BIOL371 Microbiology Lecture 17 - Microbial Infection and Pathogenesis PDF

BIOL371: Microbiology Lecture 17 – Microbial infection and pathogenesis 1 Topics of today 1. Human-pathogen interactions 2. Enzymes and toxins of pathogenesis Materials covered:  Chapter 25.1-25.8  Figures 25.1-25.6, 25.9-25.16, 25.28, 25.19  Table 25.3 2 Human-pathogen interactions 1. 2....

BIOL371: Microbiology Lecture 17 – Microbial infection and pathogenesis 1 Topics of today 1. Human-pathogen interactions 2. Enzymes and toxins of pathogenesis Materials covered:  Chapter 25.1-25.8  Figures 25.1-25.6, 25.9-25.16, 25.28, 25.19  Table 25.3 2 Human-pathogen interactions 1. 2. 3. 4. Microbial adherence Colonization and invasion Pathogenicity, virulence, and virulence attenuation Genetics of virulence and the compromised host 3 Microbial pathogenesis     Infection: a microorganism is established in a host, whether or not a host is harmed Disease: actual damage or injury that impairs host function Pathogens: microbial parasites that cause disease or tissue damage in a host Pathogenicity: the ability of a parasite to inflict damage on the host 4 Microbial adherence  Adherence: enhanced ability of microbes to attach to host tissues  Necessary but not sufficient to start disease  Pathogens typically adhere to epithelial cells through interactions between molecules on the pathogen and host tissues  Pathogens can form biofilm and adhere to the host via the biofilm  Pathogens gain access to host tissues by way of a portal of entry: mucous membranes, skin surface, puncture wounds, cuts, insect bites, or other abrasions  Could be critical in establishing an infection; e.g., Streptococcus pneumonia is killed by acid in the stomach if swallowed, but could trigger pneumonia if inhaled to reach the respiratory tract 5 Specific interactions between pathogen and host  Specific receptors on the surface of pathogens (bacteria, viruses) are used for adherence  Adhesins: receptors (made up of glycoprotein or lipoprotein) on pathogen’s surface that enable it to bind to host cells  Host receptors include: extracellular matrix, cell surface glycoproteins, membrane lipids  Influenza virus uses the surface hemagglutinin to specifically interact with mucosal cells of the upper respiratory tract Adherence through complementary receptors. The N2 and N3 domains of the Staphylococcus aureus adhesion protein SdrG binding to fibrinogen of the extracellular matrix of the host cell 6 Capsules in adherence and protection  The bacterial capsule forms a thick coating outside the plasma membrane and cell wall and serves two important functions in bacterial pathogenicity  The capsule is both sticky and contains specific receptors to facilitate attachment to host tissues  Capsules can evade the host defense system; e.g., encapsulated strains of Streptococcus pneumoniae are resistant to phagocytosis by white blood cells Encapsulated Streptococcus pneumoniae cells Encapsulated Escherichia coli cells attached to intestinal microvilli via capsule 7 Fimbriae, pili, and flagella as adherence structures  Fimbriae are structures uniformly distributed on bacterial cell surface  Implicated in specific adherence in infections by enteric bacteria (Escherichia, Salmonella, and Shigella) as well as Neisseria gonorrhoeae  Pili are involved in the attachment to urogenital epithelia by Neisseria gonorrhoeae  Flagella may also facilitate adherence to host cells Salmonella enterica (typhi) showing the numerous thin fimbriae and the much thicker flagella 8 Colonization  Colonization: growth of microorganisms after they have gained access to host tissues  Process begins at birth when exposure to a suite of harmless bacteria and viruses to establish the initial normal microbiota  Colonization usually starts in the mucous membranes  Tightly packed epithelial cells that line the surface of urogenital, respiratory and digestive tracts secrete mucous, a thick secretion of water-soluble glycoproteins  Mucous retains moisture and naturally inhibits microbial attachment through physical processes; e.g., sneezing and swallowing  Some microbes, pathogens and nonpathogens, adhere and colonize (a) Loose association upon initial exposure. (b) Penetration of mucus leading to adhesion and colonization. (c) Further colonization leading to biofilm formation. 9 Growth of microbial community and tooth decay  Saliva contains acid glycoproteins that forms a film on tooth surface providing an attachment site  First colonizers are Streptococcus sobrinus and S mutans  Sucrose (table sugar) triggers the formation of capsules and dextran to secure attachment to the tooth and gum surface  Dental plaque: thick biofilm caused by extensive bacterial growth  Besides streptococci, many other bacteria and a few archaea are found in dental plaques  Streptococcus sobrinus and S mutans are lactic acid producers  High local concentrations of lactic acid decalcifies tooth enamel, resulting in cavities 10 Invasion and systemic infection  Invasion: ability of a pathogen to grow in host tissue, spread and cause disease  Some pathogens remain localized after initial entry multiplying and invading a single site; e.g., the boil caused by Staphylococcus skin infections  Bacteremia: presence of bacteria in the bloodstream; usually asymptomatic because the immune system will remove them  Septicemia: bacteria multiplying in the bloodstream and spread systematically from an initial point and produce toxins  Usually begins at a specific organ such as intestine, kidney and lung and spread rapidly throughout the body  May lead to massive inflammation, septic shock (blood pressure drops to dangerously low level), and death 11 Virulence  Virulence: the relative ability of a pathogen to cause disease  Virulent factors: toxic or destructive compounds produced by the pathogen that directly or indirectly enhance invasiveness and host damage by facilitating and promoting infection  Virulence can be quantified in several ways including mortality, illness, or pathological lesions. For example, mortality can be estimated experimentally to determine the amount needed to kill 50% of the test animals LD50 (lethal dose50)  High virulent pathogens require a few infectious agents to cause disease; e.g., Streptococcus pneumoniae, some strains of influenza virus, Ebola virus  Example of low virulent pathogen: Vibrio cholera requiring a large inoculum to initiate disease 12 Virulence attenuation  Virulence attenuation: the decrease or loss of virulence  Attenuated strains of pathogens can be developed by culturing in the laboratory through passages rather than isolated from diseased individuals  Presumably mutated strains of pathogens (less virulent or loss of virulence) can grow faster than pathogenic strains when selective pressure is absent 13 Virulence attenuation in vaccine development  Attenuated strains of pathogens are valuable because they are used in vaccine production  First rabies vaccine developed by Louis Pasteur  Examples: measles. mumps, rubella, chicken pox/shingles, and yellow fever  Greater efficacy and generate stronger immune response than killed microbes 14 Genetics of virulence  The outcome of an infectious disease is the net result of genetic and physiological features of both the pathogen and host  Virulence of a pathogen maybe firmly encoded by chromosomal genes or by mobile elements  Example: virulence in Salmonella  Genes associated with pathogenicity are clustered in pathogenicity islands on the chromosome  Some virulence factors are carried by plasmids, which can spread rapidly in the population 15 The compromised host  The pathogen-host interaction is dependent on both the host and the pathogen  Opportunistic infections: those caused by organisms that do not cause disease in heathy hosts  Compromised hosts: individuals in whom one or more mechanisms to disease is inactive  Infection with viruses such as HIV weaken the immune system  Many hospital patients with noninfectious diseases are compromised hosts  Nosocomial infections: healthcare associated infections  Main causes; e.g., surgery, biopsy, catheterization, hypodermic injection  Affect two millions each year in the United States with ~5% mortality rate  Many factors including lifestyle choices and living conditions can make the hosts more susceptible to infections 16 Enzymes and toxins of pathogenesis 1. 2. 3. 4. Enzymes as virulent factors AB-type exotoxins Cytolytic and superantigen exotoxins Endotoxins 17 Enzymes that breakdown extracellular matrix  Infectious bacteria can produce a variety of enzymes to breakdown the host’s tissues  The polysaccharide hyaluronic acid is an extracellular matrix component that holds the cells together  Streptococcus pyrogenes produces hyaluronidase that digests hyaluronic acid to get to the deeper tissues  Clostridia that cause gangrene produce collagenase (collagen is the major protein in the connective tissues) to get to the deeper tissues with anoxic environment 18 Coagulase and streptokinase as virulence factors  Coagulase and streptokinase manipulate blood clotting  Coagulase promotes blood clotting , blocking access to the bacteria by the immune system  May account for the localized nature of infections such as boils and pimples  Streptokinase dissolves blood clot formed by the host to isolate the pathogen  Pathogens that produce streptokinase can invade deeper tissues  Streptokinase is used as a pharmaceutical to minimize blood clots following heart attacks and other treatments 19 Exotoxins  Toxicity: the ability of an organism to cause disease by means of a toxin that inhibits host cell function or kills host cells  Exotoxins: toxic proteins secreted by the pathogen  Enterotoxins: exotoxins whose site of action is the small intestine, causing secretion of fluids into the intestinal lumen, resulting in vomiting and diarrhea  Based on their mechanisms, exotoxins are grouped into three classes:  AB toxins  Cytolytic toxins  Superantigen toxins 20 Toxins  Toxicity: the ability of an organism to cause disease by means of a toxin that inhibits host cell function or kills hot cells  Exotoxins: toxic proteins secreted by the pathogen  Enterotoxins: exotoxins whose site of action is the small intestine, causing secretion of fluids into the intestinal lumen, resulting in vomiting and diarrhea  Based on their mechanisms, exotoxins are grouped into three classes:  AB toxins – two subunits: Subunit B facilitates Subunit A to cross the cytoplasmic membrane of the host; Subunit A causes damage to the host cell  Examples: toxins that cause diphtheria, tetanus, botulism, and cholera  Cytolytic toxins  Superantigen toxins 21 Diphtheria exotoxin: blockage of protein synthesis  Diphtheria toxin: an AB exotoxin produced by the bacterium Corynebacterium diphtheriae  The gene encoding the toxin is encoded by a prophage  The B subunit binds to the cytoplasmic membrane and cleaves itself to release the A subunit  The A subunit catalyzes ADP-ribosylation of the elongation factor EF-2  The EF-2-ADP is unable to transfer amino acids to the growing protein chains, thereby shutting down protein synthesis  The Shiga toxin uses the same mechanism 22 Neurological exotoxin: botulinum toxin  Botulinum toxin is produced by the anaerobic Grampositive bacterium Clostridium botulinum  Illness most likely come from consuming contaminated canned food  Muscle contraction is the result of muscle receptor interacting with the neurotransmitter acetylcholine  The botulinum toxin cleaves the proteins involved in coordinating the release of acetylcholine  Without the excitatory acetylcholine signal, muscle cannot contract  Can lead to death by suffocation if the diaphragm muscles are severely affected  Chronic pain is often caused by stress-induced muscle tension; local application of low doses of botulinum toxin can provide relief 23 Neurological exotoxin: tetanus toxin  Tetanus toxin is produced by Clostridium tetanus, an obligate anaerobe  The bacteria grow in the body in deep wounds that become anoxic  On contact with the nervous system, the toxin is transported through the motor neurons to the spinal cord  The toxin binds to the inhibitory interneurons, preventing the release of the inhibitory neurotransmitter glycine (normally required for blocking the release of acetylcholine)  The toxin results in the flooding of the neurotransmitter acetylcholine in the neuromuscular junctions, leading to muscle contraction 24 Cholera enterotoxin, and AB-type exotoxin  Enterotoxins: toxic activity affecting the small intestine  Generally cause massive secretion of fluid into the intestinal lumen, resulting in vomiting and diarrhea  Cholera starts by intake of food or water contaminated with Vibrio cholera  The bacteria travel to, and colonize, the small intestine  Subunit B of the toxin binds to the epithelial cells  Subunit A enters the cell and activates the enzyme adenylyl cyclase, converting ATP to cyclic AMP  Cyclic AMP regulates multiple cellular processes including ionic balance  Blocks the uptake of Na+, resulting in the efflux of chloride and bicarbonate and the massive secretion of water  Can be reversed by replenishing loss fluids with clean 25 rehydration solution Cytolytic exotoxins  Cytolytic exotoxins: work by degrading cytoplasmic membrane, causing cell lysis and cell death  Many are phospholipases, enzymes that degrade membrane phospholipid  Some, like Staphylococcal α-toxin, form pore to release cell contents Structure of α-toxin Scanning electron micrograph of Staphylococcus aureus cells 26 Endotoxins  Endotoxins: NOT proteins, but are part of the lipopolysaccharide component of the outer membrane of Gram-negative bacteria  Released in toxic amounts only when the bacterial cells lyse and the toxin is solubilized  Generally less toxic than exotoxins 27 Comparison of the properties of exotoxins and endotoxins Property Exotoxins Endotoxins Chemistry Proteins, secreted by certain gram-positive or gramnegative Bacteria; generally heat-labile Lipopolysaccharide–lipoprotein complexes, released on cell lysis as part of the outer membrane of gram-negative Bacteria; extremely heat-stable Mode of action; symptoms Specific; usually bind to specific cell receptors or structures; either cytotoxin, enterotoxin, or neurotoxin with defined, specific action on cells or tissues General; fever, diarrhea, vomiting Toxicity Often highly toxic in picogram to microgram quantities, sometimes fatal Moderately toxic in tens to hundreds of microgram amounts, rarely fatal Immune response Highly immunogenic; stimulate the production of neutralizing antibody (antitoxin) Relatively poor immunogens; immune response not sufficient to neutralize toxin aToxoid potential Heat or chemical treatment may destroy toxicity, but treated toxin (toxoid) remains immunogenic None Fever potential Nonpyrogenic; do not produce fever in the host Pyrogenic; often induce fever in the host Genetic origin Often encoded on extrachromosomal elements or lysogenic bacteriophages Encoded by chromosomal genes aToxoid: is a modified toxin that is no longer toxic but can still elicit an immune response against the toxin 28