# 3.5: Plating on Petri Plates for Isolation

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## Preparation

• sterile TSB test tubes (2 to grow cultures and another 3, 6, 9, or 12 [depending on how many cultures you want groups to share])
• 3 TSA Petri plates per group
• Inoculate TSB test tube with Escherichia coli and incubate at 37°C 24 hours before lab class
• Inoculate TSB test tube with pigmented strain of Serratia marcescens and incubate at 30°C 24 hours before lab class
• Just before lab class, aseptically transfer 200 μm of Escherichia coli culture to sterile TSB test tube(s) - add to 1-4 sterile TSB test tubes (E. coli cultures for students to use)
• Just before lab class, aseptically transfer 200 μm of Serratia marcescens culture to sterile TSB test tube(s) - add to 1-4 sterile TSB test tubes (S. marcescens cultures for students to use)
• Just before lab class, aseptically transfer 100-200 μm of Escherichia coli culture to sterile TSB test tube(s) and 100-200 μm of Serratia marcescens culture to the same test tube(s) - add to 1-4 sterile TSB test tubes (E. coli + S. marcescens cultures for students to use)

## Materials

• 3 TSA plates per group
• test tube racks (1 per group)
• Bunsen burner (1 per group)
• inoculating loop (1 per group)
• markers
• culture of E. coli
• culture of S. marcescens (pigmented strain)
• culture of E. coli + S. marcescens (pigmented strain)

## After Lab Class

1. Incubate quadrant streak plates at 30°C for 24-48 hours.
2. Refrigerate streak plates to prevent overgrowth if the next lab class occurs after 24-48 hours.
3. Have students examine streak plates and characterize colonies next lab class.

This page titled 3.5: Plating on Petri Plates for Isolation is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Rosanna Hartline.