Diagnosing Disease

Treatment waits on a name

Before anyone can decide how to treat an infection, they need to know what is causing it. A treatment that works on one pathogen can do nothing at all against another, so a guess is not good enough. The methods for finding out can look like a long, unrelated list: looking at symptoms, taking samples, counting cells, growing cultures, using a microscope, staining, testing with antimicrobials, analysing a genome. ⭐ They make sense once you see that each one answers a different question, and the questions get narrower. Is something wrong, and where? Symptoms, and counting cells in a sample of body fluid. What kind of organism is it? Looking at a stained sample under a microscope. Exactly which one is it? Analysing its genetic material, or a test that recognises it. What will stop it? Growing it, then testing which substances stop it growing. Is it really the cause? For plants, isolating it and infecting a healthy plant with it. ⚠️ A treatment decision needs one of the narrow answers. A broad one, like "this plant is ill", is where diagnosis starts, not where it ends. One question to carry through the module: which question does this method answer?

Five words for finding a cause

These name the things a diagnosis works with. Which question each method answers comes next.

Match each method to the question it answers

Two fields, one symptom

Two farms each have a crop with yellowing, wilting leaves. One farmer wants to spray both with the same product straight away. What is the strongest reason to identify the cause first?

• Different pathogens can produce the same symptoms, and a treatment that stops one may do nothing to another
• Identical symptoms always mean the same pathogen, so identifying it adds nothing
• A crop can only be sprayed once a laboratory has given permission
• Spraying would hide the symptoms, so the disease could never be identified afterwards

In the field, then in the lab

Diagnosis usually starts where the organism is and finishes where it can be tested. Each place answers different questions.

Two that follow from narrowing down

Select the TWO statements that are true.

• A culture is grown from a sample so that there is enough of the organism to test
• Aseptic technique matters because a contaminated sample can lead to the wrong organism being identified
• A light microscope can always show exactly which species of pathogen is present
• Once the symptoms are known, the diagnosis is complete

Looking at microorganisms properly

Microorganisms are tiny, so a light microscope is used carefully and in a fixed order. Prepare the slide. A thin sample goes on the slide, usually with a stain, because most microorganisms are nearly colourless and hard to see without one. A cover slip goes on top. Start on the lowest-power objective lens. It shows the widest area, so the sample is easy to find, and it sits furthest from the slide. Focus with the coarse focusing knob first, then the fine one. Coarse moves the stage a long way; fine sharpens the image. ⚠️ Watch from the side while moving the lens towards the slide, so the lens cannot crush it. Then change to a higher-power lens and use only the fine focus. At high power a large movement would lose the image or hit the slide. ⭐ And remember what this answers. Size and shape tell you what KIND of organism you are looking at. They rarely tell you exactly which one.

Put the microscope steps in order

Order the steps for viewing a sample of microorganisms with a light microscope.

• Put a thin, stained sample on a slide and add a cover slip
• Place the slide on the stage and select the lowest-power objective lens
• Watching from the side, bring the lens close to the slide
• Look through the eyepiece and focus with the coarse knob, then the fine knob
• Switch to a higher-power lens
• Sharpen the image again using only the fine focus

Build the reason for each lab step

Build the explanation a lab report needs for two steps in identifying a pathogen.

Proving which organism is to blame

A grower finds brown patches on the leaves of some plants. In the lab, two different fungi turn up on the damaged leaves. Both are there; only one may be causing the patches. The other could simply live on leaves, or have arrived after the damage. Finding an organism is not the same as finding the cause. So the lab runs a test that could come out either way. First, isolate. Each fungus is grown on its own, separately, so that each culture contains only one kind of organism. Then, reinfect. Healthy plants of the same kind are given one fungus each, and kept apart. Then, watch. If the plants given the first fungus develop the same brown patches and the others stay healthy, the first fungus is the cause. ⭐ The test works because only one thing differs between the groups: which fungus they received. ⚠️ And it only means something if the cultures were pure. A culture contaminated with the other fungus would make both groups look guilty.

Read the reinfection result

A bacterium is isolated from a wilting plant and grown on its own. Healthy plants given the bacterium stay healthy, and so do untreated plants. What can be concluded?

• This bacterium is probably not the cause of the wilting, so another organism or factor must be tested
• The bacterium causes the wilting, because it was found on the wilting plant
• The healthy plants must have been immune, so the result should be ignored
• The disease must be caused by a virus

The diagnosis run

Five quick calls on which method answers which question. Three lives.

A test that asks one question directly

Some diagnostic tests use monoclonal antibodies. These are many copies of one antibody, all recognising the same antigen, so a test built from them looks for one particular thing and nothing else. That gives the test two strengths the specification names. It is specific: it reacts with the antigen it was made for, not with others. It is sensitive: it can detect that antigen even when only a little is present. ⭐ Read it through the module question. A test like this answers "is this particular pathogen here?" straight away. It does not have to work down from broad questions first, which is why such tests give faster and more accurate results, and why decisions about treatment can be made sooner and on better information. ⚠️ How the antibodies themselves are made is a separate part of the course.

Spot the true diagnosis facts

Tap the TWO statements that are true.

• Choosing a treatment usually needs the exact pathogen, not just the fact that the organism is ill
• A test using monoclonal antibodies reacts with one particular antigen
• Any organism found on a diseased plant must be the cause of the disease
• A sample should first be found using the highest-power objective lens

Name each part of the diagnosis

A tomato plant has spotted leaves, which is a _____ of disease. A small piece of leaf is taken away as a _____. In the lab it is grown into a _____ so there is enough to test, and some is mixed with a _____ so that it shows up clearly under the microscope.

Three cases in the lab

Three diagnoses in progress. In each, choose the next step and say which question it answers.

• A lab has seen round cells in a stained sample from a sick animal and knows it is a bacterium. The vet now needs to choose a treatment. What next?
• A culture grown from a sample produces two different colonies. The technician suspects a stray microbe got in. What does that mean for the diagnosis?
• A clinic can either send samples away to be cultured or use a quick monoclonal antibody test for one suspected pathogen. Why is the quick test useful?

Explain how the cause is found

A grower notices that many plants in one greenhouse have spotted, wilting leaves. Explain how the cause of the disease could be identified, from the greenhouse to the lab, and why each method is used.

• What the symptoms do and do not tell you
• How samples are taken and grown
• What the microscope shows, and its limit
• How the exact pathogen could be named
• How you would show it is really the cause