Plating Bacterial Cultures from Freezer Stocks

Streak-for-Isolation Technique

1. Retrieve Freezer Stock

⚠️ Key Handling Tips

Transport on ice to minimize thaw/refreeze cycles. Work quickly and return the vial to −80°C immediately if using the scrape method. Repeated freeze-thaw cycles reduce viability.

✓ Quick Retrieval Steps

  1. Locate vial in −80°C freezer
  2. Transport on ice or work quickly
  3. Determine inoculation method (next step)

📋 What You'll Need

  • Agar plates (R2A, BHI, or TSA)
  • Sterile inoculating loops
  • Micropipette with sterile tips
  • Permanent marker or labels

2. Choose Inoculation Method

✓ Use When:

You want to preserve the stock for many future uses, or the vial is still frozen solid.

Steps:

  1. Open vial briefly in Biosafety Cabinet
  2. Using sterile inoculating loop, scrape a small amount of frozen culture from surface
  3. Return vial to −80°C immediately
  4. Proceed to streak plate (next section)
⚠️ Critical: Do not allow entire stock to thaw. Return immediately after scraping. Freeze-thaw cycles damage viability.

✓ Use When:

The vial has already thawed, or when you need a measured inoculum amount.

Steps:

  1. Gently vortex or invert thawed vial to resuspend cells
  2. Pipette 10–50 µL onto agar plate near one edge
  3. Use sterile loop to spread and streak (see next section)
  4. Return stock vial to −80°C if reusing, or discard

💡 Tip:

Gently vortexing resuspends settled cells for more even inoculation.

3. Streak for Isolation

Use a standard 3-zone streak technique to dilute inoculum and isolate individual colonies. Click each zone to see details.

1 2 3 Rotate 90° Rotate 90°
Zone 1: Heavy Inoculum
The initial heavy streak containing most of the inoculum.
Step 1: Streak the inoculum back and forth across ~1/3 of the plate. This establishes the initial bacterial concentration.
General Procedure for Zones 2 & 3:
  1. Flame or discard the loop to sterilize
  2. Rotate plate ~90° to a fresh area
  3. Streak through the edge of the previous zone, then into fresh agar

This dilution series progressively reduces bacterial density, yielding isolated colonies by Zone 3.

4. Incubate Plates

✓ Labeling Your Plates (Critical)

Use permanent marker on the agar side (bottom) of the plate:

  • Organism/strain name
  • Medium type (R2A, BHI, etc.)
  • Date
  • Initials
  • Incubation temperature

Mesophiles

e.g., Sporosarcina pasteurii

30°C
Expected growth: 1–2 days

Cold-adapted Isolates

Environmental, slow growers

15°C or 4°C
Expected growth: 3–7+ days

Incubation Setup

  1. Place plates inverted (agar side up, lid side down) in incubator
  2. Optionally seal plate edge with Parafilm to prevent desiccation during long incubations
  3. Check plates daily for colony growth

For cold-adapted isolates growing slowly, allow up to 1–2 weeks before concluding no growth.

5. After Growth: Evaluate & Store

✓ What to Look For

  • Isolated colonies: Discrete, well-separated colonies in Zone 3 confirm successful isolation
  • Colony morphology: Confirm appearance matches expected strain characteristics
  • Lawn growth: If entire plate is covered, re-streak from a single colony onto fresh plate

✓ Next Steps from Isolated Colonies

  • Pick colonies for liquid cultures or starter cultures
  • Create new freezer stocks for long-term storage
  • Confirm strain purity by visual inspection or 16S sequencing
  • Use as inoculum for additional experiments

✓ Storage & Shelf Life

  • Temperature: Store working plates at 4°C
  • Sealing: Seal with Parafilm to prevent desiccation
  • Usable window: Within 2–4 weeks of growth
  • Long-term: Create freezer stocks (−80°C with glycerol) for preservation

💡 Pro Tips

Cross-contamination: When plating multiple strains in one session, change gloves or decontaminate between strains.

No growth? Ensure correct incubation temperature and allow adequate time (especially for slow growers at 4°C or 15°C).

Medium Selection Guide

Organism Type Recommended Agar Notes
Cold-adapted environmental isolates R2A agar Low-nutrient, supports slow growers
Sporosarcina pasteurii BHI agar Rich medium, supports fast growth
General/unknown R2A or TSA Broad-spectrum options