PATHOGENESIS OF

BACTERIAL INFECTION

PATHOGENICITY TOXIGENICITY

VIRULENCE

The pathogenesis of bacterial infection includes the The pathogenesis of bacterial infection includes the initiation of the infectious process and the initiation of the infectious process and the mechanisms leading to the development of signs mechanisms leading to the development of signs and symptoms of bacterial disease. and symptoms of bacterial disease.

The outcome of the interaction between bacteria The outcome of the interaction between bacteria and host is determined by characteristics that and host is determined by characteristics that favour establishment of the bacteria within the host favour establishment of the bacteria within the host and their ability to damage the host as they are and their ability to damage the host as they are opposed by host defense mechanisms.opposed by host defense mechanisms.

Among the characterics of bacteria are Among the characterics of bacteria are adherence to host cells, invasiveness, adherence to host cells, invasiveness, toxigenity, and ability to evade the hosttoxigenity, and ability to evade the host´s imm´s immuune system. ne system.

If the bacteria or immunological If the bacteria or immunological reactions injure the host sufficiently, reactions injure the host sufficiently, disease becomes apparent.disease becomes apparent.

Pathogenesis Pathogenesis of bacterial infectionof bacterial infection

Humans and animals have abundant normal Humans and animals have abundant normal micromicroflora.flora.

Most bacteria do not produce disease but achieve a Most bacteria do not produce disease but achieve a balance with the host that ensures the survival, growth, balance with the host that ensures the survival, growth, and propagation of both the bacteria and the host.and propagation of both the bacteria and the host.

Sometimes bacteria that are clearly pathogens (e.g. Sometimes bacteria that are clearly pathogens (e.g. Salmonella typhiSalmonella typhi) are present, but infection remains ) are present, but infection remains latent or subclinical and the host is a "carrier" of the latent or subclinical and the host is a "carrier" of the bacteria.bacteria.

It can be difficult to show that a specific It can be difficult to show that a specific bacterial species is the cause of a particular bacterial species is the cause of a particular disease. disease.

In 1884, Robert Koch proposed a series of In 1884, Robert Koch proposed a series of postulates in his treatise on postulates in his treatise on Mycobacterium Mycobacterium tuberculosis and tuberculosistuberculosis and tuberculosis. .

These postulates have been applied more These postulates have been applied more broadly to link many specific bacterial broadly to link many specific bacterial species with particular diseases.species with particular diseases.

Koch´s postulates are summarized as follows:Koch´s postulates are summarized as follows:

The microorganism should be found in all cases of the The microorganism should be found in all cases of the disease in question, and its distribution in the body should disease in question, and its distribution in the body should be in accordancce with the lesions observed.be in accordancce with the lesions observed.

The microoThe microorrganism should be grown in pure culture in vitro ganism should be grown in pure culture in vitro (or outsite the body of the host) for several generations.(or outsite the body of the host) for several generations.

When such a pure culture is inoculated into susceptible When such a pure culture is inoculated into susceptible animal species, the typical disease must result.animal species, the typical disease must result.

The microorganism must again be isolated from the The microorganism must again be isolated from the lesions of such experimentally produced disease.lesions of such experimentally produced disease.

Koch´s postulates remain a mainstay of Koch´s postulates remain a mainstay of microbiology.microbiology.

However, since the late 19th century, many However, since the late 19th century, many microorganisms that do not meet the criteria microorganisms that do not meet the criteria of the postulates have been shown to cause of the postulates have been shown to cause disease. For example, disease. For example, Treponema pallidumTreponema pallidum (syphilis) and (syphilis) and Mycobacerium lepraeMycobacerium leprae (leprosy) (leprosy) cannot be grown in vitro, but there are cannot be grown in vitro, but there are animal models of infection with these agents.animal models of infection with these agents.

In another example, In another example, Neisseria gonorrhoeaeNeisseria gonorrhoeae (gonorrhea), there is no animal model of infection (gonorrhea), there is no animal model of infection even though the bacteria can readily be cultivated in even though the bacteria can readily be cultivated in vitro.vitro.  The host´s immune responses should be considered The host´s immune responses should be considered when an organism is being investigated as the when an organism is being investigated as the possible cause of a disease. possible cause of a disease.

Thus, development of a rise in specific antibody Thus, development of a rise in specific antibody during recovery from disease is an important adjunct during recovery from disease is an important adjunct to Koch´s postulates.to Koch´s postulates.

Modern-day microbial genetics has opened new Modern-day microbial genetics has opened new frontiers to study pathogenic bacteria and differentiate frontiers to study pathogenic bacteria and differentiate them from non-pathogens. them from non-pathogens. The ability to study genes associated with virulence The ability to study genes associated with virulence has led to a proposed of Koch´s postulates:has led to a proposed of Koch´s postulates:

The phenotype, or property, under investigation should The phenotype, or property, under investigation should be associated with pathogenic members of a genus or be associated with pathogenic members of a genus or pathogenic strains of a species.pathogenic strains of a species.

Specific inactivation of the gene(s) associated with the Specific inactivation of the gene(s) associated with the suspected virulence trait should lead to a measurable suspected virulence trait should lead to a measurable loss in pathogenicity or virulence.loss in pathogenicity or virulence.

Reversion or allelic replacement of the mutated gene Reversion or allelic replacement of the mutated gene should lead to restoration of pathogenicity.should lead to restoration of pathogenicity.

Analysis of infection and disease through the application Analysis of infection and disease through the application of principles such as Koch´s postulates leads to of principles such as Koch´s postulates leads to classification of bacteria as pathogenic or non-classification of bacteria as pathogenic or non-pathogenic.pathogenic.

Some bacterial species are always considered to be Some bacterial species are always considered to be pathogens, and their presence is abnormal. pathogens, and their presence is abnormal. – Examples include Examples include Mycobacterium tuberculosisMycobacterium tuberculosis

(tuberculosis) and (tuberculosis) and Yersinia pestisYersinia pestis (plague). (plague). – Other species are commonly part of the normal flora of Other species are commonly part of the normal flora of

humans (and animals) but can also frequently cause humans (and animals) but can also frequently cause disease. For example, disease. For example, Escherichia coliEscherichia coli is part of the is part of the gastrointestinal flora of normal humans, but it is also a gastrointestinal flora of normal humans, but it is also a comon cause of urinary tract infection, traveller´s diarrhea, comon cause of urinary tract infection, traveller´s diarrhea, and other diseases.and other diseases.

The infectious processThe infectious process Infection indicates multiplication of microorganisms. Infection indicates multiplication of microorganisms.

Prior to multiplication, bacteria (in case of bacterial Prior to multiplication, bacteria (in case of bacterial infection) must enter and establish themselves within infection) must enter and establish themselves within the host. the host.

The most frequent portals of entry are the respiratory The most frequent portals of entry are the respiratory (mouth and nose), gastrointestinal, and urogenital (mouth and nose), gastrointestinal, and urogenital tracts. Abnormal areas of mucous membranes and tracts. Abnormal areas of mucous membranes and skin (e.g. cuts, burns) are also frequent sites of entry.skin (e.g. cuts, burns) are also frequent sites of entry.

The infectious processThe infectious process

Once in the body, bacteria must attach or adhere to host Once in the body, bacteria must attach or adhere to host cells, usually epithelial cells. cells, usually epithelial cells.

After the bacteria have established a primary site of infection, After the bacteria have established a primary site of infection, they multiply and spread. they multiply and spread.

Infection can spread directly through tissues or via the Infection can spread directly through tissues or via the lymphatic system to bloodstream. Bloodstream infection lymphatic system to bloodstream. Bloodstream infection (bacteremia) can be transient or persistent. Bacteremia (bacteremia) can be transient or persistent. Bacteremia allows bacteria to spread widely in the body and permits them allows bacteria to spread widely in the body and permits them to reach tissues particularly suitable for their multiplication.to reach tissues particularly suitable for their multiplication.

The infectious processThe infectious process As an example of the infectious process, As an example of the infectious process, Streptococcus pneumoniaeStreptococcus pneumoniae can be can be cultured from the nasopharynx of 5-40% of healthy people. cultured from the nasopharynx of 5-40% of healthy people.

Occasionally, Occasionally, Streptococcus pneumoniaeStreptococcus pneumoniae strains from the nasopharynx are strains from the nasopharynx are aspirated into the lungs. Infection develops in the terminal air space of the aspirated into the lungs. Infection develops in the terminal air space of the lungs in persons who do not have protective antibodies against that type of lungs in persons who do not have protective antibodies against that type of Streptococcus pneumoniaeStreptococcus pneumoniae. Multiplication of . Multiplication of Streptococcus pneumoniaeStreptococcus pneumoniae strains and resultant inflammation lead to pneumonia. The strains then enter strains and resultant inflammation lead to pneumonia. The strains then enter the lymphatics of the lung and move to the bloodstream. Between 10% and the lymphatics of the lung and move to the bloodstream. Between 10% and 20% of persons with 20% of persons with Streptococcus pneumoniaeStreptococcus pneumoniae pneumonia have bacteremia pneumonia have bacteremia at the time the diagnosis of pneumonia is made. Once bacteremia occurs, at the time the diagnosis of pneumonia is made. Once bacteremia occurs, Streptococcus pneumoniaeStreptococcus pneumoniae strains can spread to their preferred secondary strains can spread to their preferred secondary sites of infection (e.g. cerebrospinal fluid, heart valves, joint spaces). The sites of infection (e.g. cerebrospinal fluid, heart valves, joint spaces). The major resulting complications of major resulting complications of Streptococcus pneumoniaeStreptococcus pneumoniae pneumonia pneumonia include meningitis, endocarditis and septic arthritis.include meningitis, endocarditis and septic arthritis.

Basic terms frequently used in describing aspects of pathogenesis:

InfectionInfection::– Multiplication of an infectious agent within the Multiplication of an infectious agent within the

body. body. – Multiplication of the bacteria that are part of Multiplication of the bacteria that are part of

normal flora of gastrointestinal tract, skin, etc, is normal flora of gastrointestinal tract, skin, etc, is generally not considered an infection.generally not considered an infection.

– On the other hand, multiplication of pathogenic On the other hand, multiplication of pathogenic bacteria (e.g. bacteria (e.g. Salmonella speciesSalmonella species), even if the ), even if the person isperson is asymptomatic, is deemed an infection.asymptomatic, is deemed an infection.

Basic terms frequently used in describing aspects of pathogenesis:

PathogenicityPathogenicity::– The ability of an infectious agent to cause disease.The ability of an infectious agent to cause disease.

VirulenceVirulence::– The quantitative ability of an agent to cause disease.The quantitative ability of an agent to cause disease. – VirulentVirulent agents cause disease when introduced into the agents cause disease when introduced into the

host in small numbers. host in small numbers. – Virulence involves invasiveness and toxigenicity.Virulence involves invasiveness and toxigenicity.

Basic terms frequently used in describing aspects of pathogenesis:

ToxigenicityToxigenicity::– The ability of a microorganism to produce a The ability of a microorganism to produce a

toxin that contributes to the development of toxin that contributes to the development of disease.disease.

InvasionInvasion::– The process whereby bacteria, parasites, The process whereby bacteria, parasites,

fungi and virusesfungi and viruses enter the host cells or enter the host cells or tissues and spread in the body. tissues and spread in the body. 

Basic terms frequently used in describing aspects of pathogenesis:

PathogenPathogen::– A microorganism capable of causing disease.A microorganism capable of causing disease.

Non-pathogenNon-pathogen::– A microorganism that does not cause disease. It may be part of A microorganism that does not cause disease. It may be part of

the normal flora.the normal flora.

Opportunistic pathogenOpportunistic pathogen::– An agent capable of causing disease only when the host´s An agent capable of causing disease only when the host´s

resistance is impaired (e.g. the patient is immunocompromised).resistance is impaired (e.g. the patient is immunocompromised).– An agent capable of causing disease only when An agent capable of causing disease only when spread from the spread from the

site with normal bacterial microflora to site with normal bacterial microflora to the the sterile tissue or organ.sterile tissue or organ.

Bacterial virulence factorsBacterial virulence factors

Many factors determine the virulence Many factors determine the virulence of bacteria, or their ability to cause of bacteria, or their ability to cause infection and disease.infection and disease.

ToxinsToxins

Toxins produced by bacteria are Toxins produced by bacteria are generally classified into two groups:generally classified into two groups:

–exotoxinsexotoxins

–endotoxinsendotoxins

Endotoxins of gram-negative Endotoxins of gram-negative bacteriabacteria

The endotoxins of gram-negative bacteria are The endotoxins of gram-negative bacteria are derived from bacterial cell walls and are often derived from bacterial cell walls and are often liberated when the bacteria lyse. liberated when the bacteria lyse.

The substances are heat-stable and can be The substances are heat-stable and can be extracted (e.g. with phenol-water).extracted (e.g. with phenol-water).

Pathophysiological effects of endotoxins are Pathophysiological effects of endotoxins are similar regardless of their bacterial origin:similar regardless of their bacterial origin:– feverfever– leukopenialeukopenia– hypotensionhypotension– impaired organ perfusion and acidosisimpaired organ perfusion and acidosis– activation of C3 and complement cascadeactivation of C3 and complement cascade– disseminated intravascular coagulation (DIC)disseminated intravascular coagulation (DIC)– deathdeath

ExotoxinsExotoxins

Many gram-positive and gram-negative Many gram-positive and gram-negative bacteria produce exotoxins of bacteria produce exotoxins of considerable medical importance. considerable medical importance.

Some of these toxins have had major role Some of these toxins have had major role in world history (e.g. toxin of in world history (e.g. toxin of Clostridium Clostridium tetanitetani).).

Diphtheria toxin

(toxin of Corynebacterium diphtheriae)

Corynebacterium diphtheriaeCorynebacterium diphtheriae strains that carry a strains that carry a temperate bacteriophage with the structural gene temperate bacteriophage with the structural gene for the toxin are toxigenic and produce diphtheria for the toxin are toxigenic and produce diphtheria toxin. toxin.

This native toxin is enzymatically degraded into This native toxin is enzymatically degraded into two fragments: A and B, linked together by a two fragments: A and B, linked together by a disulfide bound. Both fragments are necessary for disulfide bound. Both fragments are necessary for toxin activity.toxin activity.

Tetanospasmin (toxin of Clostridium tetani) Clostridium tetaniClostridium tetani is an anaerobic gram-positive rod that is is an anaerobic gram-positive rod that is widespread in the environment. widespread in the environment.

Clostridium tetaniClostridium tetani contaminates wounds, and the spores contaminates wounds, and the spores germinate in the anaerobic environment of the devitalized germinate in the anaerobic environment of the devitalized tissue. The vegetative forms of tissue. The vegetative forms of Clostridium tetaniClostridium tetani produce produce toxin tetanospasmin. The released toxin has two peptides toxin tetanospasmin. The released toxin has two peptides linked by disulfide bounds. Toxin reaches the central linked by disulfide bounds. Toxin reaches the central nervous system by retrograde transport along axons and nervous system by retrograde transport along axons and through the systemic circulation. The toxin acts by blocking through the systemic circulation. The toxin acts by blocking release of an inhibitory mediator in motor neuron synapses. release of an inhibitory mediator in motor neuron synapses. The result is initially localized then generalized, muscle The result is initially localized then generalized, muscle spasms. Extremely small amount of toxin can be lethal for spasms. Extremely small amount of toxin can be lethal for humans.humans.

Botulotoxin (toxin of Clostridium botulinum) Clostridium botulinumClostridium botulinum is found in soil or water and may is found in soil or water and may grow in foods if the environment is appropriately grow in foods if the environment is appropriately anaerobic. anaerobic.

An exceedingly potent toxin (the most potent toxin An exceedingly potent toxin (the most potent toxin known) is produced by known) is produced by Clostridium botulinumClostridium botulinum strains. It strains. It is heat-labile and is destroyed by sufficient heating. is heat-labile and is destroyed by sufficient heating. There are eight disctinct serological types of toxin. There are eight disctinct serological types of toxin. Types A, B and E are most commonly associated wih Types A, B and E are most commonly associated wih human disease. Toxin is absorbed from the gut and human disease. Toxin is absorbed from the gut and carried to motor nerves, where it blocks the release of carried to motor nerves, where it blocks the release of acetylcholine at synapses and neuromuscular junctions. acetylcholine at synapses and neuromuscular junctions. Muscle contraction does not occur, and paralysis results.Muscle contraction does not occur, and paralysis results.

Toxins of Clostridium perfringens

Spores of Spores of Clostridium perfringensClostridium perfringens are introduced into the are introduced into the wounds by contamination with soil or faeces. In the wounds by contamination with soil or faeces. In the presence of necrotic tissue (an anaerobic environment), presence of necrotic tissue (an anaerobic environment), spores germinate and vegetative cells produce several spores germinate and vegetative cells produce several different toxins. different toxins.

Many of these are necrotizing and hemolytic and favour Many of these are necrotizing and hemolytic and favour the spread of gangrene:the spread of gangrene:– alpha toxin is a lecithinase that damages cell membranesalpha toxin is a lecithinase that damages cell membranes– theta toxin also has a necrotizing affecttheta toxin also has a necrotizing affect– and other and other 

Streptococcal erythrogenic toxin

Some strains of hemolytic lysogenic Some strains of hemolytic lysogenic streptococci produce a toxin that results in a streptococci produce a toxin that results in a punctate maculopapular erythematous rash, as punctate maculopapular erythematous rash, as in scarlet fewer. in scarlet fewer.

Production of erythrogenic toxin is under the Production of erythrogenic toxin is under the genetic control of temperate bacteriophage. If genetic control of temperate bacteriophage. If the phage is lost, the streptococi cannot the phage is lost, the streptococi cannot produce toxin.produce toxin.

Toxic shock syndrom toxin - 1 (TSST-1)

Some Some Staphylococcus aureusStaphylococcus aureus strains growing on strains growing on mucous membranes (e.g. on the vagina in association mucous membranes (e.g. on the vagina in association with menstruation), or in wounds, elaborate TSST-1.with menstruation), or in wounds, elaborate TSST-1.

Although the toxin has been associated with toxic Although the toxin has been associated with toxic shock syndrome, the mechanism of action in unknown. shock syndrome, the mechanism of action in unknown.

The illness is characterized by shock, high fewer, and The illness is characterized by shock, high fewer, and a diffuse red rash that later desquamates, multiple a diffuse red rash that later desquamates, multiple other organs systems are involved as well.other organs systems are involved as well.

Exotoxins associated with diarrheal diseases

Vibrio choleraeVibrio cholerae toxin toxin

Staphylococcus aureusStaphylococcus aureus enterotoxin enterotoxin

Other enterotoxins - enterotoxins are also Other enterotoxins - enterotoxins are also produced by some strains of:produced by some strains of:– Yersinia enYersinia entterocoliticaerocolitica

– Vibrio parahaemolyticusVibrio parahaemolyticus

– Aeromonas speciesAeromonas species

EnzymesEnzymesMany species of bacteria produce enzymes that are not Many species of bacteria produce enzymes that are not intrinsically toxic but play important role in the infectious process.intrinsically toxic but play important role in the infectious process.

Collagenase:Collagenase:– degrades collagen, the major protein of fibrous connective degrades collagen, the major protein of fibrous connective

tissue, and promotes spread of infection in tissue.tissue, and promotes spread of infection in tissue.

CoagulaseCoagulase::– Staphylococccus aureusStaphylococccus aureus produce coagulase, which works in produce coagulase, which works in

conjuction with serum factors to coagulate plasma. Coagulase conjuction with serum factors to coagulate plasma. Coagulase contributes to the formation of fibrin walls around contributes to the formation of fibrin walls around staphylococcal lesions, which helps them persist in tissues.staphylococcal lesions, which helps them persist in tissues.

EnzymesEnzymesHyaluronidasesHyaluronidases::– enzymes that hydrolyze hyaluronic acid, a constituent of the ground enzymes that hydrolyze hyaluronic acid, a constituent of the ground

substance of connective tissue. They are produced bysubstance of connective tissue. They are produced by many bacteria many bacteria (e.g. staphylococci, streptococci and anaerobes) and aid in their (e.g. staphylococci, streptococci and anaerobes) and aid in their spread through tissues.spread through tissues.

StreptokinaseStreptokinase::– many hemolytic streptococci produce streptokinase (fibrinolysin),many hemolytic streptococci produce streptokinase (fibrinolysin),

substance that activates a proteolytic enzyme of plasma. This substance that activates a proteolytic enzyme of plasma. This enzyme, also called fibrinolysin, is then able to dissolve coagulated enzyme, also called fibrinolysin, is then able to dissolve coagulated plasma and probably aids in the spread of streptococci through plasma and probably aids in the spread of streptococci through tissues. Streptokinase is used intissues. Streptokinase is used in treatment of acute myocardial treatment of acute myocardial infarction to dissolve fibrin clots. infarction to dissolve fibrin clots.

EnzymesEnzymes

Hemolysins and leukocidinsHemolysins and leukocidins::– Many bacteria produce substances that are Many bacteria produce substances that are

cytolysins - they dissolve red blood cells cytolysins - they dissolve red blood cells (hemolysins) or kill tissue cells or leukocytes (hemolysins) or kill tissue cells or leukocytes (leukocidins). (leukocidins).

– Streptolysin O, for example, is produced by group Streptolysin O, for example, is produced by group A streptococci and is letal for mice and hemolytic A streptococci and is letal for mice and hemolytic for red blood cells from many animals.for red blood cells from many animals.

Antiphagocytic factorsAntiphagocytic factorsMany bacterial pathogens are rapidly killed once they are Many bacterial pathogens are rapidly killed once they are ingested by polymorphonuclear cells or macrophages. ingested by polymorphonuclear cells or macrophages.

Some pathogens evade phagocytosis or leukocyte microbidical Some pathogens evade phagocytosis or leukocyte microbidical mechanisms by adsorbing normal host componets to their mechanisms by adsorbing normal host componets to their surfaces. surfaces.

For example, For example, Staphylococcus aureusStaphylococcus aureus has surface protein A, has surface protein A, which binds to the Fc portion of IgG. Other pathogens have which binds to the Fc portion of IgG. Other pathogens have surface factors that impede phagsurface factors that impede phagoocytosis e.g. cytosis e.g. Streptococcus Streptococcus pneumoniaepneumoniae and many other bacteria have polysaccharide and many other bacteria have polysaccharide capsules.capsules.

Adherence factorsAdherence factors

Once bacteria enter the body of the host, they Once bacteria enter the body of the host, they must adhere to cells of a tissue surface. If they do must adhere to cells of a tissue surface. If they do not adhere, they would be swept away by mucus not adhere, they would be swept away by mucus and other fluids that bathe the tissue surface. and other fluids that bathe the tissue surface.

Adherence (which is only one step in the Adherence (which is only one step in the infectinfectiious process) is followed by development of ous process) is followed by development of microcolonies and subsequent complex steps in microcolonies and subsequent complex steps in the pathogenesis of infection.the pathogenesis of infection.

Adherence factorsAdherence factors

The interactions between bacteria and The interactions between bacteria and tissue cell surfaces in the adhesion process tissue cell surfaces in the adhesion process are complex. are complex.

Several factors play important role:Several factors play important role:– surface hydrophobicitysurface hydrophobicity– binding molecules on bacteria and host cell binding molecules on bacteria and host cell

receptor interactionreceptor interaction– and otherand other