Plaque halos: What causes plaque halo formation?

What is plaque halo?

Plaque halo refer to the partial-turbidy clearance surrounding the original phage induced clearance of the bacterial lawn. Not every phage can cause the formation of plaque halos.
Plate showing plaques having halo regions

How do halo regions look?

Halos appears translucent with less turbidity than the bacterial lawn region and less clear that the phage clearance zone.

Do halo regions have phage particles?

Plaque halos particularly tend to lack either phage particles or phage infections. The enzymatic activities induce their formation.

What causes halo formation?

The diffusion and subsequent enzymatic action especially of soluble (not virion associated), phage-produced EPS depolymerases causes halo formation. In comparison to the phage particles these enzymes are substantially smaller  hence they are capable of diffusing further into bacterial lawns.

Are phages with halos bad or good for phage therapy?

Up to now, phage scientists have not found any negative impact that might be initiated by depolymerase enzymes once those phages are used in therapy. In fact some scientist have taken it into advantage and try to extract these enzymes as a possible antibacterial agent by themselves.


Halos often form following plaque formation, and indeed can form during plate refrigeration. This occurs because halos, as noted, as not a direct consequence of phage population growth but instead of extra-virion diffusion along with virion-free enzymatic degradation of bacteria. Furthermore, that degradation is not of the bacterium as a whole but instead of just exterior layers that are bacterium produced and associated.

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