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Quiz: Meet the Microbiome

Test your understanding of Chapter 3. Choose the best answer, then click Show Answer to check.

Question 1

According to the 2016 estimate by Sender, Fuchs, and Milo, how do bacterial cells compare with human cells in a typical adult?

  1. Bacteria outnumber human cells about 10 to 1.
  2. Human cells outnumber bacteria about 10 to 1.
  3. There are roughly as many bacterial cells as human cells, about 1.3 to 1.
  4. Bacteria make up about half of the body's weight.
Show Answer

The correct answer is C. The estimate is about 38 trillion bacteria and 30 trillion human cells, roughly 1.3 to 1. Option A is the old "10 to 1" figure, which the 2016 analysis showed is wrong. The bacteria weigh only about 0.2 kilograms in total, so D is also wrong.

Concept Tested: Bacteria-to-Human Cell Ratio Bloom's Level: Remember

Question 2

What is a bacterial strain?

  1. A genetic variant within one bacterial species that can behave differently from other variants
  2. A large group of bacteria, such as Firmicutes or Bacteroidetes
  3. Any bacterium that causes disease
  4. A virus that infects bacteria
Show Answer

The correct answer is A. Strains of one species share most genes but differ in some, which can change what they do. E. coli is the classic example: most strains are harmless, but some cause illness. Option B describes a phylum, a much larger group.

Concept Tested: Bacterial Strain Bloom's Level: Remember

Question 3

Which method reads fragments of all the DNA in a stool sample, so researchers can see which genes are present?

  1. 16S rRNA sequencing
  2. Growing bacteria on lab dishes
  3. Counting bacteria under a microscope
  4. Metagenomic (shotgun) sequencing
Show Answer

The correct answer is D. Metagenomic sequencing reads DNA from every organism in the sample, revealing species, often strains, and possible functions. 16S rRNA sequencing (A) reads only one marker gene, so it identifies groups of bacteria but usually not their genes.

Concept Tested: Metagenomic Sequencing Bloom's Level: Remember

Question 4

Why does a healthy gut community make it harder for C. difficile to cause infection?

  1. Commensal bacteria make antibiotics that kill all pathogens.
  2. Resident bacteria take up space and nutrients and block invaders, a protection called colonization resistance.
  3. C. difficile can only live in the stomach, not the colon.
  4. Healthy people's immune systems destroy every bacterium that enters the gut.
Show Answer

The correct answer is B. Colonization resistance works like a full parking lot: when commensals already use the space and food, a newcomer struggles to settle in. That's why antibiotics, which knock down commensals, raise the risk of C. difficile. Option D is wrong because the gut normally hosts trillions of bacteria.

Concept Tested: Colonization Resistance Bloom's Level: Understand

Question 5

Exclusively breastfed babies have lower gut diversity in their first 6 months. What is the best interpretation?

  1. Breastfeeding damages the infant microbiome.
  2. These babies are likely to get sick more often.
  3. It's normal: human milk oligosaccharides feed a few milk-loving groups, so those groups dominate early on.
  4. It shows that more diversity is always healthier.
Show Answer

The correct answer is C. Human milk oligosaccharides are sugars the baby can't digest that feed certain bacteria such as Bifidobacterium, so a few groups dominate. This is a normal stage of microbiome development. Option D is the misconception the breastfeeding data disproves.

Concept Tested: Human Milk Oligosaccharides Bloom's Level: Understand

Question 6

Why does this book warn against applying germ-free mouse results directly to people?

  1. Growing up germ-free causes permanent developmental differences, and mice differ from humans in many ways.
  2. Germ-free mice actually carry more microbes than normal mice.
  3. Germ-free mice have never been used in microbiome research.
  4. Mouse experiments can never show cause.
Show Answer

The correct answer is A. Germ-free mice are powerful for testing what microbes can do, but they develop differently from normal mice, and no human lives germ-free. Option D is wrong: mouse experiments can test cause, but only in mice.

Concept Tested: Germ-Free Mouse Bloom's Level: Understand

Question 7

Leah, 15, is taking an antibiotic for a sinus infection and has mild loose stools on day 4, with no fever or blood. Based on the human studies in this chapter, what is most likely happening to her gut bacteria?

  1. Her gut bacteria are permanently destroyed and will never recover.
  2. The antibiotic is only affecting her sinuses, so her gut bacteria are unchanged.
  3. Her gut bacteria have all been replaced by C. difficile.
  4. Diversity has dropped, and most of the community will likely regrow over the following weeks, though recovery may not be perfect.
Show Answer

The correct answer is D. Human studies show antibiotics quickly reduce gut diversity, and most of the community regrows within weeks to a couple of months, sometimes with a few species missing. Option A is the "ruins forever" Hype claim; mild symptoms without warning signs don't suggest C. difficile (C).

Concept Tested: Antibiotic Disruption Bloom's Level: Apply

Question 8

A company says its home test "compares your gut to the healthy microbiome." Which fact from this chapter best challenges that claim?

  1. Stool samples can't be sequenced.
  2. No single healthy microbiome has been defined, and healthy people's microbiomes differ greatly.
  3. Bacteria outnumber human cells 10 to 1.
  4. Only 16S sequencing is allowed in home tests.
Show Answer

The correct answer is B. Without an agreed definition of a healthy microbiome, there's no validated standard to compare a sample to, and a 2024 expert consensus found no proven clinical value for direct-to-consumer tests. Option A is wrong; stool can be sequenced, but the results can't be compared to a standard that doesn't exist.

Concept Tested: Defining a Healthy Microbiome Bloom's Level: Apply

Question 9

A study finds that people with a gut disease have lower microbial diversity than healthy people. Which explanation can this study alone NOT rule out?

  1. That the disease and its treatments changed the microbes
  2. That diet or medicines affected both the microbes and the disease
  3. Both of these, because it shows a correlation, not cause
  4. Neither, because lower diversity always causes disease
Show Answer

The correct answer is C. A difference between groups is a correlation. Reverse causation (the disease changes the microbes) and confounding (a third factor affects both) remain possible until experiments or careful long-term studies test them. Option D treats correlation as proof of cause.

Concept Tested: Correlation vs Causation Bloom's Level: Analyze

Question 10

Walter and colleagues found that 36 of 38 studies transplanting human gut microbes into germ-free rodents reported that a disease trait transferred. Why did they see this as a warning sign?

  1. Such a high success rate is implausible and suggests the method or reporting overstates the microbiome's role.
  2. It proves the microbiome causes almost every human disease.
  3. It shows that germ-free rodents can't be used in research.
  4. It means human studies are no longer needed.
Show Answer

The correct answer is A. When nearly every experiment "works," it may reflect bias in methods or in which results get published, not a true causal role in humans. The authors argued for more rigorous tests of cause. Option B is exactly the overreach they warned against.

Concept Tested: Animal Model Bloom's Level: Analyze