Lecture_17_Microbial Pathogenesis_Chapter_18.pptx

Microbial Pathogenesis • Bacteria - Important component of human microbiota but many taxa can cause disease • Eukaryotes - Many members of fungal taxa play beneficial roles in human health but some cause disease; beyond disease the role of protisits are largely unknown; nematodes can be commensal bu...

Microbial Pathogenesis • Bacteria - Important component of human microbiota but many taxa can cause disease • Eukaryotes - Many members of fungal taxa play beneficial roles in human health but some cause disease; beyond disease the role of protisits are largely unknown; nematodes can be commensal but often cause disease due to their parasitic nature • Viruses - Viruses influence disease pathogenesis directly (cause disease) and indirectly (aide other microbes in causing disease) • Toxins - Compounds (usually proteins and sugars) that can cause disease even after the microbe has been destroyed • Prions - Misfolded proteins that cause fatal brain necrosis Microbial Pathogenesis • Can you name any missing groups? • Why might they be excluded in most textbooks? Bacterial Virulence • Examples of virulence factors • Effector proteins (enzymes that harm the host by disrupting normal cellular function) • Secretion systems • Cellular structures involved with: • Persistence (capsule) • Adhesion (fimbrea/pilus) Case Study - Mary Mallon • Multiple outbreaks of typhoid fever were traced to a service worker (cook) • Mary Mallon was an Irish immigrant who didn’t present any signs of illness • 8 out of 9 houses she cooked in suffered widespread outbreaks • In the end over 3,000 New Yorkers are thought to have been infected (roughly 300 of them lost their lives) Case Study - Mary Mallon • How can you explain this case? (Mary was not showing signs of Typhoid fever herself but seemed to be spreading the pathogen) • Does anyone know what organism causes Typhoid fever? Case Study - Mary Mallon • Mary was the first documented case of an “ asymptomatic carrier” • This meant that she could spread the disease but was not negatively effected by the pathogen • Unfortunately she was demonized by the press and public and forced to live a life of exile for the majority of her adult life • Doctors working on samples taken from Mary were able to also isolate the causative agent of Typhoid fever, a bacteria called Salmonella Salmonella - Typhoidal vs Non-Typhoidal • Healthy: Salmonella enterica is commonly found in humans and many of the animals we eat • Reservoirs: Food processing plants, farms (animal and plant), hospitals, soil • May play commensal roles in metabolism and homeostasis by out competing other pathogens Salmonella - Typhoidal vs Non-Typhoidal • Salmonellosis: “food poisoning”, common illness that causes gastric and water/nutrient loss due to accumulation of water in the colon • Bacteria penetrate crypts and adhere to host cells but rarely invade the epithelium itself • Release of SopB (SPI-5) via Type 3 Secretion Systems into host cells causes osmotic imbalance due to activation of chloride channels Salmonella - Typhoidal vs Non-Typhoidal • Typhoid fever: serious and potentially fatal illness that can lead to bacterial proliferation into the blood stream and organs • Bacteria enter cells lining the intestinal epithelium (SPI-1), causing immune cell activation • Macrophages and other immune cells attempt to kill the bacteria by up taking the bacterial cells (phagocytosis) and fusing them with a lysosome Salmonella - Typhoidal vs Non-Typhoidal • The Salmonella are able to evade destruction by escaping the phagocyte (SPI-2) and forming a Salmonella Containing Vacuole (SCV) • SCV can become large, complex, branching structures (SIFs = Salmonella induced filament) • Salmonella infected immune cells can then penetrate tissue and enter the blood stream • These SCV-immune cells act as internal reservoirs Salmonella - Pathogenicity Islands • Genomic island: genetic loci that contains one or more protein coding genes and different G/C content than the rest of the genome (indicating horizontal gene transfer) • These loci are normally under expressional regulation by a master regulator • Pathogenicity Islands: a genomic island that contains multiple virulence factors Salmonella - Pathogenicity Islands • SPI = Salmonella Pathogenicity Island • SPI-1: • Associated with invasion of host cells • Loci contains all of the genes required to form a functional T3SS • AvrA > induces expression of cytokines • Sip protein family > SipC binds to actin and initiates membrane ruffling; SipA enhances ruffling • Master regulator: HilA > Activated in response to environment cues Salmonella - Pathogenicity Islands • SPI-2: • Associated with survival within host cells and maintaining the SCV • Loci contains all of the genes required to form a functional T3SS • SPI-2 seems to be able to fill the functional role of SPI-1 as mutant strains of Salmonella harboring a knock out of SPI-1 can still cause disease in mice and chickens • SseF > Structuring and localization of SCV • SifA > Formation and maintenance of SIF networks • SspH1 > Inhibition of host apoptosis pathways via down regulation of JNK pathway Salmonella - Pathogenicity Islands • SPI-2: • Master regulator: SsrAB two component system • SsrA > large trans membrane protein (likely kinase) • SsrB > transcriptional regulator, becomes phosphorylation by SsrA and promotes the transcription of SPI-2 genes • Heat Shock and SPI-2: • Heat stress has been shown to increase the expression of SsrB and SPI-2 genes • Improperly cooked food could contain more virulent populations of Salmonella • Many putative small heat shock proteins share homology with SPI-2 effector proteins Salmonella - Pathogenicity Islands • SPI-5: • Function still being examined and debated but seems to play a role in both intra and exta cellular survival • Does not contain a T3SS, relies on SPI-1 or SPI-2 TS33 for translocation of effector proteins • SopB > Causes water loss from host tissue (SPI-1 T3SS) • PipD > Cystine protease that seems to be associated with long term systemic infections (SPI-2 T3SS) Secretion Systems • Allow bacteria to move compounds from the cytosol to the periplasm, extracellular environment, and host cells • Inner membrane bound proteins Tat and Sec are required for Type 1, 2 and 5 • We will focus on 2, 3, and 4 Secretion Systems • Type II Secretion Systems: • Specific (each system only allows one specific compound to pass through the membrane) • Require inner membrane bound proteins (SecA/Tat) to move compound into periplasm • Allows bacteria to secrete compounds into the extracellular environment • “Piston”; reversible polymerization causes compound to be ejected out of the second membrane/cell wall • Derived from fimbrea proteins Secretion Systems • Type III Secretion Systems: • Non-Specific (rich variety of effector proteins and other compounds can be localized) • Does not require secondary proteins to secrete compounds • Allows bacteria to secrete compounds directly into host cells • “Needle”; Consumption of ATP allows compounds to be moved against their concentration gradient • Derived from flagella proteins • Involved with virulence Secretion Systems • Type IV Secretion Systems: • Non-Specific (rich variety of effector proteins and other compounds can be localized) • Does not require secondary proteins to secrete compounds • Allows bacteria to secrete compounds directly into host cells • “Tube”; Passive diffusion relies on compounds flowing with their concentration gradient • Derived from flagella proteins • Involved with conjugation Secretion Systems • What are the main differences between Type 2 and Type 3/4 Secretion Systems? • Is there a key difference between Type 3 and 4 systems? Bacterial Virulence • Examples of virulence factors • Effector proteins (enzymes that harm the host by disrupting normal cellular function) • Secretion systems • Cellular structures involved with: • Persistence (capsule) • Adhesion (fimbrea/pilus) Persistence • Bacterial capsules are composed of dense layers of hydrated glycoproteins • This helps them evade immune detection by altering the antigenic epitopes they present to the lumenal environment • Capsules also aide in adherence Adhesion • Assembly of the pili starts with the tip protein. • It will bind to carbohydrates on the host cell surface • Pili are made up of identical pilin protein subunits • The tip of the pilus contains an adhesion protein that binds to host cell receptors • PapD > Molecular chaperon that plays an essential role in pili synthesis (must be translocated to periplasm by SecA) Adhesion • Type 1 Pili - Add in adherence to lumenal tissues to avoid host clearance mechanisms (mucus systems) • Type 4 Pili - Twitching motility (expansion) Endotoxin vs. Exotoxin • Endotoxin > Component of bacterial cell that is released when cells are destroyed • LPS = Lipopolysachride • Part of the outer membrane of the Gram-negative cell wall that includes lipopolysaccharide • After cell lysis, LPS fragments can cause inflammatory response • Fever, activation of clotting factors, activation of complement, vasodilation, shock, and death may result when endotoxin is released into the blood Endotoxin vs. Exotoxin • Exotoxin > Secreted by microbes • Two-subunit AB toxins • A subunit is toxic • B subunit binds host cell receptors; many B subunits are complexes of five units arranged as a ring • Tetanospasmin = AB exotoxin produced by Clostridium tetani Endotoxin vs. Exotoxin • Anthrax toxin: • Has two A subunits called edema factor (EF) and lethal factor (LF) • The B subunit is called protective antigen (PA) • Edema factor causes fluid loss to extracellular spaces • Lethal factor destroys regulatory cascades and cripples the immune response Endotoxin vs. Exotoxin • Toxin is produced by the bacteria: Bacillus anthracis • Soil microbe that has been used in terrorist attacks around the world • Exposure to high doses of anthrax can be fatal as it causes lung necrosis Eukaryotes - Immune System Evasion • Immune avoidance by protozoa • Antigenic masking: some protozoans coat themselves in host antigens to avoid detection by the immune system. • Antigenic variation • Intracellular location • Immunosuppression: some protozoans induce secretion of antiinflammatory cytokines to reduce the innate immune response Viruses - Latency • Human herpesviruses and latency • After a primary infection, human herpesviruses become latent in host cells (nerve cells or white blood cells) • DNA circularizes and exists as an episome • DNA integrates into host cell genome • Latent virus can reemerge after years of latency and cause a new active infection Viruses - Latency • Varicella-zoster virus > chicken pox at young age; but virus remains latent and can cause shingles later in life • Normally effects people >50 years old but can cause disease earlier Viruses - Latency • What are the differences between shingles and chickenpox? • Can both effect people of any age? End.... 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