Microbial Pathogenesis (Chapter 25) PDF

Microbial Pathogenesis Chapter 25  Bacteria Are Ubiquitous We contact numerous microorganisms daily Breathe in, ingest with food and drink, pick up on skin Relatively few are pathogens that invade tissues and cause damage Pathogenic microbes exploit those weakness...

Microbial Pathogenesis Chapter 25  Bacteria Are Ubiquitous We contact numerous microorganisms daily Breathe in, ingest with food and drink, pick up on skin Relatively few are pathogens that invade tissues and cause damage Pathogenic microbes exploit those weaknesses, and the result is disease. All pathogens have distinct characteristics that allow avoidance of body defenses – virulence factors Salmonella enterica serotype Typhimurium invading cultured human cells By NIAID: These high-resolution (300 dpi) images may be downloaded directly from this site., Public Domain, https://commons.wikimedia.org/w/index.php?curid=450281 The Normal Microbiota Normal Microbiota – community of microorganisms found in/on body of healthy individual Resident microbiota inhabit sites for extended periods Important to human health Transient microbiota inhabit temporarily (hours – days) because… Not suitable environment Host innate and adaptive defenses Competition & antagonism from By DataBase Center for Life Science (DBCLS) - https://doi.org/10.7875/togopic.2020.154, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=89430513 resident microbes The Normal Microbiota The Protective Role of the Resident Microbiota Significant contribution is protection against pathogens via competition Covering binding sites prevents attachment Consumption of available nutrients, growth factors Production of toxic compounds (bacteriocins, antibiotics) When resident microbiota killed or suppressed -> dysbiosis: Competition eliminated, pathogens may colonize, cause disease Many examples Ex: Oral antibiotics can inhibit intestinal microbiota, allow overgrowth of toxin-producing Clostridium difficile “7 to 10 times more likely to get C. diff while on antibiotics and during the month after” - CDC Kho ZY and Lal SK (2018) The Human Gut Microbiome – A Potential Controller of Wellness and Disease. Front. Microbiol. 9:1835. doi: 10.3389/fmicb.2018.01835 (CC BY)  The Normal Microbiota The Protective Role of the Resident Microbiota Stimulation of adaptive immune system When small numbers of normal skin microbiota enter our body through cuts, they stimulate an immune response The antibodies made against normal microbiota may also bind to pathogens Mice in microbe-free environment have underdeveloped mucosal-associated lymphoid tissue (MALT) – this is important in mucosal immunity and prevents microbes from entering mucous membranes The Language of Pathogenesis ▪ By definition, a parasite is an organism that receives benefits at the expense of a host. In practice, the term “parasite” refers to disease-causing protozoa and worms. Bacterial, viral, and fungal parasitic agents of disease are called pathogens. Ectoparasites live on the surface of the host; endoparasites live inside the host’s body. ▪ An infection occurs when a pathogen or parasite enters or begins to grow on a host. However, the term “infection” does not necessarily imply overt disease. The Language of Pathogenesis Disease – Any prolonged abnormal condition in which host cells are damaged and symptoms and/or signs of illness are present Infectious (contagious = caused by pathogen) vs. noninfectious (not contagious = ex: cancer) Pathogen – Any infectious biological agent that causes disease Bacteria, viruses, fungi, protozoans, helminths (worms) Host – An organism that harbors another organism either on or within itself  Disease Triangle Disease is dependent on characteristics of the host, pathogen, and environment Immunity (ability to stop pathogen) Virulence (un)Favorable for pathogen (increase virulence/abundance) (evade and reproduce in host) and/or (un)favorable for host (decrease immunity)  Environment Favorable to Pathogen/Host (Poorly Highly) Disease Pathogen Host Permeability to Pathogen (Low -> High) Host Microbiome Salamander Image by Jose Carlos Arenas-Monroy CC0 1.0 Bernardo-Cravo et al. 2020 Trends in Parasitology  The Language of Pathogenesis Disease is characterized by… Signs are objective evidence of infection (fever, rash, pus formation, swelling) Symptoms and are subjective effects experienced by patient (I feel... pain, nausea) The Language of Pathogenesis ▪ It is often “friendly fire” by our immune system reacting to a pathogen that causes major tissue and organ damage. ▪ The term immunopathogenesis applies when the immune response to a pathogen is a contributing cause of pathology and disease. ▪ To fully understand any infectious disease, researchers must study both the pathogenic mechanisms of the pathogen and the disease symptoms/signs caused by immunopathogenesis. The Language of Pathogenesis Colonization refers to a microbe establishing itself and multiplying The term infection can be used to refer to a pathogenic microbe that colonizes, establishes, and multiplies Infection does not always lead to disease Clinical Threshold Subclinical Infection = no or very mild signs or symptoms Colonization = non-pathogens Infection = pathogens Parker et al. 2022 OpenStax Microbiology CC BY 4.0.  The Language of Pathogenesis Course of Infectious Disease Subclinical infection: pathogen replicates in host but at low levels that do not produce obvious signs and symptoms Carriers may harbor and spread infectious agents in the absence of signs or symptoms Parker et al. 2022 OpenStax Microbiology CC BY 4.0. The Language of Pathogenesis Initial infection in a healthy individual is called the primary infection, caused by a primary pathogen A primary pathogen can cause disease in a healthy individual Diseases such as plague, malaria, measles, influenza, diphtheria, tetanus, and tuberculosis are caused by primary pathogens Plasmodium spp. = malaria image by Ute Frevert;false color by Margaret Shear - http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.003019 2, CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=219588  The Language of Pathogenesis Damage from primary infection (or treatment of) can predispose an individual to develop a secondary infection caused by an opportunistic pathogen Opportunistic pathogens cause disease only when the body’s immune defenses are compromised or when introduced into an unusual location Typically, members of normal microbiota or common environmental microbe Ex: Pseudomonas aeruginosa is a common environmental microbe that can cause fatal infections in people with cystic fibrosis Ex: viral respiratory illness impairing barrier defenses can result in pneumonia (bacterial lung infection) Pseudomonas aeruginosa Ex: infection that occurs as a result of surgery, cut, burn CDC/ Janice Haney Carr Public Domain The Language of Pathogenesis ▪ Pathogenicity refers to an organism’s ability to cause disease. It is defined in terms of... how easily an organism causes disease (infectivity) how severe that disease is (virulence) the specific genetic makeup of the pathogen The Language of Pathogenesis Virulence is a measure of the degree, or severity of disease The ability of an infectious agent to multiply and cause damage to the host Highly virulent bacterial strains cause severe disease Ex: Streptococcus pyogenes normally causes strep throat, but highly virulent strains can cause necrotizing fasciitis (“flesh-eating disease”) Streptococcus pyogenes on neutrophil NIAID Public Domain  The Language of Pathogenesis Measuring virulence Virulence is measured by determining the…. infectious dose ID50, - number of cells/virions required to cause infection in 50% of hosts AND/OR lethal dose LD50 - number of cells/virions required to cause death in 50% of hosts Highly virulent pathogens – the difference between LD50 and LD100 is small The Language of Pathogenesis Virulence factors are genes/products that allow microbes to cause disease. Allows microbe to… colonization of a niche in the host entry into and exit out of cells immunoevasion, evasion of the host's immune response immunosuppression, inhibition of the host's immune response obtain nutrition from the host Toxicity, ability of a pathogen to secrete a toxin Virulence genes can sometimes be transferred Capsule of Bacillus anthracis horizontally or acquired through temperate phages prevents phagocytosis by immune cells = Lysogenic conversion Larry Stauffer, Oregon State Public Health Laboratory, USCDCP CC0  Attenuation - The decrease or loss of virulence Achieved in vitro through… 1. Culturing the pathogen outside of its host (successive sub-culturing) – “use it or lose it” 2. Genetic engineering Gene deletions – mutate or knock out virulence genes or how they are regulated Achieved naturally through Natural selection Successful pathogens need to overcome host defenses long enough to multiply and then exit host to infect another If pathogen is too virulent, it causes severe illness/death before it is transmitted If not virulent enough, host defenses eliminate The Evolution of Virulence Successful pathogens need to overcome Optimal virulence… host defenses long enough to multiply …more chance and then exit host to infect another for transmission If pathogen is too virulent, it causes severe illness/death before it is transmitted If not virulent enough, host defenses Parasite fitness (R0) eliminate High virulence… No virulence… …less chance for …No chance for transmission transmission Virulence (mortality rate)  Red Queen Hypothesis Pathogens and hosts are constantly struggling to keep up with each other in an evolutionary arms race Host susceptibility high Increased resistance in host, prevents parasite reproduction non-susceptible host parasite survive, able to Only highly virulent virulence reproduce strains survive high (host susceptibility low) “It takes all the running you can do to stay in the same place” Lewis Carrol – Through The Looking Glass  Infection Cycles ▪ The infection cycle describes the route of transmission of an infectious organism. Horizontal transmission: passage from one person or animal to another within the same generation ― Can be direct (e.g., handshaking) or indirect (e.g., sharing contaminated objects) o Fomite transmission: inanimate objects - >skin contact (e.g., doorknobs, hand towels, utensils) o Vehicle transmission: ingested or inhaled materials (e.g., food, water, air) Vertical transmission: passage from a mother to her fetus during pregnancy (transplacental) or birth (parturition) © McGraw Hill, LLC Bab y image: Labora toir es Ser vier Creative Commons Attribu tion-Sha re Alike 3.0 Infection Cycles ▪ Complex infection cycles often involve vectors as intermediaries (usually arthropods like mosquitoes, ticks, mites, or flies). For example, a mosquito vector transfers the virus that causes yellow fever from infected to uninfected individuals. Akil Rolle-Rowan/Shutterstock © McGraw Hill, LLC “Figure 16.12 (a) A mechanical vector carries a pathogen on its body from one host to another, not as an infection. (b) A biological vector carries a pathogen from one host to another after becoming infected itself.” from Parker et al. 2022 OpenStax Microbiology CC BY 4.0. Infection Cycles ▪ A reservoir is an animal, bird, or arthropod that normally harbors the pathogen, often without exhibiting disease. In the case of yellow fever, the mosquito is not only the vector but the reservoir as well, because the insect can pass the virus to future generations of mosquitoes through vertical transmission. ▪ Reservoirs are critically important for the survival of a pathogen and as a source of infection.  Some pathogens are more easily transmitted (contagious) than others Contagiousness is dependent upon… Pathogen of Infectious Dose Measles 1 virions Infectivity -> Infectious dose = the number of microbes necessary to establish infection (in a healthy individual) Smallpox 10-100 virions Intestinal shigellosis results from ~10–100 ingested Plague 100-500 cells Shigella cells Salmonellosis results from at least 10 6 ingested Anthrax 8,000-50,000 spores Salmonella cells Typhoid 10,000 cells Difference partially reflects the ability to survive stomach Cholera 100,000,000 cells acid SARS-CoV2 1-7 million virions Mode of transmission Ingestion = sometimes larger ID Ability to survive outside of the host Biological and abiotic reservoirs How easily a pathogen spreads in population is measured by Ro The average number of healthy people that one sick person will infect R0 of Covid-19 variants and other infectious diseases Original Covid in Wuhan = 2.4-2.6 Delta = 5-8 Omicron variant = 8.2 Mumps = 12 Measles = 18 Sources: Lancet, Australian government J Travel Med. 2022 May 31  Infection Cycles The course of infectious disease Main periods Incubation period: time between exposure and onset of illness (signs/symptoms) Varies considerably: a few days for the common cold to years for leprosy Depends on: 1. Generation time of microbe 2. Condition of the host 3. infectious dose © McGraw Hill, LLC  Infection Cycles (Latent) Course of Infectious Disease Prodromal Stage – symptoms begin (vague symptoms – headache, malaise = general feeling of illness) Illness: period when signs and symptoms of disease appear © McGraw Hill, LLC  Infection Cycles Course of Infectious Disease Convalescence: time when person recovers from infection – some permanent damage may occur © McGraw Hill, LLC  Infection Cycles Course of Infectious Disease Incubation Period: time between exposure and onset of illness (signs/symptoms) Latent Period: time between exposure and becoming contagious Pre-symptomatic Transmission By Patilsaurabhr - Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=22904276 Infection Cycles Duration of Symptoms Acute infections: symptoms develop quickly, last a short time (strep throat) Persistent infections: Chronic infections: can develop slowly, last for months/years Latent infections: similar to acute, but pathogen never eliminated; can reactivate later Decrease in immunity may allow reactivation Chicken pox (acute); remain latent in sensory nerves (provirus), reactivated later in life Varicella-zoster virus (latent) By Isabellelyy - Own work, CC BY-SA 4.0, https://en.wikipedia.org/w/index.php?curid=63628177 Depledge DP, Sadaoka T, Ouwendijk WJD. Molecular Aspects of Varicella-Zoster Virus Latency. Viruses. 2018; 10(7):349. https://doi.org/10.3390/v10070349 icensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).  Infection Cycles Acute infections Are a result of productive infections (pathogen replication occurs immediately) - Analogous to lytic phage Usually short in duration (days-weeks) Host may develop long-lasting immunity Disease signs & symptoms result from tissue damage, infection of new cells, and immunopathogenesis By Isabellelyy - Own work, CC BY-SA 4.0, https://en.wikipedia.org/w/index.php?curid=63628177 Infection Cycles Persistent infections are a result of the pathogen continually present in the host Two types 1. Chronic infections - continuous production of low levels of virions/cells 2. Latent infections - viral genome (provirus) remains silent in host cell; can reactivate  Categories of Animal Virus Infections Latent infections: Herpes Simplex Virus Initial infection causes cold sores, sore throat Virus moves up cranial nerve near the brain and becomes latent in neuron Provirus integrated into host chromosome Cannot be eliminated Can later be reactivated (may be due to stress) and move down nerve to cause cold sore Verzosa AL, McGeever LA, Bhark S-J, Delgado T, Salazar N and Sanchez EL (2021) Herpes Simplex Virus 1 Infection of Neuronal and Non Neuronal Cells Elicits Specific Innate Immune Responses and Immune Evasion Mechanisms.Front. Immunol. 12:644664. doi: 10.3389/fimmu.2021.644664 Creative Commons Attribution License (CC BY). Infection cycles Localized infection: microbe limited to small area (boil caused by Staphylococcus aureus) Systemic infection: agent spread throughout body (measles) – circulatory/lymphatic system Usually cause severe fever, fatigue – a result of systemic immune response Bacteremia - bacteria circulating in blood Toxemia Viremia Sepsis: acute, life-threatening inflammation caused by infectious agents or products in bloodstream Identifying the pathogen of an infectious disease Koch’s Postulates 1. Microorganism must be present in every case of disease & not healthy hosts 2. Organism must be grown in pure culture from diseased host 3. Same disease must be produced when pure culture is introduced into susceptible hosts 4. Organism must be recovered in culture from experimentally infected host © McGraw Hill, LLC Establishing the Cause of Infectious Disease Koch’s Postulates (LIMITATIONS) 1. Microorganism must be present in every case of disease (not healthy hosts) Subclinical infection -> carriers (Infected individuals do not always display symptoms) 1. Organism must be grown in pure culture from diseased host Some organisms cannot be grown in laboratory medium (ex: Treponema pallidum) 2. Same disease must be produced when pure culture is introduced into susceptible hosts Suitable animal hosts not always available for testing Subclinical infection may occur 1. Organism must be recovered in culture from experimentally infected host Establishing the Cause of Infectious Disease Molecular Koch’s Postulates Rely on studying virulence factors (gene or product-toxin, protein, etc.) 1. Virulence gene or product is found in all pathogenic strains of organism 2. Mutating virulence factor gene to disrupt function should reduce virulence 3. Reversion or replacement of gene should restore virulence to the strain Limitation: often requires culturing (less so with current technology) Mechanisms of Pathogenesis From a microbe’s perspective the interior of the human body is a rich source of nutrients guarded by the innate and adaptive immune system The ability to get past host defenses is what distinguishes pathogens from other microbes Mechanisms of Pathogenesis ▪ To cause disease, all pathogens must... Enter a host Find their unique niche Avoid, circumvent, or subvert normal host defenses Multiply Transmit to a new susceptible host Infection cycles Localized infection: microbe limited to small area (boil caused by Staphylococcus aureus) Systemic infection: agent spread throughout body (measles) – circulatory/lymphatic system Usually cause severe fever, fatigue – a result of systemic immune response Bacteremia - bacteria circulating in blood Toxemia Viremia Sepsis: acute, life-threatening inflammation caused by infectious agents or products in bloodstream  Mechanisms of Pathogenesis Pathogenic mechanisms follow several general patterns: 1. Produce toxins that are ingested Microbe does not grow on or in host Produces toxins that are ingested - foodborne intoxication CDC/ George Lombard Public Domain Clostridium botulinum grows in canned foods, produces the neurotoxin botulinum 2. Colonize mucous membranes, multiplies, and produce toxins E. coli O157:H7 grows in intestines produces shiga toxin and causes bloody diarrhea CDC/ Debora Cartagena Public Domain Parker et al. 2022 OpenStax Microbiology CC BY 4.0.  Pathogenic mechanisms follow several general patterns: 3. Invade host tissues (typically wound) and then produce toxins Clostridium tetani causes tetanus Bacteria colonize wound and secrete tetanospasmin – carried to neuron cells in spinal cord and prevents inhibitory interneurons actions -> prevents muscle relaxation “Figure 15.16 Mechanisms of botulinum and tetanus toxins. (credit micrographs: modification of work by Centers for Disease Control and Prevention)” from Parker et al. 2022 OpenStax Microbiology CC BY 4.0. Pathogenic mechanisms follow several general patterns: 4. Avoid host defenses (macrophages and antibodies), invade host tissues & multiply Yersinia pestis Virulence factors turn off killing ability of macrophages once inside them; then they multiply Infected macrophages dies and release bacteria Mycobacterium tuberculosis Macrophages engulf TB cells but are unable to destroy because of the mycolic acids in cell wall prevents fusion of phagosome with lysosome Bacteria leave phagosome and multiply within macrophage © McGraw Hill, LLC  Infection Process Exposure Pathogen must be exposed to suitable tissue Adherence First line of host defense is to sweep microbes away – how do they stick around to cause disease? Pathogens must adhere to body/mucosal surface By Uhelskie - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=53717337  Establishing Infection Adherence Any microbial factor that promotes attachment is called an adhesin Viruses attach to host cells through spikes. Bacteria use a variety of strategies, including pili (fimbriae) and other nonpilus proteins, to bind to specific host cell factors. Often located at the tips of fimbriae Can also be a component of capsules or various cell wall proteins – nonpilus Adhesins attach to host cell receptor Host receptors are usually glycoproteins or glycolipids on the cell surface © McGraw Hill, LLC  Establishing Infection Adherence Binding highly specific Dictates what type of cell the pathogen can adhere to Ex: common E. coli cells have adhesins that allow them to adhere to cells that line the large intestine Pathogenic E. coli strains have adhesins (FimH) at the tips of fimbriae (Type I pilus) that allow them to bind to additional tissues urinary tract infection E. coli – fimbriae that attach to the bladder Strains that cause diarrhea can attach to cells in the small intestine Image 1 By Chen, J.; Dai, W.; Wang, H.; Lei, W.; Fang, G.; Dai, D. Cloning and Expression of Pigeon-Derived Escherichia coli Type 1 Pilus Clusters and Analysis of Amino Acid Sequence Characteristics of Functional Proteins. Genes 2024, 15, 1253. https://doi.org/10.3390/genes15101253 CCBY Image 2 By Peteruetz - PPT, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=56465855  Nonpilus Adhesins ▪ Bacteria also possess adhesins that are not fimbriae. Streptococcus pyogenes: M protein ― Binds to fibronectin Bordetella pertussis: pertactin ― Binds to host cell integrin Pseudomonas aeruginosa: multivalent adhesion molecule 7 (MAM7) ― Binds to phospholipids and fibronectin in host cell membranes © McGraw Hill, LLC Biofilms and Infections ▪ Bacteria can attach to surfaces as a population, forming a biofilm. ▪ Biofilms play an important role in chronic infections by enabling persistent adherence, resistance to host defenses, and tolerance to antimicrobial agents. © McGraw Hill, LLC Adherence ▪ Why are some people susceptible to certain infections, whereas others are not? Immunocompetence Receptor availability ▪ Pathogens rely on very specific surface structures (receptors) to recognize and attach to appropriate host cells. Person-to-person differences in receptor structures are possible. Example: HIV binds C-C chemokine receptor type 5 (CCR5); individuals with a CCR5 mutation are resistant to HIV infection!  Invasion—Breaching the Anatomical Barriers Some pathogens can cause disease while remaining on the mucosal surfaces (by producing toxins) others must penetrate anatomical barriers Once inside, the microbe multiplies in nutrient-rich tissues without competition Penetrating the Skin Most difficult barrier to penetrate Some bacteria rely on injuries Staphylococcus aureus enters via cut or wound; Others via vectors Yersinia pestis invades via flea bite “Figure 16.13 (credit “Black fly”, “Tick”, “Tsetse fly”: modification of work by USDA; credit: “Flea”: modification of work by Centers for Disease Control and Prevention; credit: “Louse”, “Mosquito”, “Sand fly”: modification of work by James Gathany, Centers for Disease Control and Prevention; credit “Kissing bug”: modification of work by Glenn Seplak; credit “Mite”: modification of work by Michael Wunderli)” from Parker et al. 2022 OpenStax Infection figure by Uhelskie - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=53717337 Microbiology CC BY 4.0.  Invasion—Breaching the Anatomical Barriers Penetrating Mucous Membranes Entry point for most pathogens Directed Uptake by Cells By NIAID: These high-resolution (300 dpi) images may be downloaded directly from this site., Public Domain, https://commons.wikimedia.org/w/index.php?curid=450281 Pathogen induces cells to engulf via endocytosis after attachment to cell Salmonella uses type III secretion system (T3SS -injectisome) to deliver effector proteins to intestinal epithelial cell Causes the host cell’s actin molecules to rearrange leading to ruffling of the membrane, ruffles enclose bacteria, bringing them into the cell © McGraw Hill, LLC  Invasion—Breaching the Anatomical Barriers Penetrating Mucous Membranes Exploiting Antigen-Sampling Processes Simplified overview of how the adaptive immune system works Lymphatic system brings lymphocytes into contact with antigens Sites where lymphocytes gather to contact antigens Lymph nodes, spleen, tonsils, Peyer’s patch Peyer’s patches allow host to sample contents from lumen of intestine Microbes enter Peyer’s patch via M cell Macrophage destroys microbes and presents antigens to B cells and T cells, which make antibodies Tindemans I, Joosse ME, Samsom JN. Dissecting the Heterogeneity in T-Cell Mediated Inflammation in IBD. Cells. 2020; 9(1):110. https://doi.org/10.3390/cells9010110 Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Invasion—Breaching the Anatomical Barriers Penetrating Mucous Membranes Exploiting Antigen-Sampling Processes Most microbes destroyed by macrophages in Peyer’s patch Some pathogens use M cells to cross intestinal barrier Shigella survives phagocytosis by macrophages and induces apoptosis Causes host cell to polymerize actin to propel bacteria into adjacent cells Other pathogens invade by means of alveolar macrophages (Mycobacterium tuberculosis) © McGraw Hill, LLC  Establishing Infection Colonization A microbe must also multiply in order to colonize the host; To do this the pathogen must: 1. Obtain resources Ex: Some microbes secrete siderophores that binds/strips and transports iron Host cells produce lactoferrin and transferrin which bind to iron making it unavailable to microbes. Vinuesa, V.; McConnell, M.J. Recent Advances in Iron Chelation and Gallium-Based Therapies for Antibiotic Resistant Bacterial Infections. Int. J. Mol. Sci. 2021, 22, 2876. https://doi.org/10.3390/ijms22062876 Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Establishing Infection Colonization 2. Compete with normal microbiota Outcompete others for space and nutrients Tolerate antimicrobials produced by normal microbiota Pathogens can grow in biofilms to circumvent these obstacles © McGraw Hill, LLC  Establishing Infection Colonization A microbe must also multiply in order to colonize the host; To do this the pathogen must: 3. Deal with/avoid host defenses © McGraw Hill, LLC  Avoiding the Host Defenses Avoiding Destruction by Phagocytes – many mechanisms: Phagocytes - Major role in innate immunity – they eliminate invading microbes. Contribute to adaptive immunity - present antigens to lymphocytes. i. Microbes invade -> complement system activated ii. C5a secreted -> phagocytes attracted iii. microbes covered in C3b iv. Phagocytes destroy microbes llustration of Enhanced Attachment of Bacteria to Phagocytes by the Opsonin C3b.jpg by Gary E. Kaiser, Ph.D. This work is licensed under a Creative Commons Attribution 4.0 International License. Parker et al. 2022 OpenStax Microbiology CC BY 4.0.  Avoiding the Host Defenses Avoiding Destruction by Phagocytes – many mechanisms: 1. Preventing Encounters with Phagocytes Secrete C5a peptidase - degrades the chemoattractant C5a (recruits phagocytes) Streptococcus pyogenes Secrete streptolysin O - membrane-damaging toxin kill phagocytes, other cells S. pyogenes Parker et al. 2022 OpenStax Microbiology CC BY 4.0.  Avoiding the Host Defenses 2. Avoiding Recognition and Attachment Phagocytes recognize and engulf foreign material more efficiently when coated by c3b or antibodies Bacterial capsules (Streptococcus pneumoniae) and M protein (S. pyogenes) inactivate C3b receptors on phagocytes a) modification of work by Centers for Disease Control and Prevention) from Parker et al. 2022 OpenStax Microbiology CC BY 4.0.  Avoiding the Host Defenses 2. Avoiding Recognition and Attachment Consequences of Antigen-Antibody Binding Precipitation Formation of immune complexes when antibodies (Abs) bind to soluble antigens (Ag). Effect: Results in visible precipitate, aiding in immune recognition. Opsonization Antibodies bind to antigens on cells, making them easier for phagocytes to recognize. Effect: Enhances phagocytosis, improving immune response efficiency. Agglutination Antibodies cross-link antigens, creating large, insoluble complexes. Effect: Traps pathogens, limiting their mobility and making them easier to eliminate. Complement Fixation Immune complexes activate the complement cascade. Effect: Leads to lysis of bacterial cells, increasing pathogen destruction. Neutralization Antibodies bind to toxins or viruses, blocking them from binding to host cells. Effect: Prevents infection and neutralizes pathogenic toxins. © McGraw Hill, LLC  Avoiding the Host Defenses 2. Avoiding Recognition and Attachment Fc receptors & Protein A – proteins that bind Fc region of antibodies Normally, Fab region (of antibody) attaches to bacteria/antigen and Fc region sticks out and serves as marker for phagocytes If Fc region binds to bacteria, then phagocytes cannot recognize Staphylococcus aureus, Streptococcus pyogenes © McGraw Hill, LLC Avoiding the Host Defenses 2. Avoiding Recognition and Attachment IgA protease: cleaves the antibody IgA (found in mucus and other secretions) Produced by Neisseria gonorrhoeae and others Antigenic variation: alter structure of surface antigens Stay ahead of antibody production by altering the molecules that antibodies recognize Neisseria gonorrhoeae varies antigenic structure of pili Mimicking host molecules: cover surface with molecules similar to those found in host cell, appear to be “self” Streptococcus pyogenes form capsule composed of hyaluronic acid, a polysaccharide found in human tissues © McGraw Hill, LLC  Avoiding the Host Defenses 3. Avoiding Destruction by Phagocytes Some bacteria use phagocytes to hide from antibodies and as mechanism to be transported throughout body Surviving Within Phagocytes Escape from phagosome: before it fuses with lysosomes and then can multiply within cytoplasm of phagocyte Listeria monocytogenes forms pores in phagosome membrane Shigella species lyse phagosome © McGraw Hill, LLC  Avoiding the Host Defenses 3. Avoiding Destruction by Phagocytes Surviving Within Phagocytes Prevent phagosome-lysosome fusion: avoid destruction Mycobaterium tuberculosis prevents fusion of phagosome with lysosome via mycolic acid Survive within phagolysosome: few can survive destructive environment Coxiella burnetii (causes Q fever) survives phagolysosome © McGraw Hill, LLC Damage to the Host Damage due to infection can be the result of direct or indirect effects Direct = toxins produced Indirect = immune response (immunopathogenesis) Damage typically helps the pathogen to exit the host and spread to others Vibrio cholerae induces diarrhea which contaminates water supplies Bordetella pertussis triggers severe coughing, and pathogens are released into the air Toxins Subvert Host Functions ▪ Bacterial toxins can be divided into two main types. 1. Exotoxins ― Proteins produced and secreted by various types of bacteria ― Kill host cells and unlock their nutrients 2. Endotoxin ― A part of lipopolysaccharide (LPS) of Gram-negative bacteria ― Hyperactivate host immune systems to harmful levels  Damage to the Host Bacteria produce exotoxins: proteins with specific tissue- damaging effects Either secreted or leaked into tissue following bacterial lysis Pathogen must colonize and multiply in great numbers to produce enough toxin Foodborne intoxication results from consumption of toxin Ex: C. botulinum grows in food, produces toxin, consumed by animal – even tiny amounts can type II secretion system - Proteins to be secreted cause paralysis first enter the periplasm, then they get folded and secreted via an outer membrane pore. © McGraw Hill, LLC The type III secretion system (T3SS) is a reengineered flagellar synthesis mechanism that uses a molecular syringe to inject proteins from the bacterial cytoplasm directly into the host cell. Secretion is normally triggered by cell-cell contact between host and bacterium. Found in Salmonella, Yersinia, Shigella, and Escherichia species. © McGraw Hill, LLC Damage to the Host Antigenic - Because exotoxins are proteins, the immune system can generate neutralizing antibodies Vaccines are critical because toxins can cause death before a natural immune response occurs Toxoids (inactivated toxins – via heat/chemical) are injected and cause the production of specific toxin-inhibiting antibodies (botulism, diphtheria, pertussis, tetanus) Antibodies will immediately bind if toxin is encountered again Antitoxin is suspension of neutralizing antibodies and can be taken if person develops symptoms of toxin-mediated disease Parker et al. 2022 OpenStax Microbiology CC BY 4.0. Damage to the Host Exotoxins can be grouped into functional categories according to the tissues they affect: Neurotoxins damage nervous system Enterotoxins cause intestinal disturbance Cytotoxins damage variety of cell types by either interfering with cellular mechanisms or lysing cells Exotoxins can also be categorized by their structure and mechanism of action: A-B toxins Membrane-damaging toxins Superantigens Damage to the Host A-B toxins have two parts A (active) subunit is toxic, usually an enzyme and is responsible for the effects on the host cell B (binding) subunit binds to cell, determines cell type to be infected Structure of A-B toxins allows novel approaches for vaccines and therapies Ex: researchers are developing medications attached to B subunits to deliver medications directly to specific cells Ex: B subunit of cholera toxin is used as vaccine against cholera toxin © McGraw Hill, LLC  Two-Subunit AB Exotoxins Diphtheria Toxin Structure and Mechanism of Action Diphtheria toxin is a two-subunit AB toxin. The B subunit binds to a host cell receptor, facilitating entry of the A subunit. The A subunit inhibits protein synthesis by ADP- ribosylating EF-2. © McGraw Hill, LLC Membrane-Damaging Toxins ▪ Some exotoxins disrupt host cell membranes by forming pores that cause leakage of cell constituents (host cell lysis). Hemolysins lyse red blood cells (and sometimes other cells). Leukocidins lyse white blood cells (leukocytes). Some membrane-disrupting exotoxins function as both hemolysins and leukocidins. ― Streptolysin O of Streptococcus pyogenes © McGraw Hill, LLC Membrane-Damaging Toxins ▪ Two types of exotoxins disrupt host cell membranes. Pore-forming toxins insert themselves into membranes, form pores (channels) that allow fluids to enter/exit the cell o Alpha toxin of Staphylococcus aureus Phospholipase enzymes hydrolyze phospholipids into fatty acids, lyse cells o Phospholipase C of Clostridium perfringens © McGraw Hill, LLC  Damage to the Host Superantigens: Proteins (exotoxins) produced by bacterial cells that… Stimulate production of helper T cells (TH cells) TH cells secrete cytokines which increase the activity of other immune cells Overstimulation causes a “cytokine storm” Leads to overactive immune response – causing fever, nausea, vomiting, diarrhea, organ failure, and circulatory collapse Examples Include: toxic shock syndrome toxin and several other toxins from Staphylococcus aureus highly specific interaction SAg circumvents Abdurrahman G, Schmiedeke F, Bachert C, Bröker BM, Holtfreter S. Allergy—A New Role for T Cell Superantigens of Staphylococcus aureus? Toxins. 2020; 12(3):176. normal interaction https://doi.org/10.3390/toxins12030176 Licensee MDPI, Basel, Switzerland. This article is by crosslinking APC an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). with T cell Damage to the Host Superantigens: Proteins (exotoxins) produced by bacterial cells that… Stimulate production of helper T cells (TH cells) TH cells secrete cytokines which increase the activity of other immune cells Overstimulation causes a “cytokine storm” Leads to overactive immune response – causing fever, nausea, vomiting, diarrhea, organ failure, and circulatory collapse Examples Include: toxic shock syndrome toxin and several other toxins from Staphylococcus aureus © McGraw Hill, LLC  Damage to the Host Endotoxin is the lipid A portion of LPS (lipopolysaccharide) Makes up outer layer of the outer membrane of G- bacteria As bacteria die, they release endotoxin, a microbe-associated molecular pattern (MAMP) molecule that binds to receptors on macrophages or B cells. Receptor binding triggers a massive cytokine release that can trigger fever, inflammation, shock, and death. A widespread immune response to LPS can have harmful effects © McGraw Hill, LLC  Damage to the Host Endotoxin Lipid A triggers inflammatory response When localized, response helps clear infection When systemic, causes inflammation throughout the body Called septic shock or endotoxic shock Lipid A only causes response when released following cell lysis of bacteria as a result of phagocytosis or antibiotics Lipid A is heat-stable; autoclaving does not destroy Therefore, solutions intended for IV use must be not only sterile, but also free of endotoxin Credit: Gregory Breese/USFWS Public Domain Limulus amoebocyte lysate (LAL) assay – protein from horseshoe crab blood that surrounds endotoxin and creates a clot Damage to the Host Comparison of Exotoxins and Endotoxin Exotoxins from Gram-positives and Gram-negatives Protein; potent; usually heat-inactivated; may or may not be secreted Endotoxins only from Gram-negatives Lipid A component of LPS; small localized amounts yield appropriate response, but systemic distribution can be deadly; heat-stable Review Questions 1) The microorganisms that are occasionally found in 3) Growth of a pathogen in or on the host is or on the body are called referred to as A) normal microbiota. B) abnormal microbiota. A) pathogenism. B) colonization. C) variant microbiota. D) transient microbiota. C) infection. D) mutualism. 2) The number of organisms necessary to ensure infection is 4) People who carry and may spread pathogenic termed the organisms without any symptoms of illness are A) lethal dose. B) pathogenic number. called C) infectious dose. D) minimum inhibitory concentration. A) mutualists. B) primary infections. C) secondary infections. D) carriers.