Lecture 2: Pathogenesis of Microbial Infection PDF

Pathogenesis of microbial infections Dr. Monica Agromayor Department of Infectious Diseases Faculty of Life Sciences and Medicine The host-microbe relationship Types of host-microbe associations Symbiosis is the close and often long-term interaction between two different biological species. Types...

Pathogenesis of microbial infections Dr. Monica Agromayor Department of Infectious Diseases Faculty of Life Sciences and Medicine The host-microbe relationship Types of host-microbe associations Symbiosis is the close and often long-term interaction between two different biological species. Types of symbiotic associations: Commensalism: one organism benefits and the other derives neither benefit nor harm. Mutualism: association which is beneficial to both organisms involved Parasitism: one organism, the parasite, benefits at the expense of the other. Obligate intracellular parasites (e.g. intracellular bacteria like Chlamydia and all viruses) can only reproduce within host cells whilst facultative parasites do not rely on its host to continue their life-cycle as can live and reproduce inside and outside cells (e.g. Salmonella) Normal Flora Definition: population of microorganisms that reside in the skin, mucous membranes and intestinal tract of healthy human body Functions: 1.Helps development of mucosal immunity. Respiratory tract: Nose – Staph. aureus Skin: Staph. epidermidis 2.Protects host from colonisation with pathogenic Throat – Strep. viridans Mouth – Strep. mutans Propionibacterium acnes microbes. 3.Aids in digestion of food. Intestinal tract: Human microbiome: collection of genes of all the Bacteroides spp. Escherichia coli microbes in normal flora. Genitourinary tract: Vagina -Lactobacilli Urethra: S. epidermidis Normal Flora Knowledge of the normal flora of the human body allows the prediction of the likely causes of disease: Prediction of the pathogens causing infection as bacteria tend to grow in specific body sites. Respiratory tract: Nose – Staph. aureus Skin: Staph. epidermidis e.g. Pneumococci frequently colonize the Throat – Strep. viridans Mouth – Strep. mutans Propionibacterium acnes upper respiratory tract, and are believed to act as a reservoir for infection of the lower respiratory tract. Intestinal tract: Investigation for underlying abnormalities in Bacteroides spp. Escherichia coli specific areas of the body when bacteria are isolated from normally sterile sites. e.g. alpha-hemolytic streptococci in the Genitourinary tract: throat are considered part of the normal flora Vagina -Lactobacilli Urethra: S. epidermidis whereas the same organism in the blood are likely to be the cause of bacterial endocarditis.  Hand hygiene in health care: skin Flora Resident Flora Transient Flora If disturbed, re-establish themselves Do not usually re-establish themselves Usually not removed by routine hand wash Easily removed by routine hand wash Usually not associated with transmission of Usually associated with transmission of infection infection Staph. epidermidis is the most common agent Staph. aureus is the most common agent The aim of hand washing is to remove transient and some resident flora to prevent transmission of pathogenic microorganisms to patients. Changes in flora due to hospitalisation Main Factors: Exogenous environmental infections (e.g. hospital flora found on linen, equipment, water supply) Invasive techniques employed increase risk of infection (e.g. urinary catheters, I.V. lines) Use of antibiotics  Harmful effects of flora: opportunistic infections Caused by non-pathogenic microorganisms that act as a pathogen in certain circumstances: Normal flora moves to other parts of the body causing infection: e.g. Escherichia coli can ascend urethra and cause urinary tract infections (UTIs) Urinary strip tests used for detection of bacterial products in urine If host defence mechanisms are weakened (immunocompromised patients): e.g. neutropenia/mucosal injury from chemotherapy predisposes to fungal or bacterial infections such us Candida spp., Aspergillus spp. or Peseudomonas aeruginosa Lack of competition from normal flora due to its loss from body: e.g. vaginal yeast infections due to use of Candidiasis antibiotics.  Harmful effects of flora: opportunistic infections Caused by non-pathogenic microorganisms that act as a pathogen in certain circumstances: Normal flora moves to other parts of the body causing infection: e.g. Escherichia coli can ascend urethra and cause urinary tract infections (UTIs) Urinary strip tests used for detection of bacterial products in urine If host defence mechanisms are weakened (immunocompromised patients): e.g. neutropenia/mucosal injury from chemotherapy predisposes to fungal or bacterial infections such us Candida spp., Aspergillus spp. or Peseudomonas aeruginosa Lack of competition from normal flora due to its loss from body: e.g. vaginal yeast infections due to use of Candidiasis antibiotics. Mechanisms of pathogenesis Pathogenesis: Basic definitions and concepts  Infection is the presence of microorganisms in the body  Colonisation describes when a new microorganism grows on superficial body sites (skin, mucous membranes and GI tract) without invading the body.  A carrier is a person who harbours a microorganism and can be a source of infection for others.  A pathogen is a microorganism capable of causing disease.  Microbial pathogenesis is the process by which infection leads to disease Course of infectious diseases Organism Exposure Infection Clinical syndrome Host Course of infectious diseases Organism Exposure Infection Clinical syndrome Host Immunological Response Immunopathogenesis is a primary cause of cell death in many infections is due to killing of infected cells by the host immune system: Cytotoxic T cell mediated attack (e.g. Hepatitis B virus and liver damage) Antibody-mediated damage to the host (e.g. Streptococcus pyogenes and rheumatic fever) Antibody-mediated complement fixation (e.g. Hepatitis C virus exploits the complement system to establish persistence) Course of infectious diseases Organism Exposure Infection Clinical syndrome Host Immunological Response Immunopathogenesis is a primary cause of cell death in many infections is due to killing of infected cells by the host immune system: Cytotoxic T cell mediated attack (e.g. Hepatitis B virus and liver damage) Antibody-mediated damage to the host (e.g. Streptococcus pyogenes and rheumatic fever) Antibody-mediated complement fixation (e.g. Hepatitis C virus exploits the complement system to establish persistence) Clinical manifestations of disease: Symptoms: Subjective features of disease experienced only by the patient. Infections can be asymptomatic (sub-clinical) or symptomatic Sign: Objective manifestations of disease that can be observed and measured by others Syndrome: Group of symptoms and signs characteristic of a disease Host response to infection SYSTEMIC LOCAL EFFECTS EFFECTS Inflammation Fever (swelling) Altered immune Tissue necrosis response Nasal congestion Muscle pain (myalgia) Obligatory steps for infectious organisms: Phenomenon Step How Entry and spread Attach and enter with local or Evade host’s primary defenses (e.g. skin general spread in the body barrier; stomach acid or mucociliary epithelium in airways) Evasion Evasion of host’s immune system Evade immune defenses long enough for full cycle in host to be completed (e.g. extracellular bacterial capsules prevent phagocytosis) Multiplication Increase numbers Rapid growth/replication Transmission Exit from body into next portal Leave body at a site and on a scale that of entry ensures spread to fresh host Stages of infectious disease progression 1 3 2 4 Number of microorganisms or Intensity of signs & symptoms Time 1.- Incubation: Time between exposure and onset of a specific clinical sign. 2.- Prodrome: Period during which non specific “constitutional” symptoms occur (e.g. fever, malaise, loss of appetite). Not all infectious diseases have a prodromal stage. 3.- Illness: Period during which the clinical features of the infection are manifests 4.- Recovery (convalescence): Period during which the illness abates and patient returns to the healthy state Stages of infectious disease progression 1 3 2 4 Number of microorganisms or Intensity of signs & symptoms Time 1.- Incubation: Time between exposure and onset of a specific clinical sign. 2.- Prodrome: Period during which non specific “constitutional” symptoms occur (e.g. fever, malaise, loss of appetite). Not all infectious diseases have a prodromal stage. 3.- Illness: Period during which the clinical features of the infection are manifests 4.- Recovery (convalescence): Period during which the illness abates and patient returns to the healthy state Stages of infectious disease progression 1 3 2 4 Number of microorganisms or Intensity of signs & symptoms Time 1.- Incubation: Time between exposure and onset of a specific clinical sign. 2.- Prodrome: Period during which non specific “constitutional” symptoms occur (e.g. fever, malaise, loss of appetite). Not all infectious diseases have a prodromal stage. 3.- Illness: Period during which the clinical features of the infection are manifests 4.- Recovery (convalescence): Period during which the illness abates and patient returns to the healthy state Types of infections: based on transmission Communicable: The infection can spread from host to host either directly (e.g. HIV) or indirectly (e.g. noroviruses). The term contagious is applied to a highly communicable disease spread by contact Non-communicable: The infection is non transmissible between humans (e.g. botulism) Types of infections: based on occurrence Sporadic: the disease occurs only occasionally (e.g. tetanus) Endemic: the disease is continously present in a population, community or country (e.g. TB and chickenpox) Map showing TB endemic distribution Epidemic: the disease has greater number of cases than normal in an area within a short period of time i.e. outbreaks occur (e.g. SARS in 2003) Pandemic: epidemic disease that has worldwide distribution (e.g. flu pandemic in Ward with flu patients in 1918 1918 and 2009) Types of infections: based on site of infection Superficial: self limiting, microorganisms replicate in epithelium at the site of entry and local damage is caused (e.g. impetigo or folliculitis caused by Staph. aureus and common cold caused by rhinovirus). Immune response to superficial infections can give rise to constitutional symptoms (e.g. cytokine storm and influenza) impetigo Systemic: Replication of microorganism at multiple sites due to infection of deeper tissues. Spread throughout the body primarily via lymph and blood (e.g. chickenpox, measles, Salmonella typhi) chickenpox Types of infections: based on outcome Outcome – balance between microbial replication and spread and the host’s ability to respond/resist Disease Acute Infection: Rapid onset of disease with relatively brief period of symptoms. The Number of microorganisms pathogen is cleared within days (e.g. influenza, poliovirus, measles) Recovery Persistent infection Clearance Time Types of infections: based on outcome Outcome – balance between microbial replication and spread and the host’s ability to respond/resist Disease Number of microorganisms Acute Infection Reactivation Disease Persistent infection: the pathogen is not Recovery cleared from the host following primary infection, but remains in tissues of infected Persistence individuals. Can be: o Latent: the microorganism persists after Time initial clearance, and may have Disease asymptomatic or symptomatic reactivation (e.g. latent tuberculosis, herpes simplex Number of microorganisms virus and varicella zoster) o Chronic: with continued production of the Recovery infectious organism and immune evasion (e.g. chronic diabetic foot infection, Persistence hepatitis B, HIV). Infected hosts are carriers. Time Nosocomial infections Definition - Any infection acquired in a hospital or medical facility. Can affect patients and health care workers and are common because: (i) are easily moved around by staff, patients or visitors and (ii) not always can be prevented by proper hand washing. The most common pathogens in hospitals all over the world are: Staphylococcus aureus Clostridium difficile (C-Diff) Pseudomonas aeruginosa Enterococcus spp Respiratory syncytial virus (RSV) Norovirus Influenza virus Chickenpox Measles Nosocomial infections Many of the bacteria that cause healthcare associated infections are resistant to antibiotics: Methicillin-resistant S. aureus (MRSA) Vancomycin resistant Enterococcus faecalis (VRE) Carbapenemase producing Enterobacteriaceae (CPE) Some of the main factors that contribute to the appearance of antibiotic resistance are: Mutation and transfer of resistance genes through plasmids Transmission of resistant bacteria via the hands of healthcare staff Incorrect and excessive use of antibiotics Transmission of resistant bacteria from surfaces within the healthcare facility Determinants of pathogenesis The epidemiological triad Pathogen DISEASE Host Environment The epidemiological triad Pathogen DISEASE Host Environment Determinants of Pathogenesis: Host Age (e.g. Infants and elderly are more susceptible to influenza infection) Underlying characteristics that increase susceptibility (e.g. asthma, obesity, diabetes, malnutrition) Genetic variation (e.g. sickle cell trait provides protection to malaria; HIV long-term non- progressors) Immunodeficiency (e.g. opportunistic infections in AIDS patients) Immune response (e.g. “cytokine storm” in influenza infection) Trauma/surgery/foreign body (e.g. surgical implants make it easier for bacteria to cause infection and more difficult to eradicate it ) Determinants of Pathogenesis: Host Age (e.g. Infants and elderly are more susceptible to influenza infection) Underlying characteristics that increase susceptibility (e.g. asthma, obesity, diabetes, malnutrition) Genetic variation (e.g. sickle cell trait provides protection to malaria; HIV long-term non- progressors) Immunodeficiency (e.g. opportunistic infections in AIDS patients) Immune response (e.g. “cytokine storm” in influenza infection) Trauma/surgery/foreign body (e.g. surgical implants make it easier for bacteria to cause infection and more difficult to eradicate it ) Determinants of Pathogenesis: Host Age (e.g. Infants and elderly are more susceptible to influenza infection) Underlying characteristics that increase susceptibility (e.g. asthma, obesity, diabetes, malnutrition) Genetic variation (e.g. sickle cell trait provides protection to malaria; HIV long-term non- progressors) Immunodeficiency (e.g. opportunistic infections in AIDS patients) Immune response (e.g. “cytokine storm” in influenza infection) Trauma/surgery/foreign body (e.g. surgical implants make it easier for bacteria to cause infection and more difficult to eradicate it ) Determinants of Pathogenesis: Host Age (e.g. Infants and elderly are more susceptible to influenza infection) Underlying characteristics that increase susceptibility (e.g. asthma, obesity, diabetes, malnutrition) Genetic variation (e.g. sickle cell trait provides protection to malaria; HIV long-term non- progressors) Immunodeficiency (e.g. opportunistic infections in AIDS patients) Immune response (e.g. “cytokine storm” in influenza infection) Trauma/surgery/foreign body (e.g. surgical implants make it easier for bacteria to cause infection and more difficult to eradicate it ) Determinants of Pathogenesis: Host Age (e.g. Infants and elderly are more susceptible to influenza infection) Underlying characteristics that increase susceptibility (e.g. asthma, obesity, diabetes, malnutrition) Genetic variation (e.g. sickle cell trait provides protection to malaria; HIV long-term non- progressors) Immunodeficiency (e.g. opportunistic infections in AIDS patients) Immune response (e.g. “cytokine storm” in influenza infection) Trauma/surgery/foreign body (e.g. surgical implants make it easier for bacteria to cause infection and more difficult to eradicate it ) Iatrogenic infections Factors that can disrupt the body’s non-specific mechanical barriers to infection make it easier for microorganisms to cause infection: Injury associated with therapy (e.g. damage sustained through procedures such as endoscopy, surgery or radiotherapy) allows microorganisms to enter normally sterile parts of the body Plastic and metal “foreign bodies” such as prostheses (hip joints; heart valves ) or indwelling IV lines provide surfaces that microbes can colonize more easily than the natural equivalents e.g. coagulase-negative staphylococci like Staph. epidermidis and aureus are members of the normal flora that can produce an adhesive slime material and grow as biofilms on plastic surfaces. Other examples include alpha haemolytic streptococci and gram negative bacilli. Treatment is difficult and device must be replaced to avoid chronic infection or sepsis. Iatrogenic infections Factors that can disrupt the body’s non-specific mechanical barriers to infection make it easier for microorganisms to cause infection: Injury associated with therapy (e.g. damage sustained through procedures such as endoscopy, surgery or radiotherapy) allows microorganisms to enter normally sterile parts of the body Plastic and metal “foreign bodies” such as prostheses (hip joints; heart valves ) or indwelling IV lines provide surfaces that microbes can colonize more easily than the natural equivalents e.g. coagulase-negative staphylococci like Staph. epidermidis and aureus are members of the normal flora that can produce an adhesive slime material and grow as biofilms on plastic surfaces. Other examples include alpha haemolytic streptococci and gram negative bacilli. Treatment is difficult and device must be replaced to avoid chronic infection or sepsis. Determinants of Pathogenesis: Organism Portal of Adhesion/ Portal of Invasion Multiplication Infection Disease entry Attachment exit Opportunism Attachment Invasiveness Virulence Effects of infection on cells (Cellular Pathogenesis) Determinants of Pathogenesis: Organism Portal of Adhesion/ Portal of Invasion Multiplication Infection Disease entry Attachment exit Opportunism Attachment Invasiveness Virulence Effects of infection on cells (Cellular Pathogenesis) Determinants of Pathogenesis: Organism Portal of Adhesion/ Portal of Invasion Multiplication Infection Disease entry Attachment exit Opportunism Attachment Invasiveness Virulence Effects of infection on cells (Cellular Pathogenesis) Determinants of Pathogenesis: Attachment Bacteria have specific molecules on their surface (adhesins) that bind to specific host cell surface components (receptors). LNα2 Adhesins are often proteins, although they can also be polysaccharides or glycolipids and include glycocalyx, capsule and pili. Integrin Upon receptor binding, adhesins mediate a Cell’s series of signaling events that affect bacterial membrane uptake and can promote inflammatory events by affecting innate immune receptors. Singh et al; FEBS microbiol. Rev. (2012) Determinants of Pathogenesis: Attachment Bacteria have specific molecules on their surface (adhesins) that bind to specific host cell surface components (receptors). LNα2 Adhesins are often proteins, although they can also be polysaccharides or glycolipids and include glycocalyx, capsule and pili. Integrin Upon receptor binding, adhesins mediate a Cell’s series of signaling events that affect bacterial membrane uptake and can promote inflammatory events by affecting innate immune receptors. Singh et al; FEBS microbiol. Rev. (2012) Determinants of Pathogenesis: Attachment Viral surface proteins bind to receptors on host cell membrane. Host cell receptors serve some function for the host cell, and the pathogen takes advantage of them (e.g. HIV binds to CD4). Also, the receptor can be different for different cell types. The interaction of virus with receptor induces conformational changes that lead to membrane fusion and penetration. Attachment factors (e.g. heparan sulfate proteoglycans) help to concentrate the viral particles at the cell surface. (V) Influenza virus (C) Cilia Unlike binding to receptors, the interactions with (M) Microvilli attachment factors are often not highly specific. Influenza virus attachment to ciliated epithelium Bacterial adhesins Pathogen Adhesin Host Cell Receptor E. coli, FimH mannosylated K. pneumoniae, glycoproteins Salmonella spp. E. coli PapG Gala(1-4)Gal E. coli Curli (CsgA) Contact phase system M. leprae PGL-1 Laminin (LNα2) S. pneumoniae PsrP sialylated carbohydrates Haemophilus influenza HifE Sialylyganglioside-GM1 Klebsiella pneumoniae MrkD Type V Collagen Virus and their receptors Pathogen Viral component Host Cell Receptor Influenza virus Hemagglutinin Sialic acid Ebola virus Glycoprotein (GP) TIM-1 and NPC1 Rhinovirus Capsid protein ICAM-1 Hepatitis C virus Envelope glycoprotein E2 SR-B1 and CD81 co-receptors: CLDN-1 and OCLN Epstein–Barr virus Envelope glycoprotein CD21 and MHC-II (gp350/220) HIV Envelope Gp120 protein CD4 and co-receptors CXCR4/CCR5 on T-Cells Determinants of Pathogenesis: Tropism Host range refers to the different species of hosts a given pathogen can infect. The host specificity of viruses is predominantly defined by the interactions of viral proteins with their cognate cellular receptors. The molecular mechanisms of host specificity are less understood for bacterial pathogens. Tissue tropism refers to the different tissues within a given host that are infected by the pathogen (e.g. rhinoviruses infect only the respiratory tract). Attachment is one of several factors that determine tissue tropism as the need for a specific cell receptor narrows the species and the type of cells the pathogen can enter. A permissive cell is one that allows a pathogen to replicate. It has the required receptor, allow the pathogen to replicate and lacks defenses against the pathogen (e.g. interferon production) or the pathogen has evolved mechanisms to overcome such defenses (e.g. Vpu and Vif genes in HIV) Determinants of Pathogenesis: Tropism Host range refers to the different species of hosts a given pathogen can infect. The host specificity of viruses is predominantly defined by the interactions of viral proteins with their cognate cellular receptors. The molecular mechanisms of host specificity are less understood for bacterial pathogens. Tissue tropism refers to the different tissues within a given host that are infected by the pathogen (e.g. rhinoviruses infect only the respiratory tract). Attachment is one of several factors that determine tissue tropism as the need for a specific cell receptor narrows the species and the type of cells the pathogen can enter. A permissive cell is one that allows a pathogen to replicate. It has the required receptor, allow the pathogen to replicate and lacks defenses against the pathogen (e.g. interferon production) or the pathogen has evolved mechanisms to overcome such defenses (e.g. Vpu and Vif genes in HIV) Determinants of Pathogenesis: Tropism Host range refers to the different species of hosts a given pathogen can infect. The host specificity of viruses is predominantly defined by the interactions of viral proteins with their cognate cellular receptors. The molecular mechanisms of host specificity are less understood for bacterial pathogens. Tissue tropism refers to the different tissues within a given host that are infected by the pathogen (e.g. rhinoviruses infect only the respiratory tract). Attachment is one of several factors that determine tissue tropism as the need for a specific cell receptor narrows the species and the type of cells the pathogen can enter. A permissive cell is one that allows a pathogen to replicate. It has the required receptor, allow the pathogen to replicate and lacks defenses against the pathogen (e.g. interferon production) or the pathogen has evolved mechanisms to overcome such defenses (e.g. Vpu and Vif genes in HIV) Determinants of Pathogenesis: Invasiveness Invasiveness is the capacity of a microbe to enter and damage a tissue. Bacteria spread to deeper tissues by producing specific enzymes (invasins): Collagenase and hyaluronidase: Degrade collagen and hyaluronic acid disrupting the epithelial basal lamina and allowing bacteria to spread through subcutaneous tissues (e.g. S. pyogenes) Coagulase: Triggers the formation of a fibrin clot around the bacteria to protect them from phagocytosis (e.g. S. aureus) Leukocidins: Degrade white blood cells (e.g. S. aureus) Hemolysins: Degrade red blood cells (e.g. Streptococci, staphylococci and clostridia ) Determinants of Pathogenesis: Virulence Pathogenicity is the ability of a microbe to cause disease. Virulence is the degree of pathogenicity in a microorganism. The virulence of a pathogen is determined by its tropism, invasiveness and the production of factors that can increase toxicity or the permissive state of the cells it infects. Measures: Lethal dose 50 (LD50) – Number of pathogens that will kill 50% of an experimental group of hosts. Infectious dose 50 (ID50) – Number of pathogens that will infect 50% of an experimental group of hosts. Bacterial virulence factors Adhesins and invasins. Endotoxins: Lipopolysaccharides (LPS) present in the cell wall of Gram-negative bacteria. weakness of eye muscles due to botulinum toxin produced by Clostridium botulinum Exotoxins: Secreted proteins produced by Gram-positive and Gram-negative bacteria (e.g. diphteria, tetanus, cholera and botulinum toxins). Exotoxins have different mechanisms of action and different targets within the cell, hence cause a variety of diseases Muscle spasms due to tetanus toxin produced by Clostridium tetani Bacterial virulence factors: toxins Property Exotoxin Endotoxin Source Gram-positive and negative Gram negative bacteria Secreted from cell Yes No (cell wall component) Chemistry Polypeptide Lipopolysaccharide Location of genes Plasmid or phage Bacterial chromosome Toxicity High Low Clinical effects Various effects Fever, shock Antigenicity High Poor Heat stability Destroyed rapidly at 60ºC Stable at 100ºC for 1 hour Viral virulence factors Viral virulence genes usually identified by mutation: attenuated virus cause reduced or no disease. Viral virulence genes can: 1. Alter the ability of the virus to replicate (e.g. Attenuated Sabin vaccine strains of poliovirus contain a mutation that reduces replication in neurons) 2. Modify the host defense mechanisms (e.g. viral proteins secreted by infected cell that mimic cytokines) 3. Enable the virus to spread in the host (e.g. HIV has 2 types of co-receptors) 4. Act as toxic proteins (e.g Nonstructural glycoprotein 4, NSP4, of rotaviruses is a viral enterotoxin). Virulence factors: Superantigens (SAgs) Superantigens (SAgs) are toxins that stimulate the immune system. In contrast with classical peptide antigen recognition, SAgs do not require processing to small peptides. They can bind to MHC-II molecules non-specifically and stimulate a large number of T cells. Some examples of SAgs include: bacterial exotoxin TSST- 1 (Toxic Shock Syndrome Toxin 1) produced by S. aureus and the severe acute respiratory syndrome (SARS) coronavirus E2-spike protein.