Chapter 9: Inversion of a Transcription- Changing Region that Switches Salmonella Flagellar Antigens By Cheryl Weddle October 10, 2002

Overview of Phase Variation Define Phase variation Discovery and methods Key features Mechanisms of phase variation Significance

What is Salmonella typhimurium? It is a type of bacterium similar to that of E. coli and if ingested into the body from certain foods can cause “food poisoning”

Flagellar Antigen Variation Salmonella flagellae can exist in one of two types: H1 or H2 phase, differing in relative orientation of a small chromosomal segment Only one type is found in a given cell, but they can convert to express the other type of flagella This phenomenon is known as phase variation

Discovery of Phase Variation It was first observed in the 1940’s… by some person(s) Initial studies were performed using DNA annealing and electron microscopy

Other Methods Utilized Cloning and sequencing the genes involved Analyzing mutants defective in the switching mechanism Developing an in vitro switching system using purified proteins to direct the process

Two Key Features of Phase Variation The transition from H1 to H2 flagella occurs about once in every one-thousand cell divisions Each phenotype is completely reversible

How does the process of phase variation occur?

Antigenic Variation and Significance Regarding Infection It is important to the viability of the species due to these three factors: 1A small fraction of the bacterial cells are resistant to human antibodies 2It extends the period of infection 3Increases the # of new hosts that become exposed

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