Phages: host range, plaques and laboratory uses

Science · Microbiology · ages 19-21

Name ______________________ Date ____________

A plated portion of a diluted phage sample produces 30 well-separated plaques. How many plaque-forming units (PFU) are represented in that plated portion?
Answer: ______________
What directly allows a tailed phage to attach to a bacterial cell?
- The colour of the culture dish
- The diameter of the bacterial colony on the agar
- Matching phage proteins and cell receptors
The Hershey-Chase experiment provided evidence that DNA carries genetic information in T2 phage.
Circle one: True False
How can lytic phages reduce a susceptible bacterial strain's abundance in nature?
- They destroy the most abundant strain without infecting it
- They replicate in susceptible cells and cause lysis
- They block nutrient uptake in every strain in the community
How should a laboratory use phages to type an unknown bacterial isolate?
- Compare its full lysis pattern with reference patterns
- Name its exact strain from one spot's diameter
- Name its exact strain from how quickly one phage clears a spot
A phage can attach only through receptor R. A bacterium loses R completely. What happens to this phage's attachment to that cell?
- Attachment fails because its required receptor is missing
- Attachment improves when receptor R is lost
- Attachment continues through a different surface receptor
During generalized transduction, a phage particle supplies a recipient with a donor bacterium's gene. What DNA cargo supplies that gene?
- Only the recipient's existing chromosome, without any donor DNA
- A donor bacterial DNA fragment containing that gene
- Only the phage genome, without any donor bacterial DNA
A phage attaches normally to a resistant isolate but produces no new phages. A susceptible control supports replication under the same conditions. What is supported?
- It has lost all receptors needed for the phage to attach
- The phage cannot bind to this resistant bacterial isolate
- The block acts after attachment; its mechanism is unknown

Answer key

For grown-ups. Fold this page away before handing over the rest.

Phages: host range, plaques and laboratory uses W1-mt_vOnjSpXjrj-s1

30 · The plated portion represents 30 PFU. This measures plaque formation under the test conditions, not every physical particle or the total in the original sample.
Matching phage proteins and cell receptors · Compatible phage binding proteins and bacterial receptors allow attachment. Successful reproduction also requires later infection steps.
True · Most DNA label was associated with infected bacterial cells, while most protein label was outside them after separation. This supported DNA as T2 genetic material.
They replicate in susceptible cells and cause lysis · Lytic infection kills susceptible bacteria through cell lysis. The population effect also depends on encounters, host range and resistance.
Compare its full lysis pattern with reference patterns · Phage typing compares responses across a standard panel with reference lysis patterns. One positive spot does not uniquely identify a strain.
Attachment fails because its required receptor is missing · The prompt makes receptor R the phage's only attachment route. Losing R removes that route, so attachment fails.
A donor bacterial DNA fragment containing that gene · Generalized transduction can carry donor bacterial DNA in a phage particle. The fragment must contain the bacterial gene being transferred.
The block acts after attachment; its mechanism is unknown · Normal attachment contradicts an attachment-failure explanation. Replication succeeds in the control, but these results do not identify the isolate's later block.