Praktikum Mikrobiologi Acara 2

Summary

This video covers two important laboratory procedures: dilution and colony counting, and Gram staining. First, it demonstrates the process of diluting bacterial samples and using agar plates to calculate microbial concentration. The calculation involves determining colony-forming units based on dilution factors. The second part explains the Gram staining technique, used to differentiate bacteria by their cell wall structure, highlighting the steps of applying crystal violet, iodine, alcohol, and safranin. Both procedures emphasize aseptic techniques and the use of precise formulas for accurate microbial analysis.

Takeaways

• 🔥 Always create an aseptic environment by lighting a Bunsen burner and sterilizing surfaces with alcohol before starting microbial work.
• 💧 Use aquades (sterile water) and microtips to perform serial dilutions of samples, typically up to 10^-6, to ensure countable bacterial colonies.
• 🔄 Homogenize samples properly using a vortex to achieve even distribution before plating on the medium.
• 🍽️ Spread diluted samples evenly on agar plates to allow individual bacterial colonies to grow for counting.
• 📊 Count colonies within the 30–300 range for accurate colony-forming unit (CFU) calculations; use the highest or lowest dilution factors when outside this range.
• 🧮 Calculate CFU/mL using the formula: CFU/mL = number of colonies × 1 / dilution factor, and average when two dilutions fall within the acceptable range.
• 📈 When averaging two dilutions, adjust exponents to match before dividing to ensure correct calculation.
• 🧪 Perform Gram staining by first fixing bacteria on a glass slide, then sequentially applying Crystal Violet, Iodine, Gram C, and Safranin dyes with proper rinsing steps.
• 🔥 Use a Bunsen burner to heat the loop before and during bacterial transfer to maintain sterility.
• 🔬 Properly prepared and stained slides can then be observed under a microscope to distinguish Gram-positive and Gram-negative bacteria.

Q & A

What is the purpose of turning on the Bunsen burner at the beginning of the procedure?

The Bunsen burner is turned on to create an aseptic environment, reducing the risk of contamination during the experiment.

Why is alcohol sprayed in the work area during the experiment?

Alcohol is used to sterilize the environment, further ensuring that bacteria from the surroundings do not contaminate the samples.

What is the reason for performing serial dilutions up to 10^-6?

Serial dilutions are performed to reduce the concentration of bacteria, making it possible to count colonies accurately on the agar medium.

How is the number of colony-forming units (CFU) calculated?

CFU is calculated using the formula: CFU/mL = number of colonies × (1 / dilution factor). Adjustments are made based on whether the colony count falls within the acceptable range of 30–300.

What is done if the colony count is more than 300 or less than 30?

If the count is more than 300, the highest dilution factor is used. If the count is less than 30, the lowest dilution factor is used.

How do you handle CFU calculation when two dilution factors fall within the acceptable range?

The ratio of the highest to the lowest dilution factor is calculated. If the ratio is greater than 2, the lowest factor is used. If less than 2, the average of the CFU from both dilutions is taken.

What is the role of the vortex in the dilution process?

The vortex is used to homogenize the sample and the diluent, ensuring that bacteria are evenly distributed for accurate dilution and plating.

Why is the loop heated before taking bacterial samples for Gram staining?

Heating the loop sterilizes it, preventing contamination of the sample or spread of bacteria during transfer.

What are the steps involved in Gram staining after placing bacteria on the glass slide?

The steps include fiksation of the bacteria, applying violet crystal (Gram A), rinsing, applying Lo, rinsing, applying Gram C, rinsing, applying safranin (Gram D), rinsing, and drying the slide.

Why is safranin (Gram D) applied at the end of the Gram staining procedure?

Safranin is a counterstain that stains Gram-negative bacteria red, allowing them to be differentiated from Gram-positive bacteria, which retain the violet crystal stain.

How is the area of bacterial spread on the slide controlled during Gram staining?

Bacteria are spread on the glass slide using a loop with a radius of 1–1.5 cm, ensuring even coverage for proper staining and observation.

What units are used to report the bacterial count after CFU calculation?

The bacterial count is reported in colony-forming units per milliliter (CFU/mL).