Establishment of Infectious Disease: Pathogenicity Flashcards
What are commensals?
Non-pathogenic organisms that grow on the skin and in the mucous membrane
What is the normal flora/human microbiome?
The collective name for all the commensals in the body
What are pathogens?
Anything that tried to invade the human body by direct or indirect means
What does the outcome of the host-pathogen relationship depend on?
It depends on the pathogenicity of the pathogen and the resistance of the host
What will happen if a pathogen attacks a host with a strong resistance?
(2)
The host will remain healthy
The pathogen is either eliminated or assumes a benign relationship with the host (host = carrier)
Under what circumstances does a host become a carrier?
Is the pathogen assumes a benign relationship with the host
What will happen if a pathogen attacks a host with a weak resistance?
(2)
The host loses the competition
An infectious disease develops
What types of organisms make up the microbiome?
Mostly bacteria but some fungi
How do commensals enter a new born?
Through food or the environment (including other humans)
What four environments on the human body are the most densely populated with microorganisms?
Gastrointestinal tract
Oral cavity
Urogenital tract
Skin
Name a bacteria found in the GIT
Escherichia coli
Name the most common bacteria found in the oral cavity.
Streptococcus mutans
Name the most common bacteria found in the urogenital tract.
Lactobacillus sp
Name the most common bacteria found on the skin.
S. epidermis
Give three symbiotic benefits of the human microbiome.
Competitive inhibition - inhibits other pathogens
Immune system stimulus
Aids digestion
Important nutrients (Vit K)
What are the four harmful effects of the human microbiome?
Dysbiosis (disturbance) of gut microbiome may cause diseases such as coeliac disease
Displaced microbiome can cause blood stream infections - skin flora in bloodstream
Diminished microbiome - e.g. after antibiotics - pathogens such as Candida albicans can become established
Overgrowth - establishes infection when host becomes compromised
What do bacteria have that increase their pathogenicity?
3
Cell wall proteins
Extracellular enzymes
Toxins
What is virulence?
The degree of pathogenicity of an organism
What are the five stages of pathogenesis?
Exposure/entry
Adherence and local invasion
Colonisation
Invasion and growth
Evasion of the host immune defenses
In what four places can pathogens enter the body?
Skin that has been penetrated
Respiratory tract (inhalation)
Gastrointestinal tract (ingestion)
Urogenital tract (sexual contact)
What is the infective dose?
The number of pathogens required to successfully infect a host
Give an example of a bacteria that has a very low infective dose.
Shigella (10-100)
Give an example of a bacteria with a very high infective dose.
Staphylococcus aureus (10^3 - 10^8)
How does a bacteria adhere to a cell?
They deploy specific adherence mechanisms such as fimbriae/pili or adhesins
Give an example of a bacteria that have fimbriae.
Oral streptococci
What are adhesins?
Surface proteins that bind to specific receptors on the surface of host cells
Give an example of a bacteria that has adhesins.
N. gonorrhoeae
Other than pili or adhesins, how can bacteria stay close to target cells.
Capsules - adhere to target cell
Biofilm
Give an example of a bacteria that attaches using a capsule.
E.coli
What is colonisation?
The process by which pathogens may multiply in an area
What can be produced by some bacteria to help them obtain iron?
Siderophores
Give an example of a bacteria that stays localised after colonisation.
Staphylococcus aureus - boil
What are invasive bacteria?
Bacteria that can spread from the initial site of infection
What facilitates bacterial infection?
Degradative enzymes
Toxins
How do degradative enzymes work?
They degrade components of the extracellular matrix providing bacteria with easier access to the host cell surfaces
Give some examples of degradative enzymes.
4
Hyaluronidase
Collagenase
Coagulase
Streptokinase
Give an example of a bacteria that produces hyaluronidase.
Staphylococcus aureus
How does hyaluronidase work?
2
It breaks down the hyaluronic acid matrix between the cells
This allows bacteria to penetrate into deep layers of the skin
Give an example of a bacteria that produces collagenase.
Clostridium perfringens
How does collagenase work?
It breaks down tissue collagen network enabling bacterial spread through the body
Give an example of a bacteria that produces coagulase.
Staphylococcus aureus
What does coagulase do?
2
It causes the formation of fibrin clots in the site of infection
This provides protection for the organisms
What species of bacteria produce streptokinase?
Streptococci
What does streptokinase do?
It dissolves fibrin clots used by the body to restrict an infected area
What are the two main types of toxins that can be produced by bacteria?
Exotoxins
Endotoxins
What type of bacteria secrete exotoxins?
Primarily gram-positive bacteria
What type of bacteria secrete endotoxins?
Gram negative bacteria (component of their outer membrane)
How are exotoxins released?
They are released from pathogens as they grow
How many types of exotoxins are there?
Three
What are the three types of exotoxins?
Cytolytic
AB toxins
Superantigen toxins
What are cytolytic toxins?
They degrade integrity of cytoplasmic membranes causing cell lysis
Give an example of a cytolytic toxin.
Alpha toxin (lecithinase)
Name a bacteria that uses alpha toxin (lecithinase).
Clostridium perfringens
What are AB toxins?
3
Toxins that consists of two sub-units, A and B
B is responsible for the binding of the toxin to the host cell
A is responsible for the toxic affect
Give an example of an AB toxin.
Tetanus toxin
What bacteria produces tetanus toxin?
Clostridium tetani
What is tetanus toxin?
A neurotoxin that blocks the release of neuro-transmitters involved in muscle control - results in twitching paralysis and affected muscles are constantly contracted
What are superantigen toxins?
2
Toxins that over-stimulate the immune system
Cause extensive inflammation and tissue damage
Give an example of a bacteria that produced a superantigen toxin.
Streptococcus pyogenes
What does streptococcus pyogenes do?
4
Implicated in streptococcal Toxic-Shock Syndrome
Bloodstream infection
Necrotising
Organ failure + amputation
What part of the lipo-polysaccharide (LPS) is the endotoxin?
Lipid A component
How are endotoxins released?
They are released in large amounts into the host following bacterial cell lysis
What does Lipid A do when released?
6
Fever
Shock
Hypotension
Thrombosis (formation of blood clots)
Septic shock
Death from organ failure
What mechanisms do bacteria use to evade the innate immune system?
(2)
Capsules
Intracellular pathogens
What mechanism do bacteria use to avoid the adaptive immune system?
Antigenic switching
How do capsules help bacterial cells avoid the innate immune system?
They prevent the cells from being phagocytosed
Give an example of a bacteria that forms a capsule.
3
Streptococcus pneumoniae
Nesseria meningitidis
Encapsulated strains of Haemophilus influenzae type b
What are intracellular pathogens?
Bacteria that can resist killing and survive or multiply inside of phagocytes or other cells
Give an example of an intracellular pathogen.
Mycobacterium tuberculosis
What is antigenic switching?
This is where pathogens can periodically change their surface antigens to prevent host antibody-mediated activity
This can be used to change their surface proteins
This will prevent antibodies already made from fitting anymore
Give an example of a bacteria that can undergo antigenic switching.
Salmonella
What does the outcome of a host microbe interaction depend on?
The virulence of the particular organism
The immune status of the host being colonized
What does it mean to be a compromised host?
One or more defence mechanisms are inactive
