Grow a Bacteria Culture (Leaving Cert Agricultural Science): Revision Notes

Grow a Bacteria Culture

Introduction

Growing bacteria cultures is a fundamental laboratory technique in agricultural science that allows us to study microorganisms present in soil samples. This controlled method helps us understand the diversity and characteristics of bacteria and fungi that play crucial roles in soil health and plant nutrition.

Equipment and materials needed

For this practical activity, you'll need several specialised pieces of laboratory equipment:

• Two pre-poured agar plates - These contain nutrient agar, which provides food for bacterial growth
• Disinfectant - Used to clean work surfaces and maintain sterile conditions
• Methylated spirits - Essential for sterilising the inoculating loop
• Bunsen burner - Provides the flame needed for sterilisation procedures
• Inoculating loop - A wire tool used to transfer soil samples to the agar
• Soil sample - The source of bacteria and fungi for your culture
• Incubator or oven - Maintains the optimal temperature for bacterial growth
• Parafilm - Seals the petri dishes to prevent contamination

Safety and preparation procedures

Before beginning any microbiology work, proper preparation is essential to ensure both safety and successful results. Start by thoroughly cleaning all work surfaces with disinfectant to eliminate any unwanted microorganisms that could interfere with your experiment.

Label your two sterile agar plates clearly - designate one as "Plate A" and the other as "Plate B". This labelling system will help you track your experimental plate versus your control plate throughout the process.

Step-by-step inoculation technique

The inoculation process requires careful attention to sterile technique to prevent contamination and ensure reliable results.

Begin by sterilising your inoculating loop through a process called flaming. Dip the loop into methylated spirits, then hold it in the Bunsen burner flame until the entire loop glows red-hot. This high temperature kills any microorganisms that might be present on the loop.

After sterilisation, allow the loop to cool by holding it just outside the flame for approximately 30 seconds. This cooling period is crucial - if the loop remains too hot, it will kill the very bacteria you're trying to culture when you collect your soil sample.

Once cooled, carefully dip the inoculating loop into your soil sample, collecting a small amount of soil on the wire loop. The soil contains numerous microorganisms that will serve as the source for your bacterial culture.

Now comes the critical inoculation step. Open the lid of Petri dish A at a 45-degree angle - this specific angle minimises the risk of airborne contaminants settling on your agar surface while still allowing you to work with the plate. Gently streak the inoculating loop across the surface of the nutrient agar in a systematic pattern. This streaking action deposits bacteria and fungi from the soil onto the growth medium.

Replace the lid as quickly as possible to maintain sterile conditions within the dish. Immediately re-sterilise your inoculating loop by flaming it until red-hot again. This prevents cross-contamination if you need to use the loop for other purposes.

Control experiments and incubation

Plate B serves as your control experiment - leave it completely unopened throughout the process. This control allows you to verify that any microbial growth you observe on Plate A actually came from your soil sample rather than from contamination during the experimental process.

Seal both plates using parafilm, which creates an airtight seal while still allowing some gas exchange. Write your initials and the current date on the underside of each petri dish - this information is essential for tracking your experiment and identifying your plates later.

Place both plates upside-down in an incubator set to 30°C. The inverted position prevents condensation from dripping onto the agar surface, which could disrupt bacterial growth patterns. The 30°C temperature creates optimal conditions for most soil bacteria to multiply and form visible colonies over the next 24 hours.

Observations and expected results

After the 24-hour incubation period, examine both plates carefully for bacterial and fungal growth. You should observe distinct differences between your experimental and control plates.

On Plate A (your experimental plate), you're likely to see various types of colonies growing on the agar surface. Bacterial colonies typically appear as small, round, often coloured spots, while fungal colonies may appear larger and more fuzzy or cotton-like in texture. The diversity of colony types reflects the rich microbial community present in your soil sample.

Your control plate (Plate B) should remain clear of growth, confirming that your sterile technique was effective and that any organisms growing on Plate A originated from your soil sample.

Document your observations by drawing detailed diagrams of both plates, noting the different types, sizes, and colours of colonies present. This visual record helps you analyse the microbial diversity in your soil sample.

Key terminology

Understanding the technical vocabulary is essential for discussing and explaining this practical work:

• Inoculation - The process of introducing microorganisms to a growth medium
• Aseptic technique - Laboratory methods designed to prevent contamination
• Nutrient agar - A gel-like growth medium containing nutrients that support microbial growth
• Bacterial colonies - Visible clusters of bacteria that have multiplied from single cells
• Sterile technique - Procedures that maintain a contamination-free environment
• Control experiment - An unchanged comparison that helps validate experimental results

Remember!