Microbial Mechanisms of Pathogenicity (Chapter 15) Lecture ...

Microbial Mechanisms of Pathogenicity (Chapter 15)

Pathogenicity = ability to cause disease Virulence = degree of pathogenicity -pathogens must first gain access to the host: -must adhere and penetrate before infection is established -then must continually evade host defenses -infection usually causes host damage: disease

Lecture Materials for Amy Warenda Czura, Ph.D. Suffolk County Community College

To cause disease a pathogen must: 1. gain access to the host 2. adhere to host tissues 3. penetrate or evade host defenses 4. damage the host, either: - directly - accumulation of microbial wastes

Eastern Campus Primary Source for figures and content: Tortora, G.J. Microbiology An Introduction 8th, 9th, 10th ed. San Francisco: Pearson Benjamin Cummings, 2004, 2007, 2010.

Entry Into Host 1. Portals of Entry A. Mucous membranes (moist mucosa) -most common route for most pathogens -entry through mucus membranes: 1. respiratory tract (most common) 2. gastrointestinal tract 3. urinary/genital tracts 4. conjunctiva B. Skin (keratinized cutaneous membrane) -some pathogens infect hair follicles and sweat glands -few can colonize surface -unless broken, skin is usually an impermeable barrier to microbes

Amy Warenda Czura, Ph.D.

C. Parenteral route -penetrate skin: punctures, injections, bites, cuts, surgery, etc. -deposit organisms directly into deeper tissues

-most microbes must enter through their preferred portal of entry in order to cause disease -some can cause disease from many routes of entry -most usually also exit the host from the same original portal to spread disease 1

SCCC BIO244 Chapter 15 Lecture Notes

2. Numbers of Invading Microbes -likelihood of disease increases as the number of invading pathogens increases ID50 (Infectious Dose) = number of microbes required to produce infection in 50% of the population -different ID50 for different pathogens -different ID50 for different portals of entry for the same pathogen LD50 (Lethal Dose) amount of toxin or pathogen necessary to kill 50% of the population in a particular time frame

-pathogen has surface molecules called adhesions or ligands that bind specifically to the host surface receptors -most microbial adhesions are glycoproteins or lipoproteins located on the glycocalyx, capsule, capsid, pili, fimbriae or flagella -most host receptors are typically proteins (for virus) or carbohydrates (for bacteria) in the wall or membrane of host cell Biofilms: -formed when microbes adhere to a surface that is usually moist and contains organic matter -each microbe secretes glycocalyx allowing other microbes to adhere; a large mass is formed

3. Adherence = attachment to the host by the microbe at portal of entry -usually necessary for virulence -blocking adhesion can prevent disease

-the biofilm is resistant to disinfectants and antibiotics (outer layer protects inner layers) -problem for catheters and surgical implants: serves as chronic reservoir

2. Cell Wall Components A. M protein of Streptococcus pyogenes: -heat and acid resistant -mediates attachment of bacterium to epithelial cells -resists phagocytosis by leukocytes

http://www.primary-plus.com/wp-content/uploads/2009/12/biofilm.jpg

Penetration of Host Defenses 1. Capsules = organized glycocalyx layer (carbohydrates) outside cell wall -impairs phagocytosis: prevents engulfment and destruction by leukocytes -if present, is usually required for virulence -some nonantigenic Amy Warenda Czura, Ph.D.

http://www.nibib.nih.gov/nibib/image/Eadvances/June07/EHEgelman_neisseria_pili.jpg

B. Fimbriae + Opa (membrane protein) used by Neisseria gohorrhoeae: -promote attachment and uptake by host epithelial cells and leukocytes -Neisseria then grows inside these cells C. Mycolic acid (waxy) of Mycobacterium tuberculosis 2

SCCC BIO244 Chapter 15 Lecture Notes

-resist digestion by phagocytes -Mycobacterium then grows inside phagocyte

D. Collagenase: breaks down collagen (fibrous part of connective tissue) for invasion into muscles and organs e.g. Clostridium species E. IgA proteases: destroy host IgA antibodies found in mucous secretions to allow adherence and passage at mucus membranes e.g. Neisseria species that infect CNS 4. Antigenic Variation -pathogen alters its surface antigens to escape attack by antibodies and immune cells e.g. Neisseria gonorrhoeae -has many versions of the Opa gene -can alter which one is being expressed e.g. influenza virus -constant genetic recombination between flu viruses: always new spike proteins

http://upload.wikimedia.org/wikipedia/commons/d /d3/Mycobacterium_avium-intracellulare_01.png

3. Enzymes (exoenzymes) A. Coagulases: clot fibrin in blood to create protective barrier against host defenses B. Kinases: dissolve clots (fibrinolysis) to allow escape from isolated wounds e.g. Streptokinase (Streptococcus pyogenes) Staphylokinase (Staphylococcus aureus) C. Hyaluronidase: hydrolyzes hyaluronic acid (‘glue’ that holds together connective tissues and epithelium barriers) allowing deeper invasion e.g. Clostridium species: allows them to cause gangrene (tissue necrosis) http://www.humanillnesses.com/images/hdc_0001_0001_0_img0044.jpg

5. Penetration into Host Cytoskeleton -use actin of host cell to penetrate and move within the cell A. Invasins: surface proteins produced by bacteria to control actin e.g. Salmonella -rearrange actin: cause the cell membrane to wrap around the microbe and take it into the cell (endocytosis) -allows Salmonella to penetrate intestinal epithelium e.g. Shigella and Listeria -trigger endocytosis -polymerize actin behind bacterium to propel through host cell Amy Warenda Czura, Ph.D.

B. Cadherin: allows penetration between cells at intercellular junctions e.g. Shigella and Listeria: move between cells at cell junctions. Damage to Host Cells 1. Using Hosts Nutrients e.g. iron -required for all cells (electron transport chain: cytochromes) both host and pathogen -host usually does not have free iron available (free iron leads to easy colonization by pathogens) -humans bind unused iron to transport proteins: transferrin -pathogens can produce siderophores: secreted by bacteria to compete iron from host proteins, siderophore iron complex then absorbed by bacteria 3

SCCC BIO244 Chapter 15 Lecture Notes

2. Direct Damage To Colonized Area -growth and replication in host cells: results in host cell lysis -penetration through host cells (mucosa, organs) causes damage -lysis of host cells to obtain nutrients

3. Production of Toxins Toxins = poisonous substance produced by microbes -tend to cause widespread damage/disease in host -may be necessary for virulence A. Exotoxins -produced inside the bacteria and either secreted or released following microbe lysis -toxin genes are often found on plasmids or via lysogenic phages -most are enzymes -function to destroy certain host cell parts or inhibit particular metabolic functions -damage from toxin results in the particular signs or symptoms of a disease -can be named for the disease, type of cell attacked or organism that produces it e.g. tetanus toxin: causes tetanus (contraction) of muscle

-three types of exotoxins: 1) A-B toxins Two parts: A is the enzyme that disrupts some cell activity B binds surface receptors to bring A into the host cell e.g. botulinum & tetanus toxin

e.g. leukocidins: make protein channels in phagocytic leukocytes e.g. hemolysins: make protein channels in RBCs (!-hemolysis: Steptococcus pyogenes) 3) Superantigens -bacterial proteins that cause proliferation of T cells and release of cytokines -excessive cytokines can cause fever, nausea, vomiting, diarrhea, shock and death (septic shock) e.g. toxic shock syndrome (Staphylococcus) e.g. enterotoxins: Staphylococcal food poisoning B. Endotoxins -part of the outer membrane portion of the cell wall of gram negative bacteria: Lipopolysaccharide (LPS)

2) Membrane disrupting toxins -cause lysis of the host cell by disrupting the plasma membrane Amy Warenda Czura, Ph.D.

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SCCC BIO244 Chapter 15 Lecture Notes

-released when dead cells lyse -in blood, causes macrophages to release high levels of cytokines resulting in chills, fever, weakness, aches, small blood clots, tissue necrosis, shock and death e.g. endotoxic shock: critical loss of blood pressure due to bacterial endotoxins (LPS) Sterile solutions can contain LPS: bacteria dies in sterilization but LPS is unaltered Due to serious consequences at very low levels of LPS, it is essential to test medical devices and solutions for endotoxin -Limulus Amoebocyte Lysate Assay: -horseshoe crab blood -contains amoebocytes that will lyse and clot in the presence of extremely low levels of LPS (only way to confirm IV solutions are “endotoxin free”)

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http://3r-training.tierversuch.ch/files/mediapics/Fendrich_Pyrogen/full/Fennrich%20Bild07%20E.gif

Plasmids, Lysogeny and Pathogenicity -plasmids carry genes for resistance to antibiotics and/or virulence factors (e.g. exotoxins, fimbriae) between bacteria allowing new bacteria to become pathogenic e.g. hemorrhagic E. coli (fimbrae + shiga toxin) -prophages can result in lysogenic conversion that results in pathogenic ability of the bacteria carrying them (new production of endotoxin) e.g. Diptheria toxin (Cornebacterium) Cholera toxin (Vibrio) -phage can be transmitted to nonpathogenic strains making them virulent

Amy Warenda Czura, Ph.D.

http://www.udel.edu/PR/UDaily/2004/hcrab404lg.jpg

Pathogenic Properties of Virus 1. Mechanisms to evade host defenses A. Grow inside host cells to hide from immune defense B. Kill immune cells e.g. HIV – TH Cells 2. Cytopathic effects = visible effects of viral infection on host cell: some effects will kill the cell, some will just change the cell A. stop DNA, RNA and/or protein synthesis e.g. Herpes virus block mitosis B. lysosomal autolysis of host cells e.g. Influenza: bronchiolar epithelium C. production of inclusion bodies (visible viral parts inside the cell) can identify a particular virus e.g. Rabies virus: Negri bodies 5

SCCC BIO244 Chapter 15 Lecture Notes

D. syncytium formation (neighboring cells fuse together) e.g. Varicella Zoster virus

Eukaryotic Pathogens 1. Fungi: -produce toxins causing allergies or disease e.g. -chronic sinusitis (black molds) -Stachybotrys: headaches, vomiting, mental disturbance

E. change in cell function e.g. Measles F. production of interferons by host cell (triggers host immune response) G. induce antigenic changes on host cell surface (triggers destruction of infected cell by host immune response) H. induce chromosomal changes I. cell transformation: may activate or deliver oncogenes resulting in loss of contact inhibition (cancer) e.g. Papilloma virus

-invasive systemic mycosis in immune compromised patients e.g. Candida -mushrooms: mycotoxins may be hallucinogenic or deadly

2. Protozoa: -can grow inside host cells causing lysis e.g. Malaria (Plasmodium) -use host cells as food source -produce wastes that cause disease 3. Algae -produce neurotoxic substances e.g. shellfish poisoning (dinoflagellates)

http://wiz2.pharm.wayne.edu/module/plasmodium.jpg

Amy Warenda Czura, Ph.D.

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SCCC BIO244 Chapter 15 Lecture Notes