Welcome to the World of Microbes!

In this chapter, we are going to look at how scientists "farm" bacteria in the lab and, more importantly, how we kill them using antibiotics. This is a vital part of your Paper 1 studies for Topic 5: Health, disease and the development of medicines.

Don't worry if some of the scientific terms sound a bit strange at first—think of this as learning how to run a very tiny, very clean "mini-zoo" where the animals are too small to see!

1. Antibiotics: The Bacterial Assassins

Antibiotics are special medicines used to treat bacterial infections. They work by either killing bacteria or stopping them from growing.

The Golden Rule: Antibiotics only work on bacteria. They do not work on viruses (like the cold or flu). This is a very common exam question, so remember: Antibiotics = Bacteria only!

Why do they only work on bacteria?

Bacteria are prokaryotic cells. Antibiotics are designed to attack specific parts of these cells that human cells (or viruses) don't have, such as the bacterial cell wall. Since viruses don't have their own cell machinery, antibiotics have nothing to "attack."

Antibiotic Resistance

Sometimes, bacteria can change (evolve) so that antibiotics no longer work on them. This is called antibiotic resistance. As we learned in Topic 4, this is a great example of natural selection in action. If we use antibiotics too much or don't finish a course of medicine, the "tough" bacteria survive and multiply.

Key Takeaway: Antibiotics are powerful tools against bacteria, but they are useless against viruses.

2. Culturing Microorganisms

To study how well an antibiotic works, we need to grow a lot of bacteria. This is called culturing. We usually grow them in a Petri dish filled with agar jelly. The agar contains all the nutrients (food) the bacteria need to grow.

Aseptic Technique (Biology Only - 5.17B)

When growing bacteria, we must use aseptic techniques. This simply means "working in a way that prevents contamination." We want to grow the right bacteria and keep "bad" microbes out of our dish (and out of our lungs!).

Step-by-step for a safe culture:

  1. Sterilise the equipment: We use an autoclave (a high-pressure steam oven) to kill any microbes on the dishes and agar before we start.
  2. Sterilise the inoculating loop: This is the wire tool used to spread bacteria. We pass it through a hot Bunsen burner flame until it glows red to kill any unwanted hitchhikers.
  3. Work near a flame: The heat from a Bunsen burner creates an upward current of air, which carries away any floating microbes in the room.
  4. The "Clamshell" opening: When putting bacteria into the dish, only lift the lid a tiny bit at an angle. Never take the lid off completely!
  5. Sealing the dish: Use adhesive tape to secure the lid, but do not seal it all the way around. Bacteria need a little bit of oxygen to grow safely; if you block all air, dangerous anaerobic bacteria might grow.
  6. Storage: Store the plates upside down. This stops drops of condensation (water) from falling onto the bacteria and ruining the colony.
  7. Temperature: In school labs, we incubate cultures at \(25^{\circ}C\). This is warm enough for them to grow, but cool enough to prevent the growth of dangerous pathogens that like human body temperature (\(37^{\circ}C\)).

Quick Review: Aseptic technique is all about keeping things clean and safe!

3. Core Practical 5.18B: Testing Antibiotics

This is a Biology Only practical where you investigate the effects of antiseptics, antibiotics, or plant extracts on bacterial cultures.

The Method:

1. Prepare an agar plate with a "lawn" of bacteria spread evenly across it.
2. Take small paper discs that have been soaked in different antibiotics or antiseptics.
3. Place the discs onto the agar jelly.
4. Include a control disc (a paper disc soaked in sterile water) to show that the paper itself doesn't kill bacteria.
5. Incubate the plate for 48 hours.

The Results:

If the antibiotic works, there will be a clear area around the disc where the bacteria have died or couldn't grow. This is called the Zone of Inhibition (or the "clear zone"). The bigger the clear zone, the more effective the antibiotic is!

Did you know? Some plants, like garlic or mint, have natural chemicals that kill bacteria. You can test these in this practical too!

4. The Maths Bit: Calculating Areas (5.19B)

In the exam, you might be asked to calculate the cross-sectional area of a bacterial culture or a clear zone. Don't panic! You just need to use the formula for the area of a circle.

The Formula:
\(Area = \pi r^{2}\)

How to do it:

  1. Measure the diameter of the clear zone (the distance all the way across the circle).
  2. Divide the diameter by 2 to get the radius (\(r\)).
  3. Square the radius (\(r \times r\)).
  4. Multiply that number by \(\pi\) (usually \(3.14\)).

Example:
If a clear zone has a diameter of \(20 mm\):
The radius (\(r\)) is \(10 mm\).
\(Area = 3.14 \times 10^{2}\)
\(Area = 3.14 \times 100\)
\(Area = 314 mm^{2}\)

Common Mistake Alert: Students often forget to divide the diameter by 2! Always check if you have the radius or the diameter before you start your calculation.

Summary Checklist

  • Antibiotics: Medicines that kill bacteria only.
  • Aseptic Technique: Using heat (flaming loops) and careful handling to prevent contamination.
  • Zone of Inhibition: The clear area where bacteria have been killed.
  • Maths Skills: Using \(Area = \pi r^{2}\) to compare how well different antibiotics work.

Great job! You've covered the essentials of culturing microbes and testing medicines. Remember, staying sterile is the key to success in this chapter!