Somewhat turbid regions of partial clearing found external to localized, less-turbid phage-induced clearings of bacterial lawns.
Plaque halos particularly tend to lack either phage particles or phage infections. Instead, halos are formed via the diffusion and subsequent enzymatic action especially of soluble (not virion associated), phage-produced EPS depolymerases. Because these enzymes are substantially smaller than phage particles they are able to diffuse substantially further into bacterial lawns.
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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