Virulence Factors & Toxins PDF

Virulence Factors Toxins  Last time Virulence and Virulence factors – Adhesions – Exoenzymes (some of these act as toxins) – Exotoxins – Metabolic pathways  More About Exotoxins synthesized by specific bacteria toxin genes plasmids or prophage...

Virulence Factors Toxins  Last time Virulence and Virulence factors – Adhesions – Exoenzymes (some of these act as toxins) – Exotoxins – Metabolic pathways  More About Exotoxins synthesized by specific bacteria toxin genes plasmids or prophage among the most lethal substances known – Small LD50 are highly immunogenic – can stimulate production of neutralizing antibodies (antitoxins) – inactivated to form toxoids Passive immunity in form of antitoxin can be given as treatment  Damage to the Host Exotoxins – Can be grouped into functional categories Neurotoxins – Cause damage to nervous system – Major symptom is paralysis Tissue Specific – Enterotoxins » Damage to intestines and tissues of digestive tract Cytotoxins – Damage to variety of cells or general tissues – Damage caused by interference with cell function or cell lysis  Exotoxins Released extracellularly May travel far from focus of infection Three main modes of action – Cytolytic toxins Attack cell components causing lysis – A-B toxins (site specific, membrane disrupting) Two subunits B = binds to cell surface A = damages cell – Superantigen toxins Stimulate large immune response and subsequent inflammation  Cytolytic Toxins Two main types: – Lipases Phospholipases Lecithinases – Pore forming leucocidins streptolysins  Other cytolytic toxins Membrane-damaging toxins (enzymes) – Phospholipases are group of potent membrane-damaging toxins Remove polar heads of phospholipid Destabilizes membrane Example: Clostridium perfringens Figure 33.6 (b)  Pore-forming exotoxins (examples) leukocidins – kill phagocytic leukocytes hemolysins – kill erythrocytes, leukocytes, and many other cells – e.g., streptolysin-O (SLO) oxygen-sensitive – e.g., streptolysin-S (SLS) oxygen stable – e.g. Staphylococcus aureus produces  toxin Figure 33.6 (a)  Hemolytic reactions beta-hemolysis – complete lysis – observed as zone of clearing around colony on blood agar alpha-hemolysis – partial lysis – observed as greenish zone around colony on blood agar Beta: complete lysis Alpha: partial lysis  More Damage to the Host A-B toxins – Toxins consist of two parts A subunit – Toxic or active part B subunit – Binding part » Binds to specific host cell receptors – Structure offers novel approaches to development of vaccine and other therapies Use toxin structure as binding a delivery system  A-B toxins Example = Diphtheria toxin – Corynebacterium diphtheriae – Rats/mice resistant, humans susceptible – Single toxin molecule needed to kill a cell Fragment B – Promotes binding to host cell and allows entry of Fragement A Fragment A – blocks translation – Prevents tRNA binding to ribosomes – Prevents protein synthesis  Other Examples of A-B toxins Clostridium toxins – Clostridium tetani – tetanus toxin – Clostridium botulinum – botulinum toxin Release toxins from site of wound – Influence muscle contractions – Botulinum toxin prevents muscle contraction – Tetanus toxin prevents muscle relaxation  Compare and explain Compare notes with your neighbor on the actions of the two examples of exotoxins Cytolytic A-B toxins Give examples of the modes of action of each  Superantigen Toxins – Elicit very strong immune response – Induce extensive release of cytokines – Systematic inflammatory response Vomiting Diarrhea Fever Systemic shock – Superantigens also suspected in contributing to autoimmune disease  Superantigens Examples: – Toxic shock syndrome toxin Staphylococcal and Streptococcal – Staphylococcal enterotoxin A Staphylococcus aureus – Scarlet Fever Toxin Streptococcus pyogenes  Superantigens Trick T cells – think they have been activated by specific antigens – Bridge MHCII of APC to CD4-T cell – T – Cell release cytokines – Activate large number of T-Cells (up to 30%) Over stimulates immune system – Systemic inflammation – Capillary leakage – Shock Some synergistic with endotoxin  Immune Responses Damaging effects of the immune response – Damage associated with antibodies Antigen-antibody complexes – Complexes form and settle in joints and kidneys » Causes destructive inflammation – Cross-reactive antibodies Some antibodies produced in response to infection bind to body’s own tissues – Promotes autoimmune response – Streptococcus pyogenes - rheumatoid arthritis  Endotoxins Gram neg bacteria Lipopolysaccharide layer on envelope – Lipid A Releases when cells are lysed Endotoxin symptoms – Fever – cause release of pyrogens – Diarrhea – Inflammation  Damage to the Host Endotoxins – Endotoxins are LPS of gram negative cell wall Toxin fundamental part of gram negative organism – Endotoxins are heat stable and therefore not suitable for use as toxoids – Lipid A responsible for toxic properties – Symptoms associated with vigorous immune response – Toxin responsible for septic shock a.k.a endotoxic shock  More about Endotoxins heat stable toxic (nanogram amounts) weakly immunogenic generally similar, despite source Responsible for “injection fevers” Used rabbits as an assay – Rabbit test or pyrogen test Endotoxins…Exogenous Pyrogens usually capable of producing general systematic effects – fever – shock – blood coagulation – weakness – diarrhea – inflammation – intestinal hemorrhage – fibrinolysis  Damage to the Host Other bacterial cell wall components – PTG and other cell wall components can elicit symptoms similar to those seen with endotoxic shock (PAMP) – These include Fever Drop in blood pressure  How do they compare? Endotoxins less toxic than exotoxins Mouse LD50 endotoxin 200-400 ug/kg Mouse LD50 exotoxin 25 pg/kg 1,000,000 pg in a ug Even though less toxic than exotoxins still toxic enough to cause major clinical problems! Presence of endotoxin can enhance sensitivity to TSS toxins up to Million fold!  Limulus test Limulus amebocyte lysate (LAL) Limulus is genus of horseshoe crab – Amebocytes – “white blood cells” of crab – Have no advanced immune system – Blood or lysate clots in response to pathogens (endotoxins) – Used as presumptive test for endotoxin  Endotoxemia Examples – Bacterial infection – Burns and severe wounds Significance – 20 to 30% of intensive care patients – 100,000 to 200,000 deaths a year in NA alone Young L, Glauser M, editors. Gram-negative septicemia and septic shock. Philadelphia: WB Saunders; 1991. Limulus Amebocytes  Limulus Test Problems – Can’t discriminate between live and dead cells – Can’t discriminate toxins of different species  Explain What is an endotoxin? What group of bacteria produce this? What are the symptoms What is an assay for the presence of endotoxin  Next Time Chapter Diseases Examples