What Causes Phage Plaque Halo?

What is plaque halo?

Plaque halo refers to the partial-turbidly clearance surrounding the original phage-induced clearance of the bacterial lawn. Not every phage infection can cause the formation of plaque halos.
Culture petri dish showing bacteriophage plaque halos on a bacterial lawn
Plate showing plaques having halo regions

How do halo regions look?

Halos appear translucent with less turbidity than the bacterial lawn region and less clear than the phage clearance zone. This layer usual follows the shape of the original phage cleared zone (Refer to the photo above)

Do halo regions have phage particles?

Plaque halos, in particular, are devoid of phage particles or phage infections. Their formation is purely triggered by enzymatic activities. These proteins (enzymes) are released from the phage-induced clearance zone. Although some phage particles may be tested in the plaque's vicinity,

What causes halo formation?

The diffusion and subsequent enzymatic action, especially of soluble (not virion-associated) phage-produced EPS depolymerase, causes halo formation. In comparison to the phage particles, these enzymes are substantially smaller; hence, they can diffuse 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 scientists have taken it to an advantage and tried to extract these enzymes as possible antibacterial agents by themselves (Read about phage lysins).

Halos often form following plaque formation, and indeed can develop during plate refrigeration. This occurs because halos, as noted, are not a direct consequence of phage population growth but instead of extra-virion diffusion and 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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