Activity 1 - Student Learning Strategies:
TESTING PHIX174 HOST RANGE

(Image from Virology.net, The Big Picture Book of Viruses, 2002)

PURPOSE
The first step in a successful viral infection is the attachment of the virus to its host cell. This can only take place when the virus and the host cell are compatible. Some viruses can only infect one particular host, while others can infect a number of different host species (the collection of susceptible host species is known as the host range of the virus). Compatibility hinges on whether or not the virus can bind to any of the host cell receptor molecules on the outside of the cell. These receptor molecules of course do not exist for the convenience of the virus. Rather, the virus has found a way to take advantage of cellular receptors that pre-exist for other reasons. We in turn can take advantage of viral specificity to begin to identify the virus. For bacteriophages such as Phi X 174, we can use a so-called plaque assay to test for host range. This assay is based on the ability of some viruses to lyse, or destroy the host cell. In the lab, a dilute solution of the bacteriophage is mixed with a dense bacterial culture and plated on a petri dish. Fed by the nutrients in the petri dish, the bacteria multiply to cover the entire dish, forming a visible bacterial lawn. Wherever a viral particle is present, if the virus and host are compatible, the virus particle will infect one of the bacterial cells. Viral replication followed by host cell lysis spews out many more infectious viral particles, each of which can go on to infect a neighboring cell. When enough neighboring host cells have been lysed, a characteristic clear area called a plaque will show up on the opaque bacterial lawn. The higher the number of plaques we see, the more compatible the host.

KEY CONCEPTS:
After this activity, students will be able to:
• define bacteriophage, bacterial lawn, host range, lysis, plaque, soft agar, plaque assay, (negative) control, serial dilution, and plaque morphology
• use the pour plate method to plate bacteria
• perform and interpret a plaque assay
• optional: define PFU, and calculate bacteriophage titer.