Bacteriophages as Predators and as Laboratory Tools
Bacteriophages infect bacteria. Their host ranges shape which bacterial populations they affect and allow phage typing. Plaque assays measure infectious units, and phages also provide tools for studying and transferring genes.
What a learner can do afterwards
- Explains host range and why receptor binding alone does not guarantee productive infection.
- Interprets separated plaques as plaque-forming units in the plated portion under stated assay conditions.
- Explains how phages support bacterial typing, tests of genetic material and transfer of bacterial genes.
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You have studied lysis. A bacteriophage, or phage, infects bacteria. Its host range is the strains in which it can reproduce under given conditions. Many tailed phages use binding proteins, including tail fibres, to recognize bacterial surface receptors. Compatible receptors permit attachment, but later steps may be blocked by host defences. Host range can include one strain or several.
A plaque is a patch of reduced growth in a bacterial lawn caused by phage infection. New phages spread to nearby cells, so one plaque involves many infections. Count well-separated plaques as plaque-forming units (PFU): 46 plaques represent 46 PFU in the portion plated. PFU measures infectivity on the chosen host under the assay conditions, not every physical particle. An aggregate of particles may form one plaque; some particles form none.
Phage typing compares a bacterial isolate's responses to a standard panel of phages with reference lysis patterns. An isolate is a bacterial culture obtained from a sample. Different host ranges give different patterns of lysis. A matching pattern supports a phage type, but one positive spot does not uniquely identify a strain. Use standard conditions and controls when comparing patterns.
Hershey and Chase labelled T2 phage DNA and protein in separate preparations. After infection, they removed the attached phage coats and separated bacterial cells from the fluid. Most DNA label was with the cells; most protein label was in the fluid. This supported DNA as T2 phage's genetic material. It did not show that all viruses have DNA genomes.
Phages can transfer bacterial genes by transduction. In generalized transduction, a particle can package a fragment of the donor bacterium's DNA during phage production. It can deliver that fragment to another bacterium, the recipient. If the DNA is retained and expressed, the recipient may gain a trait. The DNA carrying the transferred bacterial gene comes from the donor, rather than being only the usual phage genome.
Lytic phages reproduce inside susceptible bacteria and release new particles when infected cells lyse. This can reduce susceptible strains and change competition among bacteria. A common strain is not necessarily susceptible. Host range, encounter rates and resistance affect the outcome, so phages do not always remove the most abundant strain.
Resistance can prevent attachment or block a later step. Losing or altering a receptor can stop a phage that needs it from attaching. If attachment remains normal but no new phages are produced, the block can be after attachment. Compare with a susceptible control under the same conditions. These results locate a stage, not the exact defence mechanism.
Host range requires successful reproduction. Plaques measure PFU under assay conditions. Phages can type bacteria, investigate genetic material and transfer bacterial DNA.
Lesson, questions, worked solutions and flashcards written for this dataset.
Readings and flashcards written for this dataset. Linked videos are credited to Khan Academy in the supplied source records: Evidence that DNA is genetic material 2 and Viral replication: lytic vs lysogenic. Video licensing terms have not been verified.
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Testing the limits of host range
A phage can encounter a bacterial cell without completing an infection. Host range describes the bacterial strains that support its reproduction under specified conditions.
For many tailed phages, attachment depends on a match between phage binding proteins and bacterial surface receptors. Removing a required receptor can prevent attachment. A change elsewhere in the cell may instead stop an infection after the phage has bound.
These possibilities lead to different predictions. If a resistant isolate no longer binds the phage, a change affecting attachment is a possible explanation. If attachment remains normal but no new phages are produced, attachment failure does not explain the result. Comparison with a susceptible control helps interpret the observation, but further tests are needed to identify the mechanism.
Host range is therefore not fixed by receptor matching alone. It depends on whether the phage can complete the steps needed for reproduction. Some phages reproduce in one bacterial strain; others reproduce in several.
Reading plaques and lysis patterns
A bacterial lawn is a dense layer of bacterial growth. Local phage infection can produce a plaque, where growth is reduced. Released phages infect nearby cells, so a visible plaque records the effects of successive infections.
Count well-separated plaques as plaque-forming units, or PFU, in the portion plated. For example, 52 separated plaques represent 52 PFU in that portion. This is an infectivity measurement for the chosen host and test conditions. It is not an exact count of physical particles. Some particles cannot form plaques in the assay, and an aggregate of particles may produce one plaque. Overlapping plaques cannot be counted reliably as separate plaques. Estimating the original sample's concentration also requires the dilution and plated volume.
Phage typing uses a panel-wide comparison rather than a plaque total. A laboratory records which phages lyse an isolate and compares the pattern with reference patterns under standard conditions. A match supports a phage type. One positive spot does not uniquely identify a bacterial strain.
Studying and moving genes with phages
Hershey and Chase used T2 phage to investigate genetic material. They labelled DNA and protein in separate preparations. After infection, they removed the attached phage coats and separated bacterial cells from the surrounding fluid. Most DNA label was associated with the cells, while most protein label was in the fluid. These observations supported DNA as T2 phage's genetic material. They did not establish that every virus has a DNA genome.
Phages can also transfer bacterial genes through transduction. During generalized transduction, a particle can package a fragment of the donor bacterium's DNA. It can then deliver that fragment to another bacterium. If the recipient retains and expresses the transferred DNA, it may gain a trait.
The DNA-label experiment and transduction answer different questions. One investigated which phage material carried genetic information. The other describes how a particle can move bacterial DNA between cells. Neither use requires every phage to infect exactly one bacterial strain.
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