biological agents of disease Flashcards
pathogen =
an organism that causes disease
e.g. viruses, bacteria, protozoan, fungi, worms
not all microbes are ______ as lots are ______ to health
pathogens, beneficial
this type of primary pathogen can only survive in host and have specific host species.
obligate pathogen
this type of secondary pathogen is present in environmental reservoirs waiting for host
facultative pathogen
this type of secondary pathogen is normally benign but can cause disease in compromised host
opportunistic pathogen
2 closely related species can be pathogenic and harmless. they only differ by what type of genes?
virulence genes
where are virulence genes found?
clustered together on the bacterial chromosome in large clusters called pathogenicity islands
they are then incorporated into bacterial DNA
proteins that virulence genes encode are called?
virulence factors
virulence genes can be carried on _______ by ________ gene transfer
bacteriophages
horizontal
describe virulence genes in vibrio cholera
100s of different strains but only those infected by bacteriophages
→ bacteriophage transfer genes that encode the cholera toxin into bacterium
→ combination of the virus and bacteria that individually are harmless
what does the cholera toxin do?
causes diarrhoea → dehydrates intestinal cells
→ spreads infected bacteria to new host
have a complex life cycle and often show dimorphism
fungal pathogens
histoplasmosis capsulatum
grows as mold in low temps in soil → switches to yeast when inhaled in warm lungs → engulfed by alveolar macrophages in yeast form
why are anti fungal treatments less effective than antibiotics?
eukaryotic nature of fungal cells and dimorphism
what type of pathogens often have more than 1 host?
protozoan pathogens
describe how the malaria plasmodium falciparum parasite works
→ virus grows inside mosquito vector → sporozoites injected into human host when blood is sucked → replication in liver → infects RBCs → production of gametocytes in blood → gametocytes sucked up by new mosquito vector when fertilisation occurs → spreads the plasmodium falciparum
what is a vector?
an insect that transmits disease
what diseases caused by viruses need to replicate in insect cells to use them as vectors?
dengue and yellow fever
how do successful pathogens work?
enter the host → find niche that is nutritionally compatible → deal with the hosts immune system → replicate → move from one host to another
successful pathogens need to:
invade protective barriers and breach the cell membrane
what are the protective barriers?
epithelia, flora, mucous
how do epithelial cells work to protect against pathogens?
held together by tight junctions that prevent pathogens from squeezing between cells
where is mucous secreted?
epithelial areas without flora secrete mucous e.g. lower lung, small intestines, bladder
what are the different ways that pathogens overcome protective barriers?
open wounds in epithelial barriers give access to opportunistic pathogens (but breaks in epithelia are quickly recognised by WBCs)
some bacteria have P pili and special adhesin proteins → anchor them to epithelia
(adhesions have receptors on host cell that have other good functions so treatment can’t target these protein receptors)
breach the cell membrane → necessary to inject toxins or replicate inside the cell
toxins kill host cells
kill WBCs to evade the immune system
many bacteria have type 3 secretion systems → act like syringes to inject toxins or effector proteins
