Welcome to the Invisible World: Microbial Techniques
Hello there! Welcome to one of the most practical and exciting parts of your Biology B course. In this chapter, we are going to learn how scientists "farm" and count creatures that are too small to see with the naked eye. Whether it’s developing life-saving antibiotics or ensuring our food is safe to eat, microbial techniques are the tools that make it possible.
Don't worry if the math or the long names of techniques seem a bit scary at first. We’ll break everything down into simple steps, use some handy analogies, and point out exactly what you need to know for your Edexcel exams.
1. Aseptic Techniques: Staying Clean in a Messy World
Imagine you are trying to grow a specific type of blue flower in a garden, but the soil is full of weed seeds. Your flowers will quickly be overtaken! In microbiology, the "weeds" are unwanted bacteria and fungi from the air, your breath, or your hands. Aseptic technique is simply the set of rules we follow to keep our cultures sterile (free from living microorganisms) and prevent contamination.
How we keep things sterile:
1. The Autoclave: This is like a giant, super-powered pressure cooker. It heats equipment to \(121^{\circ}C\) using high-pressure steam. This kills even the toughest bacterial spores.
2. Flaming: We pass metal tools, like inoculating loops, through a hot Bunsen burner flame until they glow red. This incinerates any microbes instantly.
3. The "Neck" Trick: When opening a bottle of nutrient broth, we pass the neck of the bottle through the flame. This creates outward air currents that prevent dust and microbes from falling inside.
4. Lids and Angles: When working with an agar plate (a Petri dish), we only lift the lid slightly and at an angle. Think of it like using an umbrella to keep the "rain" of microbes out!
Quick Review Box:
- Sterile: Totally free of all living organisms.
- Aseptic: Procedures used to prevent the introduction of unwanted microbes.
- Contamination: When "bad" microbes get into your "good" culture.
Key Takeaway: Aseptic techniques protect your experiment from the environment and, more importantly, protect you from the bacteria!
2. Feeding the Microbes: Culture Media
Bacteria won't grow unless they have a "menu" of nutrients. We call the stuff they grow on culture media.
Types of Media:
- Broth: A liquid "soup" containing nutrients. Great for growing large amounts of bacteria quickly.
- Agar: This is broth that has been turned into a solid jelly using agar (which comes from seaweed!). It provides a flat surface for bacteria to grow into visible clumps called colonies.
- Selective Media: Think of this as a "VIP list" for bacteria. This media contains specific chemicals (like antibiotics or specific sugars) that only allow certain types of bacteria to grow while stopping others.
Did you know? A single "colony" on an agar plate started from just one individual bacterium that divided millions of times!
Key Takeaway: Scientists choose different media depending on whether they want a lot of bacteria (broth), to see individual types (agar), or to find a specific "needle in a haystack" (selective media).
3. Measuring Growth: How Many Are There?
If you’re doing Core Practical 12, you need to know how to count bacteria. Since we can't see them individually, we have to be clever.
Method A: Cell Counts (The Direct Way)
We use a special microscope slide called a haemocytometer. It has a tiny grid etched into it. We count the cells in the grid and use math to figure out the total concentration.
Common Mistake: This counts *all* cells, including dead ones! This is called a total count.
Method B: Dilution Plating (The "Viable" Way)
If we put a drop of "thick" bacterial soup on a plate, it would just grow into one big smear. To count them, we do a serial dilution:
- Take \(1ml\) of the sample and add it to \(9ml\) of sterile water (This is a 1 in 10 dilution).
- Repeat this several times until the "soup" is very thin.
- Spread the thin liquid on agar.
- Count the colonies. Since each colony came from one living cell, this gives us a viable count (only the ones that were alive and able to divide).
Method C: Turbidity (The "Cloudiness" Way)
As bacteria grow in a broth, the liquid gets cloudy (turbid). We use a machine called a colorimeter to shine light through the tube.
- Clear liquid = High light transmission.
- Cloudy liquid = Low light transmission (the bacteria are blocking the light!).
Memory Aid: "Viable" sounds like "Alive-able." Dilution plating only counts the living!
Key Takeaway: Total counts (haemocytometer) include everyone; viable counts (dilution plating) only include the living "survivors."
4. The Bacterial Growth Curve
When you put bacteria into a new flask of broth, their population follows a very predictable pattern. It's like a party in a house with a limited supply of snacks.
- Lag Phase: No increase in numbers. The bacteria are "getting ready"—making enzymes and adjusting to their new home.
- Log (Exponential) Phase: The population doubles at a constant rate. There is plenty of food and lots of space. This is the "baby boom."
- Stationary Phase: The birth rate equals the death rate. Food is running low, and toxic waste products are building up.
- Death Phase: The death rate is higher than the birth rate. The food is gone, and the waste is now poisonous.
Calculating Growth Rate
During the Log Phase, we can calculate the exponential growth rate constant (\(k\)) using this formula:
\(k = \frac{\log_{10}N_t - \log_{10}N_0}{0.301 \times t}\)
Where:
- \(N_t\) is the number of organisms at the end.
- \(N_0\) is the number of organisms at the start.
- \(t\) is the time interval.
Don't worry if this math looks tough! Just remember that \(k\) tells us how many "doublings" happen per hour.
Key Takeaway: Bacterial populations grow incredibly fast until they run out of resources or drown in their own waste.
5. Core Practical 13: The Streak Plate
In Core Practical 13, you are asked to isolate a single species from a mixed culture. We use a technique called streak plating.
Step-by-Step Isolation:
1. Sterilize your loop in the flame.
2. Dip it in the mixture and make three or four short streaks in one corner of the agar (Area 1).
3. Flame the loop again! (This is the most important step).
4. Drag the loop once through Area 1 and make a new set of streaks (Area 2).
5. Repeat this process until you have 4 or 5 areas.
By the time you get to the last area, the bacteria are so spread out that they grow as separate, pure colonies.
Quick Review Box:
- Pure Culture: Contains only one species of microorganism.
- Mixed Culture: Contains multiple different species.
- Purpose of Streaking: To dilute the bacteria across the plate until they are physically separated.
Key Takeaway: Flaming the loop between every set of streaks is the secret to getting isolated colonies. If you don't flame, you'll just drag a thick "smear" across the whole plate!
Final Encouragement: You've made it through the basics of microbial techniques! Remember, the exam often asks *why* we do certain steps (like flaming or using specific media), so always think about the logic behind the technique. You've got this!