Observing Yogurt Bacteria

Students prepare microscope slides of live yogurt cultures to observe probiotic bacteria such as Lactobacillus and Streptococcus.

1. Place a clean glass slide on your work surface.
2. Use a toothpick to collect a tiny drop of yogurt and smear it thinly across the slide.
3. (Optional) Add a drop of methylene blue solution to improve visibility of bacteria; if used, wear gloves and handle carefully.
4. Gently lower a cover slip over the sample, avoiding air bubbles.
5. Blot excess liquid from the edges of the cover slip with a tissue or paper towel.
6. View under a compound microscope, starting at 4x or 10x magnification before moving to higher power.
7. Look for different bacterial arrangements: single cells, pairs (diplo), chains (strepto), and clusters.

Yogurt Microscope Experiment - Sci Files:

Yogurt Under the Microscope - Sci- Inspi:

📄 Yoghurt bacteria - MRC Laboratory of Molecular Biology: https://www2.mrc-lmb.cam.ac.uk/microscopes4schools/yoghurt.php

• Compare different yogurt brands or types (Greek, natural, flavored, probiotic drinks).
• Try viewing without stain versus with methylene blue to see differences in contrast.
• Compare yogurt with “live cultures” versus sterilized or pasteurized dairy products.
• Examine other fermented foods (kefir, sauerkraut, kimchi) for bacterial diversity.

• Use gloves if handling methylene blue; it is a mild irritant and stains skin and clothing.
• Do not ingest microscope samples.
• Wash hands thoroughly after preparing and observing slides.
• Supervise children closely when using stains or glass slides.

• Which bacterial shapes are visible in the yogurt sample? (Rod-shaped bacilli, spherical cocci, chains, or clusters.)
• Why is methylene blue sometimes used when viewing bacteria? (It stains cells, increasing contrast under the microscope.)
• How do bacteria transform milk into yogurt? (They ferment lactose into lactic acid, which curdles proteins and gives yogurt its tangy flavor.)
• What are some examples of probiotic bacteria commonly found in yogurt? (Lactobacillus bulgaricus, L. acidophilus, Streptococcus thermophilus, Bifidobacterium bifidus.)
• How does the presence of beneficial bacteria in yogurt relate to human gut health? (They support digestion and contribute to a healthy microbiome.)