Mechanism of Bacterial Toxins: Cholera and Anthrax Insights

Mechanism of Bacterial Toxins Choleragen and Anthrax Toxin Vibrio cholerae Bacillus anthracis

Vibrio cholera Vibrio cholerae is a gram-negative, comma-shaped rod belonging to the family Vibrionaceae. Its natural habitat consists of fresh-water and salt-water environments. Based on differences in the composition of the major cell wall antigen (O), 139 serotypes have been differentiated.

Vibrio cholerae, which belongs to either serogroup O1 which is responsible for the seventh cholera pandemic, is divided into three serotypes, Inaba, Ogawa and Hikojima (a variant of the Ogawa serotype) , or serogroup O139, has been associated with epidemic cholera.

Vibrio cholerae Vibrio cholerae Produces A + 5B exotoxin Gram negative vibrio Unusual disease Cholerae does not invade tissue Cholerae does not damage tissue Lives in estuaries on copepods Humans are incidentally infected when ingesting contaminated food or water

Symptoms of Vibrio cholerae Symptoms Secretory or watery diarrhea No blood in diarrhea Large watery bowel movements Loss of electrolytes Muscle cramps Low blood pressure Rapid heart rate & feeble pulse Vomiting White blood cell count usually normal Treatment Usually self limiting symptoms as long as IV fluids are administered with oral rehydration solutions

Mechanism of Action of cholerae toxin Normally Adenylate cyclase (AC) enzyme which makes cAMP Epithelial cells secrete digestive fluid (HCO3-) in response to small increases in cAMP levels cholerae exotoxin ADP ribosylation of G-protein Over activation of AC Causes 100X increase in cAMP Causes huge amounts of water and Cl- to leave via channels

Bacillus anthracis Bacillus anthracis Produces 2A + B exotoxin Gram positive spore forming bacteria Found in soil Anthrax disease direct exposure to spores Inhalation pulmonary Ingestion gastrointestinal Invasion into surface wound cutaneous No cases involve person to person spread

Symptoms of Bacillus anthracis Cutaneous Spores enter abrasions or cut in skin Germination of spore causes local ulceration of the skin Painless black eschar with edema Antibiotics prevent invasion into blood stream Usually heals completely without scarring

Symptoms of Bacillus anthracis Pulmonary Life cycle Macrophages engulf spores Travel to nodes Spores germinate en route Cells are released spreading toxins and vegetative cells into the blood stream Symptoms caused by toxins Fever and chills Shortness of breath, & cough Massive pleural effusions Sepsis, shock & death

Mechanism of Action of anthracis Toxins Two primary toxins & capsule gene All three genes are located on plasmids Edema Factor A toxin Adenylyl cyclase enzyme increase in cAMP Causes edema and pro-inflammatory response Lethal Factor A toxin Metalloprotease Cleaves MAP kinase (Mitogen Activated Protein kinase) required from cell division and signaling Causes an overall suppression of immune system

Mechanism of Action of anthracis Toxins EF LF EDEMA B Increased expression of pro-inflammatory mediators EF cAMP Endosome B Acidic Environment MAPK IMMUNE SUPPRESSION WBCs do not divide in the presence of pathogens; overall decrease in phagocytosis

Normal Signaling Pathway Adenylate Cyclase Receptor GDPGTP cAMP P PKA Catalytic Subunit PKA Regulatory Subunit Leads to a temporary increase in pro-inflamatory proteins and an overall activation of the immune system. Nucleus

Mechanism of Action of Edema Toxin Cell Membrane toxin has already entered through RME Over expression of cAMP by edema factor A EF PKA Catalytic Subunit cAMP P Results in overall systemic edema Leads to an abnormal increase in pro-inflamatory proteins, increased capillary permeability & decrease in phagocytosis Nucleus

Intracellular Signaling Pathways Terms Adenylate Cyclase (AC) Membrane enzyme which makes cAMP cAMP Inracellular second messenger Binds to the regulatory subunit of PKA and releases an active catalytic subunit G proteins A group of proteins which activates AC When GDP is bound not active When GTP is bound is active short lived GTP is self hydrolyzed by the G protein back to GDP This ensures that the levels of cAMP are tightly regulated Exchange of GDP to GTP requires hormonal activation

Intracellular Signaling Pathways Terms PKA protein kinase A Enzyme which phosphorylates (P) other proteins Phosphorylation activates some proteins Phosphorylation inactivates other proteins Transcription Factor DNA binding protein which binds to genes to increase or decrease the amount of gene expression In some cases phosphorylation (P) is necessary for the transcription factor to bind DNA

Mechanism of Action of Lethal Toxin Cell Membrane toxin has already entered through RME LF Protease Needed for cell division of WBCs during an immune response MAP Kinase Needed for expression of TNF Tumor necrosis factor which increases chemotaxis and phagocytosis Due to LF protease activity MAP kinase is not active and normal cellular responses are lost Degraded MAP kinase Protein