Human, Half Bacterium?

Open with the claim that you are mostly bacteria, then contrast it with a human body weighing tens of kilograms. Ask learners which comparison they think best captures the claim: number of cells, number of nucleated cells, or mass.

• Microscopic bacteria can collectively outnumber human cells without making up most of body mass
• The result depends on what counts as a cell and what kind of comparison is being made
• The investigation tests a specific 70 kg reference estimate

Before revealing the revised estimate, let learners set a body weight and predict how the bacterial-to-human ratio changes under three definitions: all cells, only nucleated human cells, and total mass. The ratio display should update immediately and invite a commitment to one likely result.

• Adjust body weight from 40 kg to 100 kg
• Switch among all cells, nucleated cells, and mass
• Watch bacteria-to-human ratio and absolute totals move together
• Notice that switching the definition changes the interpretation more dramatically than changing weight

Present the 2016 Sender, Fuchs, and Milo reconstruction as a layered evidence diagram. Compare the familiar 10:1 claim with the revised all-cell estimate for a 70 kg reference man: about 3.8 × 10^13 bacterial cells and 3.0 × 10^13 human cells, or roughly 1.3:1.

• Sender, Fuchs, and Milo published the revised estimates in PLOS Biology in 2016
• The 70 kg reference man contains about 3.8 × 10^13 bacterial cells
• The same reference contains about 3.0 × 10^13 human cells
• The revised all-cell ratio is approximately 1.3:1

Break the old calculation into three evidence cards. Explain that multinucleated muscle cells were effectively counted by nuclei rather than by whole cells; gut bacteria were undersampled rather than estimated from their dense colon habitat; and stool counts included many dead bacteria.

• Muscle is a large body fraction, but its cells contain multiple nuclei
• Counting one cell per nucleus greatly overestimates muscle-cell numbers
• Bacteria are densely concentrated in the colon, not represented well by sparse gut sampling
• Roughly one third of bacteria in faeces are dead, so stool counts overestimate living bacteria

Use a manipulable 3D-style cell landscape to compare dominant human cell types and their bacterial habitats. Learners can isolate human-cell categories and inspect bacterial density across skin, oily facial areas, and the colon, connecting microscopic abundance with body-scale totals.

• Red blood cells make up roughly 90% of the human-cell total
• The haematopoietic lineage dominates the human-cell count
• Skin averages about 7.8 million bacteria per square centimetre
• Oily facial areas exceed about 78 million bacteria per square centimetre
• The colon contains on the order of 10^11 bacteria

Use a three-column comparison to show exactly how each claim is valid. Count all cells, including bacterial cells and non-nucleated red blood cells: about 1.3:1. Count only nucleated human cells against bacteria: about 10:1. Compare mass: roughly 0.2 kg of bacteria versus a much larger human-cell mass.

• Red blood cells have no nucleus, so excluding them greatly raises the apparent bacteria-to-human ratio
• A count-based statement and a mass-based statement answer different questions
• Neither definition is universally superior; the wording of the claim must state which one it uses
• Bacterial cells are not equivalent to bacterial mass because bacterial cells are tiny

Define the microbiome as the community of microorganisms and their genetic material, and microbiota as the organisms themselves. Introduce symbiosis, commensal organisms, host, and germ-free animals, then use the immune-deprived mouse example to show that gut bacteria can become harmful when normal host–microbe balance is disrupted.

• Microbiome refers to the microbial community and its genetic material; microbiota refers to the organisms
• Symbiosis describes a close, long-term association; commensal organisms benefit without harming the host in that context
• A germ-free animal has no normal microbes and is used to study microbial effects
• Removing a mouse's immune defenses can allow its own gut bacteria to kill it from within
• Host–microbe relationships are context-dependent rather than simply parasitic or harmless

Return to the driving question and resolve it explicitly. By all-cell count, the 70 kg reference person is about half bacterial cells; by nucleated-cell count, the ratio is around 10:1; by mass, bacteria contribute only about 0.2 kg, roughly the weight of a grape. Conclude that both “mostly bacteria” and “mostly human” can be true depending on the stated comparison.

• By all-cell count: about 1.3 bacterial cells for every human cell
• By nucleated-cell count: roughly 10 bacterial cells for every nucleated human cell
• By mass: about 0.2 kg of bacteria, less than a grape
• The answer is definition-dependent, and the revised count is lower than the old 10:1 or 100:1 claims