Chapter 9:

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

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

3. 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”

4. 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

5. 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

6. 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

7. 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

8. How does the process of phase variation occur?

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

12. THE END