microorganisms in disease and infection Flashcards
define pathogenicity
the capacity of a micro-organism to cause an infection
what are the main characteristics which an organism must show to be pathogenic
transmissibility
establisment in or on a host
Harmful effects
Persistence
what are the stages in the chain of infection
pathogenic organism- in adequate numbers.
reservoir or source that allows the organism to survive and multiply.
mode of exit from the source
mode of transmission from the source to the host
Portal of entry through which the pathogen can enter the host
Suceptible host
define virulence
the degree to which a microorganism is able to cause disease.
what are the main routes of transmission for infection
faecal oral
blood borne
respiratory
direct contact- hand to hand to mucous membrane.
define LD50- Lethal dose
lethal dose- dose of organism which causes death in 50% of the exposed population.
define ID50- Infection dose
ID50-dose or organism which causes infection in 50% of the organism exposed.
how do pathogens colonise ( attachment to to what on surface of host cell)
Ligands in the organism binds to receptors on the surface of the host cell
Complementary ligand- receptor interaction.
define infectivity
The ability of a microorganism to become established in/on a host.
Examples of ligand receptor interactions
E. coli P fimbriae: glycolipids on human uroepithelial cells
S. pyogenes protein-F: fibronectin (Large multifunctional glycoprotein)
Influenza haemagglutinin: respiratory epithelial sialic acid receptors
what are the virulence mechanisms
adhesion, toxic affect, tissue damage, interference with host defence mechanisms, facilitation of invasion
modulation of host cytokine response.
what is a important toxin produced by the gram -ve bacterial cell wall and name infections where endotoxins are used
endotoxin
E.coli
Neisseria meningitidis.
In which layer of the gram -ve bacterial wall in endotoxin an active part of
active component of the liposaccaride membrane
when is an endotoxin released
released from damaged or dead cells.
what is a lipopolysaccharide made from
Lipid A, polysaccaride and a specific O antigen.
what is the host response to an endotoxin
uncontrollable T lymphocyte response- cytokine release, shock and cardiac and renal failure.
uncontrollable clotting cascade activation- DIC, depletion of clotting factors and a tendency to bleed.
uncontrollable complement activation.
what effects do the neisseria meningitidis endotoxin have on the host
mediated increase in vascular permeability causes loss of protein, fluid and plasma into the tissues, with pathological compensatory vasoconstriction.
peripheral necrosis.
Are exotoxin produced by living or dead bacteria
living
Are endotoxin produced by living or dead bacteria
dead
how does the Boutulism exotoxin work (where does it work)
affects the presynaptic membrane so that the neurotransmitter cannot be relased.
How does the preformed botulism toxin colonise the host.
Ingestion of pre-formed toxin Contaminated food Infection of dirty wounds May be trivial wounds Gastrointestinal colonisation (infants)
what is the clinical presentation of botulism
diplopia, dysphagia, dysarthria, dry mouth, death, rigid paralysis (respiratory failure)
How does clostridium tetani colonise the host
infects wounds
what is the toxin in tetanus called
tetanospasim.
How does the tetanospasim (tetanous toxin work)
binds to nerve synapses
inhibited release of inhibitory neurotransmitter in the central nervous system.
what are the main clinical presentations of tetanus
Jaw spasm
rigid paralysis
opisthotonos (body thrown into shapes which depends on muscle strength)
death caused by respiratory paralysis
what is the treatment for tetanus
metronidazole and tetanus antitoxin.
what effect do the strep pyogenes virulence factors have
promote connective tissue breakdown and invasion.
what infections can strep pyogenes cause
streptococcal sore throat
Erysipelas- skin infection
Necrotizing fascilitis.
Scarlet fever- streptococcus (sore throat caused by erthyrogenin toxin.)
what is the function of Hyaluronidase and streptokinase virulence factor of S pyogenes
Break down connective tissue components – facilitate tissue invasion
what is the function of C5a peptidase virulence factor of S pyogenes
inactivates complement component C5a
what is the function of streptolysisn O and H virulence factors of S pyogenes
Lyses RBC, WBC and platelets.
what is the function of the Erythrogenic toxin (phage-encoded) virulence factor of S pyogenes
rash and scarlet fever.
what is the function of the toxic shock syndrome toxin virulence factor of S pyogenes
Streptococcal toxic shock syndrome is superficially similar to a syndrome of endotoxin release
what are the symptoms of infection of S pyogenes
mild- red and swelling around cheeks
severe- necrosis, necrotizing fascialis
how does S pyogenes inhibit phagocytosis
M Protein binds fibrinogen and masks bacterial surface, blocking complement binding and opsonisation.
how does S pneumonia inhibit phagocytosis
Polysaccharide capsule inhibits opsonisation and therefore phagocytosis
which organisms are intracellular. This means they invade cells of host to clear, so you must kill your own cells to get rid of it.
Mycobacterium tuberculosis
Salmonella typhi
Listeria monocytogenes
are viruses intracellular or extracellular organisms
intracellular,
what is the composition of a virus
Genome - RNA or DNA
Capsid - protein
(Envelope – lipid bilayer)
