takeaways

- An auxotroph is an organism that can't synthesize a compound it needs and must get it from the growth medium.
- Auxotrophy is caused by a mutation that breaks an enzyme in a biosynthesis pathway, a direct example of how genotype changes phenotype (EK 6.7.A.1).
- A prototroph makes everything it needs on minimal medium; an auxotroph requires the missing nutrient to be added.
- Whether losing the pathway is harmful or neutral depends on the environment, which ties into EK 6.7.B.1.
- On the test, a strain that grows on complete medium but not minimal medium is the classic signal of an auxotroph.

## FAQs

### What is an auxotroph in AP Biology?

It's an organism, usually a bacterium, that carries a mutation disabling its ability to make a needed molecule like an amino acid, so it can only grow when that molecule is provided in its medium. It's a Topic 6.7 example of a mutation changing phenotype.

### Is being an auxotroph always bad for the organism?

No. If the missing nutrient is always available in the environment, the auxotroph is fine and may even save energy by not building it. The cost only shows up when that nutrient is gone, which is the environment-dependent idea in EK 6.7.B.1.

### What's the difference between an auxotroph and a prototroph?

A prototroph is the wild-type that can synthesize everything it needs and grows on plain minimal medium. An auxotroph lost one of those abilities to a mutation and needs the missing compound supplied to grow.

### What kind of mutation creates an auxotroph?

Any mutation that disables an enzyme in a biosynthesis pathway can do it, including point, nonsense, or frameshift mutations. The broken enzyme means the pathway stops, so the organism can no longer make its product.

### How do you tell if a bacterial strain is an auxotroph on a test question?

Look for a strain that grows on complete (or supplemented) medium but fails to grow on minimal medium. That gap means it can't make something on its own, which signals an auxotroph.

## Related Study Guides

- [6.7 Mutations](/ap-bio/unit-6/mutations/study-guide/WIuGA11Yy2RsVq8JpSnt)

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