Bacterial Virulence Factors

Dongwoo Shin

Laboratory of Molecular BacteriologyDepartment of Molecular Cell Biology

Sungkyunkwan University School of Medicine

What is a pathogen?

-An organism capable of colonizing a host organism where the interaction results in disease

Bacteria and Disease

-opportunistic pathogens

-strict pathogens

Bacteria and Disease

Establishing connection: Koch’s Postulates

• Proving cause and effect in infectious disease research

-first raised in the 1800s by Robert Koch

• Koch’s postulates

-association of the bacteria with the lesions of the disease

-isolating the bacterium in pure culture

-showing that the isolated bacterium causes disease in humans or animals

-reisolating the bacterium from the intentionally infected animal

Bacteria and Disease

What is virulence (pathogenicity)?

-The capacity of a pathogen to cause damage or disease in the host

-Virulence factors: Cell wall components, DNA, Proteins

Bacteria and Disease

Virulence factors: Extracytoplasmic components

- Cell wall components: Teichoic (Lipoteichoic) acid, LPS

- Lipoproteins

- Capsule

- Pili

Bacteria and Disease

Virulence factors: Extracytoplasmic components

- Cell wall structure: Gram (+) bacteria

Bacteria and Disease

Virulence factors: Extracytoplasmic components

- Gram (+) bacteria cell wall: Teichoic acid (polymers of modified ribose or glycerol connected by phosphate

Bacteria and Disease

Virulence factors: Extracytoplasmic components

- Cell wall structure: Gram (-) bacteria

Lipopolysacchride (LPS)

-Somatic O polysacchride + Core polysaccharide + Lipid A

Bacteria and Disease

Bacteria and Disease

Virulence factors: Various proteins with different functions

Bacteria and Disease

Biology of Salmonella Infection

Bacteria and Disease

“Environmental Signals inside Host”

Expression of Necessary Proteins Expression of Necessary Proteins (Virulence Proteins) in the Correct Tissues(Virulence Proteins) in the Correct Tissues

Virulence Genes

Genetic screenings of bacterial virulence factors

1. Pre-genomic era: 1990’s

2. Post-genomic era: 21C

In Vivo Expression Technology (IVET)

1. Isolation of Salmonella genes whose expression is induced inside the host (i.e. genes whose products are necessary for host infection)

2. Auxotrophic selection method (Science, 1993) and Differential fluorescence induction method (Science, 1997)

Auxotrophic selection method

Step 1: Creating transcriptional fusions of random fragments of the Salmonella chromosome with a promoterless purA gene; introducti

on of this library into a purA mutant

a mutant that cannot synthesize purines (auxotroph)

Auxotrophic selection method

Step 2: Integration of a plasmid construct into chromosome of a purA

mutant via single crossover

Auxotrophic selection method

Step 3: Host infection with the pool of fusion strains and selection

X-gal plate

Differential fluorescence induction (DFI)

Rationale: trapping the gene promoters that are activated inside macrophages; using GFP

macrophage

Activation of mgtC transcription

Salmonella

PmgtC gfp

Differential fluorescence induction (DFI)

Step 1: Cloning of random fragments of Salmonella chromosome into a promoterless gfp plasmid; introduction of plasmids into Salmon

ella

Differential fluorescence induction (DFI)

Step 2: Infection of macrophages with Salmonella harboring gfp fusio

n plasmids; sorting GFP-active Salmonella with FACS

Validation of a screened candidate for virulence

1. In vitro method: Gentamicin protection assay for evaluations of Salmonella invasion into and survival within host cells

2. In vivo method: Animal experiments for evaluations of Salmonell

a’s ability to infect host

Validation of a screened candidate for virulence

- Gentamicin protection assay:

Infection of epithelial cells (e.g. Hep-2) or macrophages (e.g. J774.A)

with Salmonella

Incubation allowing for Salmonella to invade into epithelial cells or for

macrophages to engulf Salmonella (i.e. phagocytosis)

Gentamicin treatment to kill bacteria outside host cells

Detergent treatment to lyse host cells; plating onto agar plate to coun

t Salmonella

Validation of a screened candidate for virulence

- Gentamicin protection assay:

a mutant that cannot produce SPI-2 TTSS

Validation of a screened candidate for virulence

- Animal experiments: Mouse infection model

- Oral infection and Intraperitoneal infection

- Immunocompromised mouse and Immunocompetent mouse

Validation of a screened candidate for virulence

- Animal experiments:

Genetic screenings of bacterial virulence factors

1. Pre-genomic era: 1990’s

2. Post-genomic era: 21C

Announcement of Salmonella genome sequence: Nature (2000)

“Now, one can predict which genes are important for Salmonella virulence and experimentally test them.”

In this paper, the authors evaluated the role of every single transcription factor in Salmonella virulence

- A revolutionary method for construction gene deletion mutants in E. coli: PNAS (2000)

- Applicable to other enteric bacteria: Salmonella, Klebsiella, Yersinia, Enterobacter etc.

- Accurate, fast, and cheap method