Phage bind to specific receptors on the host cell. See the following link for an electron micrograph of phage binding to bacteria.
The receptors are bacterial structures exposed to the environment. For example, many phage bind to the lipopolysaccharide (LPS) or outer membrane proteins on the surface of Gram negative bacteria, some phage bind to flagella, and some phage bind to fimbriae. Phage receptors are often identified by mutations that make the bacteria resistant to phage lysis.
The first step of attachment is called adsorption. This is a diffusion limited process that relies on a protein on the phage surface colliding with and binding to a molecule on the bacterial cell surface. This process does not require energy and thus (in most cases) can occur at low tempertures (in an ice bath or on the lab bench). At this initial stage the phage can be dissociated from the host cell. After the phage is attached to the host cell, entry requires secondary energy requiring steps that alter the host cell, and ultimately result in entry of the phage nucleic acid into the bacterial cytoplasm, leaving the phage protein capsid outside of the host cell.
Evidence that the phage nucleic acid enters the host cell and the phage protein capsid remains outside the host cell was first provided by the Hershey and Chase Experiment. This experiment is famous because it provided strong biological support that DNA is the genetic material.
Electron micrographs of phage T4 which seems to have a long extended tail before binding to E. coli, and a compressed tail upon release of the DNA into E. coli suggested that the DNA is injected into the host cell like a hypodermic syringe. However, several lines of evidence indicate that this model is oversimplified: hypodermic model predicted that pressure in the phage head propelled the DNA across the cytoplasmic membrane, but measurements of the pressure in phage heads indicte that the pressure is too low to push the DNA into the cell; the hypodermic model predicted that the host is a passive participant in the transfer of DNA, but uptake of phage across the cytoplasmic membrane requires energy; many phage have very short, non-contractile tails that could not provide a long enough needle. Thus, although this process was discovered well over 50 years ago, it is still not clear how DNA from most phage crosses the cytoplasmic membrane into the host cell.
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Last modified July 10, 2004