Micro-organisms, Grade 11: A Safe Version of the 30-Mark Investigation, and Six Corrections

There is a 30 mark investigation in this topic that tells learners to press their nose into the growth medium, add a mouthful of saliva to a second dish, and leave the lot in a warm place for several days.

There is a photograph of someone doing it. The only safety note is to wash your hands and nose afterwards.

Growing whatever lives in a learner's nose and mouth, at the temperature that suits it best, in an open classroom, with no seal on the dish and no word about how to throw it away, is the one thing school microbiology guidance everywhere tells you not to do. This page gives the same investigation rewritten so it teaches the same five variables and is safe to run, plus six places where the printed account is wrong, starting with a bacterium described as a fungus in a topic about classifying micro-organisms.

Where this fits in the curriculum

Subject Life Sciences
Grade 11
Term 1
Topic Topic 1, Biodiversity and classification of micro-organisms
Units Unit 1 the biodiversity of micro-organisms. Unit 2 roles in the web of life. Unit 3 diseases. Unit 4 immunity and the use of drugs. Unit 5 uses in medicine and food
Formal assessment None. The growing investigation is a 30 mark activity, not one of the six formal assessment tasks
Marks Our worksheet is 80

Finding 1: the investigation cultures the learners themselves

The activity sets up five dishes of gelatine and then does this:

Dish What the learner is told to do
1 Add a mouthful of saliva
2 Press your fingers or nose into the gelatine
3 Leave exposed to the air for 30 minutes
4 Place a coin on the gelatine
5 Leave covered, as the control

The dishes are then covered with plastic wrap, left in a warm place for several days, and afterwards opened again so that the learner can write down what grew on each one and draw it.

Dishes 3, 4 and 5 are fine. Air, a coin and a sealed control teach the whole lesson. Dishes 1 and 2 are the problem, and they are the two the photograph is of.

Why this matters, in four lines

  • A warm place is roughly body temperature, which is exactly what human pathogens are selected by. School plates are kept at or below 30 degrees for that reason, and never at 37
  • Saliva and nasal material are the two richest sources of human pathogens a school has. Everything that grows is already adapted to living in a person
  • Plastic wrap is not a seal, and the method has the dish opened again afterwards. A plate grown for several days and then opened in a classroom releases whatever is in it into the room
  • There is no disposal instruction at all. A plate of several days' growth goes in the bin as it stands

The same investigation, rewritten

It tests the same hypothesis, has the same five dishes, the same three variables and the same 30 marks. Only the two sources change.

Dish Do this instead Why it still works
1 Press an unwashed coin or a door handle swab onto the medium Still a contact sample, still rich, not from a person
2 Touch the medium with a finger that has been deliberately left unwashed, then immediately wash and touch a second dish This is a better investigation than the original: it adds a before and after and makes the point of the lesson
3 Leave exposed to the air for 30 minutes Unchanged
4 Place a coin on the medium Unchanged
5 Leave covered Unchanged, the control

The finger before and after washing is the version worth running. It answers a question a fifteen year old actually has, it gives two plates that can be put side by side, and nobody has to put their face in anything.

The four rules that go with any plate

  1. Tape the lid to the base at four points. Not all the way round: a fully sealed plate goes short of oxygen and favours exactly the organisms you least want
  2. Incubate at room temperature, and never above 30 degrees. A warm cupboard, not an incubator set to body heat
  3. Never open an incubated plate. Count and draw the colonies through the lid. That is what the lid is for
  4. Kill it before you bin it. Autoclave, or flood the taped plate with household bleach and leave it overnight, then bag it

Finding 2: a bacterium is called a fungus, in the topic about classification

You will read that antibiotics are secreted by fungi, and that "two examples of such fungi" are Penicillium, which grows on fruit, and Streptomyces, which grows in the soil. The same claim is repeated in the content summary.

Streptomyces is not a fungus. It is a bacterium.

Penicillium Streptomyces
Kingdom Fungi Monera, the bacteria
Cell type Eukaryote Prokaryote
Gave us Penicillin Streptomycin, tetracycline, erythromycin, neomycin

It grows in branching threads, which is why it gets mistaken for a mould, and it lives in soil, which is why soil smells the way it does after rain. But it is a prokaryote: no nucleus, no membrane-bound organelles, a bacterial cell wall.

This is the worst possible place to get a kingdom wrong. The whole topic is about sorting micro-organisms into kingdoms, and the example chosen to illustrate the Fungi is a member of the Monera. A learner who has understood the topic properly could catch this, which is the best reason to put it in front of them.

There is a second reason it matters. The reason given for fungi making antibiotics is to stop bacteria competing with them for food. That explanation is reasonable for Penicillium. Applied to Streptomyces it says a bacterium poisons bacteria to stop bacteria competing with it, which is true, but it is not the sentence that was written.

A detail the material gets right, and it is a good one

The same section notes that a Sudanese Nubian population was using a tetracycline antibiotic as early as 350 CE, long before anybody understood what an antibiotic was. That is real: tetracycline has been detected in their skeletal remains, and it is thought to have come from Streptomyces growing in the stored grain they brewed beer with.

So the ancient tetracycline came from the organism the page has just miscalled a fungus. Teach the two facts together and both stick.

Finding 3: the definition of a micro-organism contradicts its own list

"Micro-organisms are unicellular organisms that are found in the kingdoms Monera, Protista and Fungi."

Two bullets later in the same list: "Some protists are unicellular and some are multicellular." One bullet after that: "Fungi are eukaryotes and can be unicellular or multicellular."

The problem What it should say
Unicellular in the definition, multicellular in two of the three kingdoms it names Micro-organisms are organisms too small to see without a microscope. Most are unicellular; some are not
Viruses are left out entirely, although the topic devotes a whole section to them and four of its diseases are viral Viruses are not placed in any kingdom, because they are not cells and are not considered living. They are studied alongside micro-organisms, not inside the definition

Learn the exam-safe version: size is what makes something a micro-organism, not cell number. And if you are asked which kingdom a virus belongs to, the answer is none, and the reason is that it is not a cell.

Finding 4: immunity is defined so as to exclude half the topic's own diseases

"Immunity is having resistance to infection and diseases caused by bacteria and viruses."

The unit immediately before it teaches malaria, caused by a protist, and thrush, caused by a fungus. Neither is covered by that definition.

Pathogen group Disease the topic teaches In the printed definition of immunity
Virus Rabies Yes
Bacterium TB Yes
Protist Malaria No
Fungus Thrush No

Immunity is resistance to infection and disease caused by any pathogen. Three words removed from the printed sentence would have fixed it.

Finding 5: "the first link in all aquatic food chains"

"Autotrophic Protista are the first link in all aquatic food chains."

Two counter-examples, and the first one is the most abundant photosynthetic organism on Earth.

Where it fails What is actually first
Open ocean Cyanobacteria, which are Monera, not Protista. Prochlorococcus alone accounts for an enormous share of ocean photosynthesis
Deep sea hydrothermal vents Chemosynthetic bacteria. There is no light, so nothing autotrophic by photosynthesis can be first at all

The corrected version: autotrophic protists and photosynthetic bacteria are the first link in most aquatic food chains.

This is the third sentence in our library that begins with "all" and is refuted by one example. We collect them on the Grade 7 biodiversity page, which has five of its own. Treat "all", "only" and "never" in a summary as an invitation to find the exception.

Finding 6: gelatine melts, and the task half knows it

The investigation uses gelatine as the growth medium, and then asks the learner, for two marks, "Why did you use gelatine to try to grow bacteria?"

The honest answer is that you should not have.

Gelatine Agar
Melts at around 35 degrees, which is inside the range the method tells you to incubate at Stays solid past 80 degrees
Many bacteria digest it, so the colonies sink into a liquefying puddle Almost nothing digests it
Cheap and in every kitchen Costs more and has to be bought

This is exactly why microbiology stopped using gelatine in the 1880s. Agar was suggested to Robert Koch's laboratory by Fanny Hesse, who knew it from preserving jams, and it is the reason plates work at all.

If the budget only runs to gelatine, run it anyway and keep the plates cool. Then ask the class why half the dishes went runny. That is the better lesson, and it is free.

Two more things worth correcting in passing

What is printed What is missing
"TB can be cured by taking antibiotics" The course has to be finished. Standard treatment runs about six months, and stopping early is how drug-resistant TB is made. Three bullets later the same summary says pathogens can become resistant to antibiotics, and the two are never connected. South Africa carries one of the heaviest TB burdens in the world, so this is not an abstract point here
Malaria "affects the liver and can also affect the brain" The red blood cells. The parasite does pass through the liver, but the illness happens when it bursts out of red blood cells in waves, which is what produces the cycles of fever and the anaemia. Cerebral malaria is a complication of that, not a separate target. Leave out the red blood cells and the symptoms cannot be explained

What the five units actually cover

Unit What is in it
1. Biodiversity of micro-organisms Classification, then the structure and characteristics of viruses, bacteria, protists and fungi in turn
2. Roles in the web of life Importance in the environment, symbiotic bacteria in digestion and vitamin formation, and bacteria in the nitrogen cycle
3. Diseases Viral, bacterial, protist (malaria) and fungal diseases, with effects and management for each
4. Immunity and the use of drugs The human immune system, immune responses in plants, vaccination, and the effect of drugs on micro-organisms
5. Uses in medicine and food Producing medicines, and traditional technology producing maas and sorghum beer

The parts worth spending teaching time on

  • Virus against bacterium. A virus has nucleic acid and a protein coat, no cell structure, no cell functions, and only reproduces inside a host. That last point is why antibiotics cannot touch it: there is no bacterial machinery to attack
  • Prokaryote against eukaryote, because every classification question in this topic comes back to it
  • Binary fission, and what it means for how fast a population grows
  • Antibiotic resistance, which is the same argument as natural selection. Variation exists, the antibiotic selects, the survivors reproduce. Our Grade 12 natural selection page works it through
  • The nitrogen cycle, where nitrogen-fixing, nitrifying and denitrifying bacteria each have a job and the marks go to naming which does which

What this topic costs

The investigation can be run for under R15 a group, and most of what we stock here is optional.

Item Price In stock Why
Petri dishes, plastic, sterile, pack of 10 R10,00 228 Two groups per pack. Sterile out of the sleeve, so the boiling water step falls away
Nutrient agar plates, pack of 10 R200,00 3 Poured and ready. Solves finding 6 outright
Potato dextrose agar, pack of 10 R135,00 5 For the fungi half of the investigation
Bacteria slide set of 12 R755,00 13 The shapes under a microscope, with nothing to grow and nothing to dispose of
Beginners microscope slide set of 12 R400,00 14 A cheaper first set
Mould wall chart R85,00 1 Fungal structure on the wall

Stock on the two agar lines is thin, three and five packs, and there is one mould chart left. We would rather say so than take an order we cannot fill.

Microscopes at microscopes, charts at educational wall charts.

If you want this topic to cost nothing at all, buy the slides and skip the growing. Prepared bacteria slides show the three shapes properly, which is what gets examined, and nothing has to be sealed, incubated or bleached.

What you should be able to do by the end

  • Say what makes something a micro-organism, and why viruses sit outside every kingdom
  • Sort an organism into Monera, Protista or Fungi from prokaryote against eukaryote and unicellular against multicellular
  • Compare a virus with a bacterium across nucleic acids, cell functions, reproduction and cell wall
  • Name the three bacterial shapes and describe binary fission
  • Explain antibiotic resistance as natural selection
  • Give the causative organism and its kingdom for TB, malaria and thrush
  • Explain how a vaccine works without causing the disease
  • Set up a growing investigation safely, and say why each of the four rules exists

Free worksheet and marking memo

Both free, no sign up, straight to the PDF.

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