A bacteriophage is a virus that infects bacteria. The Luria-Delbrück experiment uses a bacteriophage challenge to reveal resistant biological colonies.
Bacteriophage T4 is a tailed, double-stranded DNA bacteriophage that infects Escherichia coli. Its viral capsid encloses the genome, while the tail and attachment structures participate in contact with the host and DNA delivery. A complete virion, a detached enzyme such as bacteriophage T4 DNA polymerase, and DNA remaining inside a shell are distinct experimental objects.
A primary study by Bo Hu and collaborators used cryo-electron tomography to examine phage–cell complexes. It observed different T4 infection intermediates, including tail contraction and particles with different amounts of DNA remaining in their heads.[ref]
Structural remodeling of bacteriophage T4 and host membranes during infection initiation by Bo Hu and others. Abstract and Figures 2–3: structural states and DNA retention
These frozen three-dimensional observations connect structure with an inferred sequence of entry events. They do not continuously film the same particle through every transition.A constructed counting example shows why one structural marker cannot replace another. Suppose 100 attached particles include 60 with contracted tails. Among those 60, suppose 40 have empty heads and 20 retain DNA. Then the contracted fraction is , whereas the empty-head fraction among contracted particles is . Neither count alone gives the number of progeny produced. These stipulated counts illustrate different denominators and endpoints; they are not observations from the cited study.
The primary work is especially useful because it separates attachment, structural remodeling and genome delivery instead of treating them as one instantaneous event. A shell or tail image supplies evidence about particle structure; successful genome transfer and subsequent reproduction require their own observations. This distinction also explains why the biochemical activity of a purified T4 enzyme is not a measurement of intact-phage infectivity.
Bacteriophage T2 is a bacteriophage that infects Escherichia coli. Its DNA and external protein components made it an experimental system for separating attachment from delivery of genetic material in the Hershey-Chase experiment.[ref]
Independent functions of viral protein and nucleic acid in growth of bacteriophage by A. D. Hershey and Martha Chase. Experimental material and abstract, points 1–8
A complete infectious particle and its individual components need not have the same abilities. In the original experiments, disrupting T2 particles separated much of their DNA from sulfur-containing material that retained specific attachment and antigenic properties. Thus observing attachment alone does not establish successful intracellular reproduction.[ref]
For a conceptual example, compare intact particles with particles whose entry machinery has been impaired. If both attach but only the intact preparation yields progeny, an attachment count alone cannot serve as a progeny count. This separates distinct stages of virus infection rather than treating every particle touching a bacterium as a completed infection.
A double-stranded DNA bacteriophage of Bacillus subtilis, widely discussed as a model of active genome packaging. A portal-associated molecular motor uses ATP hydrolysis to oppose the physical costs of dense DNA confinement.[ref]
Experimental Test of Connector Rotation during DNA Packaging into Bacteriophage phi29 Capsids by Thorsten Hugel and others. Introduction identifies the host and the packaging motor
A small single-stranded-DNA bacteriophage used in molecular-genetics research. Its compact genome made it useful for testing relationships among sequence, genetic organization and viral function.
