Mic 101: L 21 & 22 Microbial Pathogenesis Ref: Tortora

Basic Definitions PathogenicityThe ability to cause disease VirulenceThe extent of pathogenicity When a microbe overpowers the hosts defenses, disease results ! Pathogens need to gain entry, adhere, penetrate and cause damage to cause disease. Entry means infection of the host. Adherence means biochemical interaction between host and pathogen that allow pathogens to stay with the host cell. Penetration means escaping the defense mechanism of the host and multiplication. Damage is the final effect of the pathogen which we see in the sign and symptoms of disease.

Basic Mechanism of Pathogenicity

Portals of entry Entry of a Microbe Mucous membranes of – Respiratory tract (nasal mucosa, airway epithelia, larynx, pharynx, trachea, bronchea, pulmonary alveoli) – Gastro-Intestinal tract (mouth, buccal cavity, oesophagus, gastric mucosa, intestinal epithelium, colonic epithelium, rectal epithelium) – Urogenital tract (the skin in genitalia, genital mucosa) – Conjunctiva in the eye Skin – Tough to penetrate, so rare portal of entry. However broken skin in an easy portal of entry. Hair follicle and sweat glands ca act as portals of entry under unhygienic conditions. Parenteral route – Puncture or injection – bites – scratches – surgery – splitting of skin due to swelling or dryness

Factors influencing preferred portal of entry Just because a pathogen enters the body it does not mean it’s going to cause disease. pathogens - 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

Quantification of Infectious Dose ID 50 : Infectious dose for 50% of the test population LD 50 : Lethal dose (of a toxin) for 50% of the test population – Example: ID 50 for Vibrio cholerea 10 8 cells (100,000,000 cells) – ID 50 for Inhalation Anthrax - 5,000 to 10,000 spores

Virulence Factors The ability to: 1. Adherence – To host surfaces and not be washed off 2. Avoid phagocytosis – Prevent host defenses from destroying 3. Penetrate – Get into host and spread 4. Produce Enzymes – Spread, prevent host defenses and cause damage at or near site of infection 5. Produce Toxins – Cause damage at distant site

Adherence Importance of Adherence 1. Attachment : To host surfaces and not be washed off 2. Avoid phagocytosis – Prevent host defenses from destroying 3. Penetrate – Get into host and spread 4. Produce Enzymes – Spread, prevent host defenses and cause damage at or near site of infection 5. Produce Toxins – Cause damage at distant site Adhesions/ligands bind to receptors on host cells so won’t get flushed off. Mechanisms to adhere and avoid host defenses: – Glycocalyxof Streptococcus mutans evade phagocytosis – Fimbriae in Escherichia coli allow tight binding which cannot be removed by mechanical means Capsules prevent phagocytosis and help with attachment (adherence) Eg. Streptococcus pneumoniae, Klebsiella pneumoniae, Haemophilus influenzae Bacillus anthracis, Streptococcus mutans, Yersinia pestis

Mechanisms of Penetration of Host Defense Many pathogens secrete enzymes and toxins that contribute to their pathogenicity: Increase virulence by use of enzymes and avoid phagocytosis – Coagulase coagulates blood ; creates a blockade in host circulatory system – Kinases digest fibrin clot; allow spreading of the pathogen from one tissue to another – Hemolysins lyse RBC’s and release nutrient for pathogen – Leukocidins destroy WBC and weaken the immune surveillance of host.

Toxins ToxinSubstances that contribute to pathogenicity Provide properties to spread and cause damage to the host. endotoxins and exotoxins Endotoxins from inside the cell. Released upon cell lysis. Exotoxins are secreted out of the cell during cell life. ToxigenicityAbility to produce a toxin ToxemiaPresence of toxin the host's blood

Exotoxins Produced inside Gram- positive bacteria as part of their growth and metabolism and released into the surrounding medium. Are proteins and many are enzymes. The genes for most exotoxins are carried on bacterial plasmids or phages.

SourceMostly Gram + Metabolic productBy-products of growing cell ChemistryProtein Water soluble FeverNo Neutralized by antitoxinYes LD 50 Small - Very potent 1 mg of Clostridium botulinum toxin can kill 1 million guinea pigs

Types of Exotoxins. Cytotoxins kill and destroys the target host cells completely Eg. Shigella dysenteriae cause bloody diarrhea due to destruction of luminal epithelia and basal cells 2. Neurotoxins interfere with normal nerve impulses Clostridium tetani produces tetanus toxin which binds irreversibly with neuro- muscular junction and maintains the muscles in continuous contraction (spastic paralysis). 3. Enterotoxins effect cells lining the G.I. Tract Eg- Cholera enterotoxin from Vibrio cholerae Many toxins have A-B subunit A - active Causes change in host B - binding

Common Mode of Action of the A-B type Enterotoxins A-B toxins are proteins with A and B subunits joined by disulphide bonds. A is the catalytic subunit which goes inside the cell and causes molecular dysfunction. Diphtheria toxin causes ADP-ribosylation and inactivation of elongation factor 2b of mammalian cells Erythrogenic toxins lyse erythrocytes Botulinum toxin binds to neuro-muscular junction and causes flaccid paralysis. Tetanus toxin binds irreversibly with neuro- muscular junction and causes spastic paralysis. Vibrio Enterotoxin enhances function of adenylyl cyclase and releases water nad ions from cells. Staphylococcal Enterotoxin is a super- antigen which stimulates T cells to release excess cytokines and cause inflaamation.

Endotoxins Endotoxins are LP or LPS from the Gram negative bacteria LPS is also called lipid A All endotoxins produce same signs and symptoms regardless of organism of origin The symptoms of endotoxins are fever, weakness, shock and death.

Pathogenesis from Endotoxins Fever: when the endotoxigenic bacteria are phagocytosed, the LPS is released and phagocytes produce interleukin-1 (IL-1) in response to LPS. IL-1 comes to the blood stream, goes to the hypothalamus of brain and the brain gets a signal to produce excess heat by accelerating metabolism. Prostaglandins released from the brain fasters energy generation and produces excess heat which is seen as fever. Weakness: The LPS often causes blood to clot is small particles, which obstructs oxygenation in major tissues, causing inability to function and weakness.

Pathogenesis from Endotoxins Shock: it means loss of blood-pressure that can lead to organ failure. LPS often induces production of tumor necrosis factor (TNF), which increases vascular permeability and water exocytosis. Serum moves out of blood into tissues and cerebrospinal fluid moves out of meninges into cavities. As a result, blood pressure drops and brain undergoes extreme dehydration. If not attended quickly, shock can lead to death by brain failure.

Portals of Exit Respiratory tract – Coughing, sneezing Gastrointestinal tract – Feces, saliva Genitourinary tract – Urine, vaginal secretions Skin Blood Biting arthropods, needles/syringes

Transmission of Pathogen Contact transmission by person-to person contact, fomites or droplets. Vehicle transmission through water, air. Vector transmission through insects and parasites.

Summary

Questions Which part of our outer surface is the most susceptible to pathogens: skin or mucous membrane? Why do pathogens have preferred portal of entry? How can we quantify virulence dose? What are the major virulence factors of pathogens? What can the bacteria to adhere effectively to its host?

Questions What are the common mechanisms of penetration of host defense? Why are toxins dangerous in pathogenesis? What types of symptoms can exotoxins cause? How can endotoxins cause fever and shock? What are the differences between endotoxins and exotoxins?

Questions How can pathogens travel from one host to another? When do we see signs and symptoms of a disease after infection? Why do exotoxins differ in their signs and symptoms but endotoxins don’t? Why can we prevent exotoxins with vaccines?