Chapter 15:Chapter 15:

Microbial PathogenesisMicrobial Pathogenesis

Microbial PathogenesisMicrobial PathogenesisEntry into the HostEntry into the HostMust access and adhere to host tissues, penetrate or Must access and adhere to host tissues, penetrate or

evade host defenses, and damage tissue to cause evade host defenses, and damage tissue to cause disease.disease.

Portals of EntryPortals of EntryThe three main portals of entry are:The three main portals of entry are: Mucous membranesMucous membranes SkinSkin ParenteralParenteral

Microbial Mechanisms of Pathogenicity: How Microorganisms Cause Disease

I. Mucous MembranesI. Mucous Membranes

Epithelial tissue lining the:Epithelial tissue lining the: Respiratory tractRespiratory tract: Easiest and most frequently : Easiest and most frequently

used entry site for microbes.used entry site for microbes. Gastrointestinal tractGastrointestinal tract: Another common entry : Another common entry

site. Enter through water, food, contaminated site. Enter through water, food, contaminated fingers and fomites. Must survive stomach HCl, fingers and fomites. Must survive stomach HCl, enzymes, and bile.enzymes, and bile.

Genitourinary tractGenitourinary tract: Entry site for most sexually : Entry site for most sexually transmitted diseases (STDs).transmitted diseases (STDs).

ConjunctivaConjunctiva: Membrane covering eyes and : Membrane covering eyes and eyelidseyelids..

II. SkinII. SkinUnbroken skin is impenetrable by most Unbroken skin is impenetrable by most microbes.microbes.

Some microbes gain access through hair Some microbes gain access through hair follicles and sweat glands.follicles and sweat glands.

Nectator americanusNectator americanus (hookworm) can (hookworm) can bore through intact skin.bore through intact skin.

Certain fungi (dermatophytes) grow on Certain fungi (dermatophytes) grow on skin and produce enzymes that break skin and produce enzymes that break down keratin.down keratin.

III. Parenteral Route III. Parenteral Route Microbes are deposited directly into the Microbes are deposited directly into the tissues beneath the skin or mucous tissues beneath the skin or mucous membranes.membranes.

Examples: Injections, bites, cuts, wounds, Examples: Injections, bites, cuts, wounds, surgery, punctures, and splitting due to swelling surgery, punctures, and splitting due to swelling or drying.or drying.

Preferred Portal of EntryPreferred Portal of EntryMany microbes have a preferred portal of entry Many microbes have a preferred portal of entry which is a prerequisite to cause disease.which is a prerequisite to cause disease. Example: Example: Streptococcus pneumoniaeStreptococcus pneumoniae that are inhaled can that are inhaled can

cause pneumonia; if swallowed generally don’t cause cause pneumonia; if swallowed generally don’t cause disease.disease.

Number of Invading MicrobesNumber of Invading Microbes Higher number of pathogens increase the Higher number of pathogens increase the

likelihood of developing disease.likelihood of developing disease. LDLD5050: Lethal dose for 50% of hosts. : Lethal dose for 50% of hosts.

Number of microbes that will kill 50% of Number of microbes that will kill 50% of inoculated test animals.inoculated test animals.

IDID5050: Infectious dose for 50% of hosts. : Infectious dose for 50% of hosts.

Number of microbes that will cause a Number of microbes that will cause a demonstrable infection in 50% of demonstrable infection in 50% of inoculated test animals.inoculated test animals.

AdherenceAdherence Attachment between of microbe to host tissue requires:Attachment between of microbe to host tissue requires: Adhesins or LigandsAdhesins or Ligands: Surface molecules on pathogen that bind specifically to host cell surface molecules. May be : Surface molecules on pathogen that bind specifically to host cell surface molecules. May be

located on glycocalyx, fimbriae, viral capsid, or other surface structure.located on glycocalyx, fimbriae, viral capsid, or other surface structure. ReceptorsReceptors: Surface molecules on host tissues to which pathogen adhesins bind.: Surface molecules on host tissues to which pathogen adhesins bind.

Cell Wall ComponentsCell Wall Components M protein:M protein: Found on cell surface and fimbriae of Found on cell surface and fimbriae of Streptococcus pyogenesStreptococcus pyogenes. Mediates attachment an dhelps resist . Mediates attachment an dhelps resist

phagocytosis.phagocytosis. Waxes:Waxes: In cell wall of In cell wall of Mycobacterium tuberculosis Mycobacterium tuberculosis helps resist digestion after phagocytosis.helps resist digestion after phagocytosis.

EnzymesEnzymes Extracellular enzymes (exoenzymes) lyse cells, form or dissolve clots, and dissolve materials in tissue.Extracellular enzymes (exoenzymes) lyse cells, form or dissolve clots, and dissolve materials in tissue.

.. Leukocidins: Leukocidins: Destroy white blood cells that are phagocytes. Produced by staphylococci and streptococci. Destroy white blood cells that are phagocytes. Produced by staphylococci and streptococci. . Hemolysins: Destroy red blood cells. Produced by . Hemolysins: Destroy red blood cells. Produced by clostridium perfringensclostridium perfringens (gangrene) and streptococci. (gangrene) and streptococci. . Coagulases: Produce blots in blood. Clots may protect bacteria from host immune system, by walling off site of infection. Produced by . Coagulases: Produce blots in blood. Clots may protect bacteria from host immune system, by walling off site of infection. Produced by

some staphylococci.some staphylococci. . Bacterial Kinases: Break down clots produced by body to isolate infection. Made by streptococci and staphylococci.. Bacterial Kinases: Break down clots produced by body to isolate infection. Made by streptococci and staphylococci. . Hyaluronidase: Breaks down hyaluronic acid which holds cells together in connective tissue. Made by some streptococci and gangrene . Hyaluronidase: Breaks down hyaluronic acid which holds cells together in connective tissue. Made by some streptococci and gangrene

causing clostridia.causing clostridia. . Collagenase: Breaks down collagen which forms connective tissue of muscles, skin, and other organs. Produced by several clostridia.. Collagenase: Breaks down collagen which forms connective tissue of muscles, skin, and other organs. Produced by several clostridia. . Necrotizing Factors: Kill body cells.. Necrotizing Factors: Kill body cells. . Hypothermic factors: Decrease body temperature.. Hypothermic factors: Decrease body temperature. . Lecithinase: Destroys plasma membrane of cells.. Lecithinase: Destroys plasma membrane of cells. . Proteases: Break down proteins in tissue.. Proteases: Break down proteins in tissue.

Penetration into Host CellsPenetration into Host Cells Invasins: Invasins: Surface proteins that alter actin filaments of host cell cytoskeleton, allowing microbes to enter cells.Surface proteins that alter actin filaments of host cell cytoskeleton, allowing microbes to enter cells. Examples:Examples: Salmonella typhinuriumSalmonella typhinurium and and E. coli.E. coli. Cadherin: Cadherin: A glycoprotein that bridges junctions between cells, allowing microbes to move from one cell to A glycoprotein that bridges junctions between cells, allowing microbes to move from one cell to

another.another.

Microbial Mechanisms of Pathogenicity: How Microorganisms Cause Disease

How Bacterial Pathogens Penetrate How Bacterial Pathogens Penetrate Host DefensesHost Defenses

CapsulesCapsules Increase the virulence of many pathogens.Increase the virulence of many pathogens. Examples: Examples: Streptococcus pneumoniae, Klebsiella Streptococcus pneumoniae, Klebsiella

pneumoniae, Hemophilus influenzae, Bacillus pneumoniae, Hemophilus influenzae, Bacillus anthracis, anthracis, and and Yersinia pestisYersinia pestis..

Resist host defenses by impairing phagocytosis.Resist host defenses by impairing phagocytosis. Host can produce antibodies to capsule, which Host can produce antibodies to capsule, which

attach to microbe and allow phagocytosis.attach to microbe and allow phagocytosis.

Cell Wall ComponentsCell Wall Components M proteinM protein: Found on cell surface and fimbriae : Found on cell surface and fimbriae

of of Streptococcus pyogenesStreptococcus pyogenes. Mediates attachment . Mediates attachment and helps resist phagocytosis.and helps resist phagocytosis.

WaxesWaxes: Cell wall of : Cell wall of Mycobacterium tuberculosis Mycobacterium tuberculosis helps resist digestion after phagocytosis.helps resist digestion after phagocytosis.

Microbial EnzymesMicrobial EnzymesExtracellular enzymes (exoenzymes) lyse cells, form Extracellular enzymes (exoenzymes) lyse cells, form or dissolve clots, and dissolve materials in tissue.or dissolve clots, and dissolve materials in tissue. LeukocidinsLeukocidins: Destroy white blood cells that are : Destroy white blood cells that are

phagocytes. Produced by staphylococci and streptococci.phagocytes. Produced by staphylococci and streptococci. HemolysinsHemolysins: Destroy red blood cells. Produced by : Destroy red blood cells. Produced by

clostridium perfringensclostridium perfringens (gangrene) and streptococci. (gangrene) and streptococci. CoagulasesCoagulases: Produce clots in blood, which may wall off : Produce clots in blood, which may wall off

site of infection from immune response. Produced by some site of infection from immune response. Produced by some staphylococci.staphylococci.

Bacterial KinasesBacterial Kinases: Break down clots produced by body to : Break down clots produced by body to isolate infection. Made by streptococci and staphylococci.isolate infection. Made by streptococci and staphylococci.

HyaluronidaseHyaluronidase: Breaks down hyaluronic acid which holds : Breaks down hyaluronic acid which holds cells together in connective tissue. Made by some cells together in connective tissue. Made by some streptococci and gangrene causing clostridia.streptococci and gangrene causing clostridia.

Severe gangrene caused by Clostridium perfringens.Source: Tropical Medicine and Parasitology, 1997

Tissue Damage Caused by Microbial Enzymes of Clostridium perfringens

Microbial Enzymes (Continued)Microbial Enzymes (Continued) CollagenaseCollagenase: Breaks down collagen which forms : Breaks down collagen which forms

connective tissue of muscles, skin, and other organs. connective tissue of muscles, skin, and other organs. Produced by several clostridia.Produced by several clostridia.

Necrotizing FactorsNecrotizing Factors: Kill body cells.: Kill body cells. Hypothermic factorsHypothermic factors: Decrease body temperature.: Decrease body temperature. LecithinaseLecithinase: Destroys plasma membrane of cells.: Destroys plasma membrane of cells. ProteasesProteases: Break down proteins in tissue.: Break down proteins in tissue.

Necrotizing fasciitis with blood filled vesicles.Source: Perspectives in Microbiology, 1995

Tissue Damage Caused by Enzymes of Flesh-Eating Streptococcus pyogenes

Penetration into Host CellsPenetration into Host Cells InvasinsInvasins: : Surface proteins that alter actin Surface proteins that alter actin

filaments of host cell cytoskeleton, allowing filaments of host cell cytoskeleton, allowing microbes to enter cells.microbes to enter cells. Examples:Examples: Salmonella typhinuriumSalmonella typhinurium and and E. coli.E. coli.

CadherinCadherin: : A glycoprotein that bridges A glycoprotein that bridges junctions between cells, allowing microbes to junctions between cells, allowing microbes to move from one cell to another.move from one cell to another.

How Bacterial Cells Damage Host CellsHow Bacterial Cells Damage Host Cells

Three mechanisms:Three mechanisms: Direct DamageDirect Damage Toxins*Toxins* Hypersensitivity ReactionsHypersensitivity Reactions

* Most bacterial damage is carried out by toxins.* Most bacterial damage is carried out by toxins.

1. Direct Damage1. Direct Damage Some bacteria can induce cells to engulf them (Some bacteria can induce cells to engulf them (E. coli, E. coli,

Shigella, Salmonella, Shigella, Salmonella, and and Neisseria gonorrhoeaeNeisseria gonorrhoeae).). Microbial metabolism and multiplication kills host cells.Microbial metabolism and multiplication kills host cells. Other microbes enter the cell by excreting enzymes or Other microbes enter the cell by excreting enzymes or

through their own motility.through their own motility.

2. Toxin Production2. Toxin Production ToxinsToxins: Poisonous substances produced by microbes.: Poisonous substances produced by microbes. Frequently toxins are the Frequently toxins are the main pathogenic factormain pathogenic factor.. ToxigenicityToxigenicity: Ability of a microbe to produce toxins.: Ability of a microbe to produce toxins. ToxemiaToxemia: Presence of toxins in the blood.: Presence of toxins in the blood. Toxin effectsToxin effects: May include fever, cardiovascular : May include fever, cardiovascular

problems, diarrhea, shock, destruction of red blood cells problems, diarrhea, shock, destruction of red blood cells and blood vessels, and nervous system disruptions.and blood vessels, and nervous system disruptions.

Of 220 known bacterial toxins, 40% damage eucaryotic Of 220 known bacterial toxins, 40% damage eucaryotic cell membranes.cell membranes.

Two types of toxins: Two types of toxins: ExotoxinsExotoxins EndotoxinsEndotoxins

Exotoxins versus Endotoxins

A. ExotoxinsA. Exotoxins ProteinsProteins: Enzymes that carry out specific reactions. : Enzymes that carry out specific reactions. SolubleSoluble in body fluids, in body fluids, rapidly transportedrapidly transported throughout throughout

body in blood or lymph.body in blood or lymph. Produced mainly by Produced mainly by gram-positive bacteriagram-positive bacteria.. Most genes for toxins are carried onMost genes for toxins are carried on plasmids plasmids or or phagesphages.. Produced inside bacteria and released into host tissue.Produced inside bacteria and released into host tissue. Responsible for disease symptoms and/or death.Responsible for disease symptoms and/or death.

CytotoxinsCytotoxins: Kill or damage host cells.: Kill or damage host cells. NeurotoxinsNeurotoxins: Interfere with nerve impulses.: Interfere with nerve impulses. EnterotoxinsEnterotoxins: Affect lining of gastrointestinal tract.: Affect lining of gastrointestinal tract.

Antibodies called Antibodies called antitoxinsantitoxins provide immunity. provide immunity. ToxoidsToxoids: Toxins that have been altered by heat or : Toxins that have been altered by heat or

chemicals. Used as vaccines for diphtheria and tetanus.chemicals. Used as vaccines for diphtheria and tetanus.

Important ExotoxinsImportant Exotoxins Diphtheria ToxinDiphtheria Toxin: : Corynebacterium diphtheriaeCorynebacterium diphtheriae when infected by a when infected by a

phage carrying tox gene. phage carrying tox gene. CytotoxinCytotoxin inhibits protein synthesis in inhibits protein synthesis in eucaryotic cells. Two polypeptides: A (active) and B (binding).eucaryotic cells. Two polypeptides: A (active) and B (binding).

Erythrogenic ToxinsErythrogenic Toxins: : Streptococcus pyogenes Streptococcus pyogenes produces three produces three cytotoxinscytotoxins which damage blood capillaries, causing a red rash. which damage blood capillaries, causing a red rash.

Botulinum ToxinsBotulinum Toxins: Produced by : Produced by Clostridium botulinum.Clostridium botulinum. NeurotoxinNeurotoxin that inhibits release of neurotransmitter acetylcholine and that inhibits release of neurotransmitter acetylcholine and prevents prevents transmission of nerve impulses to musclestransmission of nerve impulses to muscles, causing flaccid paralysis. , causing flaccid paralysis. Extremely potent toxins.Extremely potent toxins.

Tetanus ToxinTetanus Toxin: Produced by : Produced by Clostridium tetaniClostridium tetani. A . A neurotoxinneurotoxin that that blocks relaxation of skeletal musclesblocks relaxation of skeletal muscles, causing uncontrollable muscle , causing uncontrollable muscle spasms (lockjaw) and convulsions.spasms (lockjaw) and convulsions.

Vibrio EnterotoxinVibrio Enterotoxin: Produced by : Produced by Vibrio choleraeVibrio cholerae. Two polypeptides: . Two polypeptides: A (active) and B (binding). The A subunit of A (active) and B (binding). The A subunit of enterotoxinenterotoxin causes causes epithelial cells to discharge large amounts of fluids and electrolytes. epithelial cells to discharge large amounts of fluids and electrolytes.

Staphylococcal EnterotoxinStaphylococcal Enterotoxin: : Staphylococcus aureus Staphylococcus aureus produces an produces an enterotoxinenterotoxin similar to cholera toxin. Other enterotoxins cause toxic similar to cholera toxin. Other enterotoxins cause toxic shock syndrome.shock syndrome.

Rash of Scarlet Fever Caused by Erythrogenic Toxins of Streptococcus pyogenes

Neonatal Tetanus (Wrinkled brow and risus sardonicus)Source: Color Guide to Infectious Diseases, 1992

Muscle Spasms of Tetanus are Caused by Neurotoxin of Clostridium tetani

Rice-water stool of cholera. The A subunit of enterotoxin causesepithelial cells to discharge large amounts of fluids and electrolytes.Source: Tropical Medicine and Parasitology, 1995

Vibrio Enterotoxin Causes Profuse Watery Diarrhea

Diseases Caused by Staphylococcal Toxins

Scalded Skin Syndrome Toxic Shock Syndrome

EndotoxinsEndotoxins Part of outer membrane surrounding gram-negative Part of outer membrane surrounding gram-negative

bacteria.bacteria. Endotoxin is lipid portion of lipopolysaccharides (LPS), Endotoxin is lipid portion of lipopolysaccharides (LPS),

called called lipid Alipid A.. Effect exerted when gram-negative cells die and cell Effect exerted when gram-negative cells die and cell

walls undergo lysis, liberating endotoxin.walls undergo lysis, liberating endotoxin. All produce the same signs and symptoms:All produce the same signs and symptoms:

Chills, fever, weakness, general aches, blood clotting Chills, fever, weakness, general aches, blood clotting and tissue death, shock, and even death. Can also and tissue death, shock, and even death. Can also induce miscarriage.induce miscarriage.

FeverFever: Pyrogenic response is caused by endotoxins.: Pyrogenic response is caused by endotoxins.

Endotoxins (Continued)Endotoxins (Continued) Endotoxins do not promote the formation of Endotoxins do not promote the formation of

effective antibodieseffective antibodies.. Organisms that produce endotoxins include:Organisms that produce endotoxins include:

Salmonella typhiSalmonella typhi Proteus spp.Proteus spp. Pseudomonas spp.Pseudomonas spp. Neisseria sppNeisseria spp..

Medical equipment that has been sterilized may Medical equipment that has been sterilized may still contain endotoxins. still contain endotoxins. LimulusLimulus amoebocyte assay (LAL) is a test used to amoebocyte assay (LAL) is a test used to

detect tiny amounts of endotoxin.detect tiny amounts of endotoxin.

Events leading to fever:Events leading to fever: Gram-negative bacteria are digested by Gram-negative bacteria are digested by

phagocytes.phagocytes. LPSLPS is released by digestion in vacuoles, causing is released by digestion in vacuoles, causing

macrophages to release macrophages to release interleukin-1interleukin-1 (IL-1). (IL-1). IL-1 is carried via blood to IL-1 is carried via blood to hypothalamushypothalamus, which , which

controls body temperature.controls body temperature. IL-1 induces hypothalamus to release IL-1 induces hypothalamus to release

prostaglandinsprostaglandins, which reset the body’s , which reset the body’s thermostat to higher temperature.thermostat to higher temperature.

Endotoxins and the Pyrogenic (Fever) Response

ShockShock: Any life-threatening loss of blood : Any life-threatening loss of blood pressure. pressure.

Septic shockSeptic shock: : Shock caused by Shock caused by endotoxinsendotoxins of gram-negative bacteria. of gram-negative bacteria.

Phagocytosis of bacteria leads to secretion of Phagocytosis of bacteria leads to secretion of tumor necrosis factortumor necrosis factor (TNF), which alters the (TNF), which alters the permeability of blood capillaries and causes them permeability of blood capillaries and causes them to lose large amounts of fluids. to lose large amounts of fluids.

Low blood pressure affects kidneys, lungs, and Low blood pressure affects kidneys, lungs, and gastrointestinal tract.gastrointestinal tract.

Plasmids, Lysogeny, and PathogenicityPlasmids, Lysogeny, and Pathogenicity

Plasmids: Plasmids: Small, circular pieces of DNA that Small, circular pieces of DNA that are not connected to chromosome and are are not connected to chromosome and are capable of independent replication.capable of independent replication. R (resistance) factors contain antibiotic R (resistance) factors contain antibiotic

resistance genes. resistance genes. Other plasmids contain genes for toxins and Other plasmids contain genes for toxins and

pathogenic factors: tetanus toxin, pathogenic factors: tetanus toxin, staphylococcal enterotoxin, staphylococcal enterotoxin, E. coli E. coli enterotoxin enterotoxin (heat-labile), adhesins, and coagulase.(heat-labile), adhesins, and coagulase.

BacteriophagesBacteriophages: : Can incorporate genetic material into Can incorporate genetic material into chromosomal DNA and remain latent (chromosomal DNA and remain latent (lysogenylysogeny). ). Bacterial cell can change characteristics (Bacterial cell can change characteristics (lysogenic lysogenic conversionconversion) and produce certain toxins or ) and produce certain toxins or pathogenic factors: pathogenic factors:

Diphtheria toxinDiphtheria toxin

Capsules in Capsules in S. pneumoniaeS. pneumoniae

Botulinum neurotoxinBotulinum neurotoxin

Staphylococcal enterotoxinStaphylococcal enterotoxin

Cholera toxin.Cholera toxin.

Cytopathic Effects (CPE) of VirusesCytopathic Effects (CPE) of VirusesViral infection may result in one or several of the Viral infection may result in one or several of the

following following cytocidalcytocidal or or noncytocidalnoncytocidal effects in effects in infected cells:infected cells:

1.1. Inhibit macromolecular synthesisInhibit macromolecular synthesis (DNA, RNA, (DNA, RNA, protein). Some viruses irreversibly stop mitosis protein). Some viruses irreversibly stop mitosis (herpes simplex virus).(herpes simplex virus).

2.2. Release ofRelease of lysosomallysosomal enzymesenzymes, resulting in cell , resulting in cell death.death.

3.3. Inclusion bodies Inclusion bodies: Granules in cytoplasm or : Granules in cytoplasm or nuclei of infected cells. May contain viral parts.nuclei of infected cells. May contain viral parts.

4.4. Syncytium Syncytium: Fusion of several adjacent cells to : Fusion of several adjacent cells to form a single giant cell.form a single giant cell.

Cytopathic Effects of Viruses (Cont.)Cytopathic Effects of Viruses (Cont.)5.5. Metabolic changes Metabolic changes in host without visibly in host without visibly

damaging infected cells. May increase hormone or damaging infected cells. May increase hormone or protein production by infected cells, which in turn protein production by infected cells, which in turn affects other cells.affects other cells.

6.6. Interferon production Interferon production: Interferon produced by : Interferon produced by infected cells, protects neighboring cells from infected cells, protects neighboring cells from infection.infection.

7.7. Antigenic changesAntigenic changes on cell surface, causing on cell surface, causing destruction of infected cells by immune system.destruction of infected cells by immune system.

8. 8. Chromosomal changesChromosomal changes: Breakage and : Breakage and incorporation of oncogenes.incorporation of oncogenes.

9. 9. TransformationTransformation: Abnormal cells that have lost : Abnormal cells that have lost contact inhibition.contact inhibition.

Microbial Mechanisms of Pathogenicity: How Microorganisms Cause Disease