Low-Cost Microbial Cryopreservation and Viability

ISEF Category: Microbiology

Subcategory: Other  ·  Difficulty: Advanced  ·  Setup: School Lab  ·  Time: Full Year

The Hook

A freezer can act like a time machine for cells, but only if the storage mix protects them well. Tiny changes in temperature can decide whether microbes wake up later or stay dead. That makes cryopreservation a strong science fair topic, because you can measure survival, compare conditions, and build a real model from your data. You also get a direct link to how labs keep strains alive for months or years.

What Is It?

Cryopreservation means storing living cells at very low temperatures so they keep their structure and can grow again later. Think of it like parking a bike in a garage before winter, except the bike is a cell, and the garage has to stop water damage, ice crystals, and chemical stress. In this project, you would test whether a mix of skim milk, glycerol, and trehalose protects microbes during freezing better than simpler mixes.

The biology matters because freezing can tear cell membranes, dehydrate cells, and concentrate salts around them. Glycerol lowers ice damage by helping water stay less organized. Trehalose, a sugar, can help stabilize proteins and membranes. Skim milk adds extra protection from a complex mixture of proteins and sugars. You then compare how many cells stay alive after storage at different temperatures, which gives you a way to connect storage conditions to survival.

Why This Is a Good Topic

This is a strong science fair topic because you can change one variable at a time and measure a clear outcome, survival. It connects to real problems in microbiology, like storing lab strains, probiotic cultures, and starter organisms. You can also learn core research skills, including controls, plating or growth-based counting, replication, and data modeling. The topic feels advanced, but the basic idea is easy to explain.

Research Questions

• How does storage temperature change the viability of B. subtilis after freezing in skim-milk, glycerol, and trehalose mixes? ?
• What is the effect of adding trehalose on post-thaw survival compared with glycerol alone? ?
• Does skim milk improve recovery after freezing more than a simple buffer does? ?
• To what extent does repeated freeze-thaw exposure reduce colony recovery in each protectant mix? ?
• Which cryoprotectant mix gives the highest long-term survival for S. cerevisiae under the same storage conditions? ?
• How does the survival pattern differ between a bacterium and a yeast in the same freezer condition? ?

Basic Materials

• B. subtilis culture from a school lab source.
• S. cerevisiae culture from a school lab source.
• Sterile culture plates and agar suitable for each organism.
• Sterile tubes or cryovials.
• Household freezer with stable temperature around -18 °C.
• Dry ice container or access to a one-time dry-ice storage setup through a school lab.
• Skim milk powder or sterile skim milk solution.
• Glycerol.
• Trehalose.
• Micropipettes and sterile tips.
• Digital balance with 0.01 g or better resolution.
• Incubator or warm growth area approved by your lab supervisor.
• Marker, lab notebook, and labels.
• Biohazard waste container and disinfectant approved by the lab.

Advanced Materials

• Reference strain of B. subtilis with known growth characteristics.
• Reference strain of S. cerevisiae with known growth characteristics.
• Controlled-rate freezing container or programmable freezer, if available.
• Vacuum-insulated dry-ice shipping container with temperature logging.
• Temperature logger or data logger with freezer probe.
• Spectrophotometer or microplate reader for growth-based recovery measurements.
• Colony counter or imaging setup for plate counting.
• Cryovials with secure caps.
• Sterile filtration setup for protectant solutions.
• Statistical software for survival analysis and regression.
• pH meter for checking protectant mix stability.
• Incubator with consistent temperature control.

Software & Tools

• Google Sheets: Organizes survival counts, calculates percent recovery, and makes graphs.
• ImageJ: Measures colony size or plate coverage from images when manual counting gets crowded.
• R or Python: Fits models, compares groups, and tests whether temperature predicts viability.
• GraphPad Prism: Makes clean survival plots and basic statistics if your school has access.
• PubMed: Helps you find review articles and prior cryopreservation studies.

Experiment Steps

1. Define the cell type, storage conditions, and survival metric you will compare.
2. Choose one cryoprotectant mix as your baseline and plan a small set of contrasts against it.
3. Decide how you will separate storage effects from handling effects with matched controls.
4. Build a recovery readout that turns growth after thawing into a countable number.
5. Plan a replication scheme that gives you enough samples to compare temperatures and formulations.
6. Choose the statistical test or model that will let you compare survival across conditions.

Common Pitfalls

• Using a freezer that opens often, which creates temperature swings that blur the effect you are trying to measure.
• Mixing protectants by volume without checking the final composition, which makes one condition unfairly stronger than another.
• Comparing thawed samples with unequal starting cell loads, which makes survival numbers impossible to trust.
• Counting crowded plates by eye without a standard method, which can hide real differences between mixes.
• Treating dry ice as a simple cold source instead of a different storage condition, which can make the interpretation of your results messy.

What Makes This Competitive

A competitive version of this project goes beyond simple before-and-after survival counts. You would compare multiple protectant mixes, use enough replicates to support real statistics, and model the temperature effect instead of only describing it. You could also compare two species, or test whether one mix protects bacteria and yeast differently. Strong controls and clear data handling will matter more than fancy equipment.

Project Variations

• Test whether honey, skim milk, or both improve freezing survival for yeast in a home freezer.
• Compare the post-thaw recovery of B. subtilis in glycerol-only versus glycerol-plus-trehalose mixes.
• Measure whether repeated freeze-thaw cycles damage yeast more than one long storage period at the same temperature.

Learn More

• NIH PubMed: Search review articles on cryopreservation, membrane damage, and protectant chemistry.
• NCBI Bookshelf: Read free textbook chapters on microbial growth, stress responses, and preservation methods.
• ASM Microbe Library: Find free educational articles and lab-focused background on microbiology techniques.
• USDA National Agricultural Library: Search for preservation and storage research related to microbes and food systems.
• MIT OpenCourseWare: Look for free materials on biochemistry and cell biology that explain membranes, proteins, and freezing damage.