Bacteriophage Purification by Plaque Streaking Protocol

Plaque streaking is a core laboratory procedure used to isolate and purify a single viral strain (bacteriophage or phage) from a mixed phage population obtained during initial enrichment steps.
Outcomes of initial spot assay screening on enrichments:
- No Lytic Infection: The bacterial lawn remains entirely intact with no visible clearing (e.g., soil enrichments tested against Bacillus subtilis showing zero lysis).
- Complete Lytic Clearing: Defined spots or broad clear zones where filtered enrichment successfully lysed the bacterial host (e.g., Caulobacter crescentus / Colobacter crescentis assays showing defined cleared zones, indicating one or more phage types present).
- Incomplete Clearing / Discrete Plaques: Sub-confluent infection resulting in clusters of small, individual plaques rather than total lawn destruction, serving as direct evidence of phage activity.

Inoculating Tool:
- Utilizes a specialized inoculating needle, consisting of a thin metal wire.
- The wire is blunt and not sharp enough to puncture skin.
Aseptic Technique:
- Sterilize the wire needle in a Bunsen burner flame until the entire wire glows red hot.
- Allow the needle to air-cool for approximately $15\,\text{seconds}$ prior to contacting biological samples to avoid heat-inactivating the bacteriophages.

Plaque Sampling Procedure:
- Direct the cooled, sterile needle tip into a cleared lytic zone or poke a single, well-defined plaque on the assay plate.
Streaking Procedure:
- Lightly drag the needle tip across the surface of a fresh, sterile agar plate to spread and deposit phage particles.
- Although care should be taken to avoid gouging or cutting into the agar medium, light surface cuts frequently occur and do not disrupt the protocol.
- Surface streaking physically dilutes and separates phage particles across the agar face.

Achieving viral purity requires sequential passes known as "streaking to isolation":
- Round 1 (Spot Assay Plate): Initial screening plate containing localized spot transfers from environmental enrichment samples.
- Round 2 (First Streak Plate): Produced from Round 1. Typically yields mixed results featuring regions of confluent lysis alongside regions with separated, individual plaques (e.g., observed with Pseudomonas fluorescence soil isolates).
- Round 3 (Second/Third Streak Plate): Produced by sampling a single isolated plaque from Round 2 and streaking onto fresh agar to ensure clonal isolation.
Adapting to Phenotypic Variations:
- Discrete Plaques Present: Target and poke a single, well-isolated plaque (e.g., Bacillus subtilis or Caulobacter crescentus streak plates) to isolate a single genetic line.
- Confluent Lysis Without Discrete Plaques: Certain highly efficient lytic phages (e.g., phages infecting Bacillus licheniformis) may clear agar surfaces without forming distinct individual plaques. In such instances, poke directly into the cleared agar area to transfer phages into the subsequent round of streaking.

Matching Host Cultures:
- Each streaked plate requires its specific bacterial host strain corresponding to the original enrichment (Caulobacter crescentus / Colobacter croissantis, Pseudomonas fluorescence, Bacillus licheniformis, or Bacillus subtilis).
- Dispense $500\,\mu\text{L}$ ( $500\,\text{microliters}$ ) of host liquid culture into an individual sterile glass slip-cap tube for every streak plate prepared.
Top Agar Overlay Protocol:
- Using a $5\,\text{mL}$ serological pipette paired with a pipette pump, draw up approximately $4.5\,\text{mL}$ of molten top agar.
- Transfer the $4.5\,\text{mL}$ of top agar directly into the glass culture tube containing the $500\,\mu\text{L}$ host bacterial suspension.
- Gently mix the host and top agar solution.
- Immediately pour the mixture evenly over the top of the agar plate that was previously streaked with phage particles.
- Allow the top agar overlay to set and solidify, creating a host lawn within which transferred phages can infect host cells and form clear plaques.

Step 1: Initial phage isolation from environmental samples using host enrichment.
Step 2: Screening enrichments for lytic phage presence via spot assay testing.
Step 3: Purifying phages through repeated needle streaking and targeted single-plaque selection.
Step 4: Select a single pure plaque from the final streak round, propagate viral progeny from that clone, and utilize the resulting pure phage isolate for downstream experimental assays.