Some bacterial viruses penetrate into their host by fusing their viral membranes with the host outer membrane. The virus lipid membrane involved can be either external (e.g. Cystoviridae) or internal (e.g. Corticoviridae) which in the latter case requires the preliminary dissociation of the surrounding capsid.
The fusion events lead to the release of respectively the viral genome or the capsid into the host periplasmic space 
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