Microbial Mechanisms of Pathogenicity Prof. Khaled H. Abu-Elteen Infection and Disease A. Definitions B. Generalized Stages of Infection C. Virulence Factors and Toxins A. Definitions Disease and Infectious Disease • Disease • Any deviation from a condition of good health and well-being • Infectious Disease A disease condition caused by the presence or growth of infectious microorganisms or parasites A. Definitions Pathogenicity and Virulence • Pathogenicity • The ability of a microbe to cause disease • This term is often used to describe or compare species • Virulence • The degree of pathogenicity in a microorganism • This term is often used to describe or compare strains within a species Definitions Acute infection vs. chronic infection • Acute Infection • An infection characterized by sudden onset, rapid progression, and often with severe symptoms • Chronic Infection • An infection characterized by delayed onset and slow progression Definitions Primary infection vs. secondary infection • Primary Infection • An infection that develops in an otherwise healthy individual • Secondary Infection • An infection that develops in an individual who is already infected with a different pathogen Definitions Localized infection vs. systemic infection • Localized Infection • An infection that is restricted to a specific location or region within the body of the host • Systemic Infection • An infection that has spread to several regions or areas in the body of the host Definitions Clinical infection vs. subclinical infection • Clinical Infection • An infection with obvious observable or detectable symptoms • Subclinical Infection • An infection with few or no obvious symptoms Definitions Opportunistic infection • An infection caused by microorganisms that are commonly found in the host’s environment. This term is often used to refer to infections caused by organisms in the normal flora. Definitions The suffix “-emia” • A suffix meaning “presence of an infectious agent” • Bacteremia = Presence of infectious bacteria • Viremia = Presence of infectious virus • Fungemia = Presence of infectious fungus • Septicemia = Presence of an infectious agent in the bloodstream Definitions The suffix “-itis” • A suffix meaning “inflammation of” • Examples: – Pharyngitis = Inflammation of the pharynx – Endocarditis = Inflammation of the heart chambers – Gastroenteritis = Inflammation of the gastointestinal tract Definitions Epidemiology • The study of the transmission of disease Communicable Disease • A disease that can be transmitted from one individual to another Noncommunicable Disease • A disease that is not transmitted from one individual to another Definitions Endemic Disease • A disease condition that is normally found in a certain percentage of a population Epidemic Disease • A disease condition present in a greater than usual percentage of a specific population Pandemic Disease • An epidemic affecting a large geographical area; often on a global scale Definitions Reservoir of Infection • The source of an infectious agent Carrier • An individual who carries an infectious agent without manifesting symptoms, yet who can transmit the agent to another individual Fomites • Any inanimate object capable of being an intermediate in the indirect transmission of an infectious agent Definitions • Animal Vectors • An animal (nonhuman) that can transmit an infectious agent to humans • Two types: mechanical and biological • Mechanical animal vectors: The infectious agent is physically transmitted by the animal vector, but the agent does not incubate or grow in the animal; e.g, the transmission of bacteria sticking to the feet of flies • Biological animal vectors: The infectious agent must incubate in the animal host as part of the agent’s developmental cycle; e.g, the transmission of malaria by infected mosquitoes Definitions Direct Mechanisms of Disease Transmission • Directly From Person to Person • Examples: Direct Skin Contact Airborne (Aerosols) Definitions Indirect Mechanisms of Disease Transmission • Examples: Food & Waterborne Transmission Fomites Animal Vectors Pathogenicity - ability to cause disease Virulence - degree of pathogenicity Many properties that determine a microbe’s pathogenicity or virulence are unclear or unknown But, when a microbe overpowers the hosts defenses, infectious disease results! Molecular Determinants of Pathogenicity Attachment to host tissues Production and delivery of various factors Replication and evasion of immunity Damage to host tissues Microbial Mechanisms of Pathogenicity: How Microorganisms Cause Disease Portals of Entry 1. Mucus Membranes 2. Skin 3. Parentarel 1. Mucus Membranes A. Respiratory Tract • microbes inhaled into mouth or nose in droplets of moisture or dust particles • Easiest and most frequently traveled portal of entry Common Diseases contracted via the Respiratory Tract Common cold Flu Tuberculosis Whooping cough Pneumonia Measles Diphtheria Mucus Membranes B. Gastrointestinal Tract • microbes gain entrance thru contaminated food & water or fingers & hands • most microbes that enter the G.I. Tract are destroyed by HCL & enzymes of stomach or bile & enzymes of small intestine Common diseases contracted via the G.I. Tract Salmonellosis • Salmonella sp. Shigellosis • Shigella sp. Cholera • Vibrio cholorea Ulcers • Helicobacter pylori Clostridium botulinum Botulism • Clostridium botulinum Fecal - Oral Diseases These pathogens enter the G.I. Tract at one end and exit at the other end. Spread by contaminated hands & fingers or contaminated food & water Poor personal hygiene. Mucus Membranes of the Genitourinary System - STD’s Gonorrhea Neisseria gonorrhoeae Syphilis Treponema pallidum Chlamydia Chlamydia trachomatis HIV Herpes Simplex II Mucus Membranes D. Conjunctiva – • mucus membranes that cover the eyeball and lines the eyelid Trachoma • Chlamydia trachomatis 2nd Portal of Entry: Skin Skin - the largest organ of the body. When unbroken is an effective barrier for most microorganisms. Some microbes can gain entrance through openings in the skin: hair follicles and sweat glands, wound …etc 3rd Portal of Entry: Parentarel Microorganisms are deposited into the tissues below the skin or mucus membranes Punctures and scratches injections bites surgery Preferred Portal of Entry Just because a pathogen enters your body it does not mean it’s going to cause disease. pathogens - preferred portal of entry Preferred Portal of Entry Streptococcus pneumoniae • if inhaled can cause pneumonia • if enters the G.I. Tract, no disease Salmonella typhi • if enters the G.I. Tract can cause Typhoid Fever • if on skin, no disease Number of Invading Microbes LD50 - Lethal Dose of a microbes toxin that will kill 50% of experimentally inoculated test animal ID50 - infectious dose required to cause disease in 50% of inoculated test animals • Example: ID50 for Vibrio cholerea 108 cells (100,000,000 cells) • ID50 for Inhalation Anthrax - 5,000 to 10,000 spores ???? How do Bacterial Pathogens penetrate Host Defenses? 1. Adherence - almost all pathogens have a means to attach to host tissue Binding Sites adhesins ligands Some cells use fimbriae to adhere. Fimbriae can play a role in tissue tropism. Example attachment of Candida to vaginal epithelial cells Adhesins and ligands are usually on Fimbriae Neisseria gonorrhoeae ETEC (Entertoxigenic E. coli) Bordetello pertussis Bacteria typically employ proteins known as Adhesins to attach to host tissues, which usually are located on ends of fimbriae. Alternatively, adhesins can consist of glycocalyx. 2. Capsules K. pneumoniae Prevent phagocytosis attachment Streptococcus pneumoniae Klebsiella pneumoniae Haemophilus influenzae Bacillus anthracis Streptococcus mutans Avoidance of Phagocytosis Capsules are Involved in avoidance of phagocytemediated recognition and attachment. Cell Wall Components M protein: Found on cell surface and fimbriae of Streptococcus pyogenes. Mediates attachment and helps resist phagocytosis. M-protein is heat and acid resistant Waxes [ Mycolic Acid]: In cell wall of Mycobacterium tuberculosis helps resist digestion after phagocytosis and can multiply inside WBC. 3. Enzymes Many pathogens secrete enzymes that contribute to their pathogenicity Enzymes and toxins that harm eukaryotic cells. A. Leukocidins Attack certain types of WBC’s 1. Kills WBC’s which prevents phagocytosis 2. Releases & ruptures lysosomes • lysosomes - contain powerful hydrolytic enzymes which then cause more tissue damage B. Hemolysins - cause the lysis of RBC’s Streptococci 1. Alpha (α) Hemolytic Streptococci - secrete hemolysins that cause the incomplete lysis or RBC’s Incomplete Lysis of RBC 2. Beta (β) Hemolytic Streptococci - secrete hemolysins that cause the complete lysis of RBC’s Complete Lysis of RBC 3. Gamma (γ) Hemolytic Streptococci - do not secrete any hemolysins C. Coagulase - cause blood to coagulate Blood clots protect bacteria from phagocytosis from WBC’s and other host defenses Staphylococcus aureus - are often coagulase positive Fibrinogen ----------------- Fibrin ( Clot) D. Kinases - enzymes that dissolve blood clots 1. Streptokinase - Streptococci 2. Staphylokinase - Staphylococci Helps to spread bacteria - Bacteremia Streptokinase - used to dissolve blood clots in the Heart (Heart Attacks due to obstructed coronary blood vessels) E. Hyaluronidase Breaks down Hyaluronic acid (found in connective tissues) “Spreading Factor” mixed with a drug to help spread the drug through a body tissue Streptococci, Staphylococci, Clostridia and pneumococci. F. Collagenase Breaks down collagen (found in many connective tissues) Clostridium perfringens - Gas Gangrene • uses this to spread through muscle tissue Tissue Damage Caused by Microbial Enzymes of Clostridium perfringens Severe gangrene caused by Clostridium perfringens. Source: Tropical Medicine and Parasitology, 1997 G. Necrotizing Factor - causes death (necrosis) to tissue cells “Flesh Eating Bacteria” Necrotizing fasciitis H. Lecithinase Destroys lecithin ( phosphatidylcholine) component of plasma membrane. Allowing pathogen to spread Clostridium perfringens Summary of How Bacterial Pathogens Penetrate Host Defenses 1. Adherence 2. Capsule 3. Enzymes • • • • A. leukocidins B. Hemolysins C. Coagulase D. Kinases E. Hyaluronidase F. Collagenase G. Necrotizing Factor H. Lecithinase 4. Toxins Poisonous substances produced by microorganisms toxins - primary factor - pathogenicity 220 known bacterial toxins • 40% cause disease by damaging the Eukaryotic cell membrane Toxemia • Toxins in the bloodstream • Toxigenicity: Capacity of microorganisms to produce toxins. Two Types of Toxins 1. Exotoxins • secreted outside the bacterial cell 2. Endotoxins • part of the outer cell wall of Gram (-) bacteria. ?? Exotoxins versus Endotoxins I- Exotoxins Mostly seen in Gram (+) Bacteria Most gene that code for exotoxins are located on plasmids or phages Three Types of Exotoxins 1. Cytotoxins • kill cells e.g. Diphtheria toxin 2. Neurotoxins • interfere with normal nerve impulses.e.g. Botulinum Toxin 3. Enterotoxins • effect cells lining the G.I. Tract. e.g. Cholera toxin or choleragen. Response to Toxins If exposed to exotoxins: antibodies against the toxin (antitoxins) Exotoxins inactivated ( heat, formalin or phenol) no longer cause disease, but stimulate the production of antitoxin • altered exotoxins - Toxoids Toxoids - modified toxin by heat, chemical, radiation, that have lost their toxicity. Injected to stimulate the production of antitoxins and provide immunity. Example: DPT Vaccine D - Diphtheria • Corynebacterium diphtheriae P - Pertussis • Bordetello pertussis T - Tetanus • Clostridium tetani DPT - Diphtheria Toxoid Pertussis Antigen Tetanus Toxoid Required Immunizations in Jordan 1. Diphtheria 2. Pertussis 3. Tetanus 4. Measles 5. Mumps 6. Rubella Corynebacterium diphtheriae Bordetello pertussis Clostridium tetani Measles virus Mumps virus Rubella virus • German Measles 7. Polio 9. Hepatitis B Polio virus Hepatitis B Virus Most genes that code for exotoxins - plasmids or phages Lysogenic convergence Diphtheria Cytotoxin inhibits protein synthesis resulting in cell death Pseudomembrane • fibrin, dead tissue, bacterial cells Lysogenic Convergence Scarlet Fever Streptococcus pyogenes • lysogenic convergence cytotoxin - damages blood capillaries and results in a skin rash • Strep Thoat with a rash Rash of Scarlet Fever Caused by Erythrogenic Toxins of Streptococcus pyogenes Diseases Caused by Staphylococcal Toxins Scalded Skin Syndrome Toxic Shock Syndrome Diseases caused by Neurotoxins Botulism • Clostridium botulinum • Gram (+), anaerobic, spore-forming rod, found in soil • works at the neuromuscular junction • prevents impulse from nerve cell to muscle cell • results in muscle paralysis Tetanus (Lock Jaw) Clostridium tetani Gram (+), spore-forming, anaerobic rod neurotoxin acts on nerves, resulting in the inhibition of muscle relaxation tetanospasmin - “spasms” or “Lock Jaw” Muscle Spasms of Tetanus are Caused by Neurotoxin of Clostridium tetani Neonatal Tetanus (Wrinkled brow and risus sardonicus) Source: Color Guide to Infectious Diseases, 1992 Diseases caused by Enterotoxins Cholera • Vibrio cholerae • Gram (-) comma shaped rods Cholera toxin Converts ATP into cAMP causes cells to excrete Cl- ions and inhibits absorption of Na+ ions Electrolyte imbalance H2O leaves by osmosis H2O Loss (Diarrhea) Two polypeptides: A (active) and B (binding). The A subunit of enterotoxin causes epithelial cells to discharge large amounts of fluids and electrolytes. Severe cases, 12 - 20 liters of liquid lost in a day Untreated cases - Mortality Rate about 50% Mortality may be reduced to about 1% • administering fluids and electrolytes Vibrio Enterotoxin Causes Profuse Watery Diarrhea Rice-water stool of cholera. The A subunit of enterotoxin causes epithelial cells to discharge large amounts of fluids and electrolytes. Source: Tropical Medicine and Parasitology, 1995 EHEC (Enterohemorrhagic E. coli) E. coli (0157:H7) enterotoxin causes a hemolytic inflammation of the intestines results in bloody diarrhea • Toxin • • • • • alters the 60S ribosomal subunit inhibits Protein Synthesis Results in cell death lining of intestine is “shed” Bloody Diarrhea (Dysentary) More on Toxins II- Endotoxins • Part of outer membrane surrounding gram-negative bacteria. • Endotoxin is lipid portion of lipopolysaccharides (LPS), called lipid A. • Effect exerted when gram-negative cells die and cell walls undergo lysis, liberating endotoxin. • All produce the same signs and symptoms: • Chills, fever, weakness, general aches, blood clotting and tissue death, shock, and even death. Can also induce miscarriage. • Fever: Pyrogenic response is caused by endotoxins. Exotoxins vs. Endotoxins Endotoxin is LPS Endotoxins (Continued) • Endotoxins do not promote the formation of effective antibodies. • Organisms that produce endotoxins include: • • • • Salmonella typhi Proteus spp. Pseudomonas spp. Neisseria spp. • Medical equipment that has been sterilized may still contain endotoxins. • Limulus amoebocyte assay (LAL) is a test used to detect tiny amounts of endotoxin. Events leading to fever: • Gram-negative bacteria are digested by phagocytes. • LPS is released by digestion in vacuoles, causing macrophages to release interleukin-1 (IL-1). • IL-1 is carried via blood to hypothalamus, which controls body temperature. • IL-1 induces hypothalamus to release prostaglandins, which reset the body’s thermostat to higher temperature. Microbial Mechanisms of Pathogenicity: How Microorganisms Cause Disease III. B. The Normal Flora of Humans Types of Symbiosis • Mutualism • A symbiotic relationship in which both species benefit • Commensalism • A symbiotic relationship in which one species benefits, and the other species is neither helped nor harmed III. B. The Normal Flora of Humans Types of Symbiosis (cont.) • Parasitism • A symbiotic relationship in which one species benefits, and the other species is harmed • Generally, the species that benefits (the parasite) is much smaller than the species that is harmed (the host) III. B. The Normal Flora of Humans Normal flora is present in • skin • upper respiratory tract • oral cavity • intestine, especially large intestine • vaginal tract Very little normal flora in eyes & stomach III. B. The Normal Flora of Humans Notably absent in most all internal organs • Absent in: • lower respiratory tract • muscle tissue • blood & tissue fluid • cerebrospinal fluid • peritoneum • pericardium • meninges III. B. The Normal Flora of Humans Benefits of the normal flora • Nutrient production/processing eg Vitamin K production by E. coli • Competition with pathogenic microbes • Normal development of the immune system Normal flora and opportunistic infections III. C. Generalized Stages of Infection 1. Entry of Pathogen • Portal of Entry 2. Colonization • Usually at the site of entry 3. Incubation Period • Asymptomatic period • Between the initial contact with the microbe and the appearance of the first symptoms III. C. Generalized Stages of Infection 4. Prodromal Symptoms • Initial Symptoms 5. Invasive period • Increasing Severity of Symptoms • Fever • Inflammation and Swelling • Tissue Damage • Infection May Spread to Other Sites III. C. Generalized Stages of Infection 6. Decline of Infection 5. Convalescence Course of Infectious Disease Convalescence is a time of recuperation and recovery from illness. Incubation period is the interval between exposure and illness onset. Depending on various factors an individual may still be infectious during either incubation or convalescence.
