Superbug Lab

Superbug Lab: bacterial structure, binary fission and antibiotic resistance

This free game is part of our Pearson Edexcel iGCSE Biology (4BI1) course for exams in EXAM YEARS 2026-2029.

What are bacteria?

Bacteria are microscopic, single-celled organisms. They are prokaryotes, which means their DNA is not enclosed in a nucleus. Plants, animals, fungi and protoctists are eukaryotes: their cells keep their DNA inside a nucleus.

A few bacteria can photosynthesise, but most feed off other living or dead organisms. Some are useful, like Lactobacillus bulgaricus, and some are pathogens that cause disease, like Pneumococcus.

The structure of a bacterial cell

• Cell wall: a rigid outer layer that gives the cell its shape and stops it bursting. It is not made of cellulose, unlike a plant cell wall.
• Cell membrane: controls what goes into and out of the cell.
• Cytoplasm: the jelly-like fluid where most of the cell's chemical reactions happen.
• Circular chromosome: one loop of DNA, floating free in the cytoplasm. There is no nucleus.
• Plasmids: small extra rings of DNA that can carry useful genes, such as antibiotic resistance. Bacteria can pass plasmids to each other.
• Flagellum: some bacteria have a whip-like tail that spins like a propeller to move them.

Bacteria have no nucleus, no mitochondria and no chloroplasts. In the game, those three are decoys: try to fit one and you will find out why it does not belong.

The four rounds

• Build a Bug: drag the parts that a bacterium really has into a 3D blueprint, and learn what each one does.
• Flagellum Dash: steer your bacterium through the bloodstream, swim through the true side of each gate and grab glucose before a phagocyte engulfs you.
• Split!: put the steps of binary fission in order and do each one to the cell, then fast-forward time and predict how fast the colony grows.
• Superbug: treat a patient with pneumonia over a 7-day course of antibiotics, decide what happens when they feel better on day 3, and watch natural selection create resistant bacteria.

Binary fission and antibiotic resistance

Bacteria reproduce by binary fission: the DNA is copied, the cell grows, the two copies move to opposite ends and the cell splits into two genetically identical daughter cells. In perfect conditions some bacteria divide every 20 minutes, so one cell could become 2²¹ = 2,097,152 cells in 7 hours.

Antibiotic resistance starts with a random mutation. When an antibiotic is used, the bacteria that are not resistant die, but resistant ones survive, reproduce and pass on the resistance gene. Over time the population becomes resistant. This is natural selection, and it is how superbugs such as MRSA appear. Finishing the course and not using antibiotics for viral infections helps to slow it down.

Exam tips

• Say that bacteria have a cell wall not made of cellulose, not just "a cell wall".
• Bacterial DNA is a circular chromosome plus plasmids, free in the cytoplasm. Never put it in a nucleus.
• Learn the two named examples: Lactobacillus bulgaricus is rod-shaped and used to make yoghurt; Pneumococcus is spherical and causes pneumonia.
• For growth questions, work out the number of divisions first (time ÷ time per division), then double that many times.
• Antibiotics do not cause mutations. They kill the bacteria that are not resistant, so the resistant ones survive and multiply.