medical microbiology week 5 Flashcards
what are some microbial causes of infection
- bacteria (prokaryotic)
- viruses (unclassified)
- fungi (eukaryotic)
- parasites (usually eukaryotic)
- prions (unclassified)
what is a prion
protein of unknown function that resides on the surface of brain cells
what’s bigger a prion or a virus
virus
what’s bigger a bacteria or a virus
bacteria
what’s bigger a fungi or a bacteria
fungi
in microscopy when would you not use a stain
- to see white blood cells (e.g. urine)
- to see parasites (e.g. faeces)
in microscopy when would you use a stain
to visualise bacteria and yeasts/fungi
can you see viruses in light microscope
no
how do you detect viruses in clinical samples
molecular methods - real time/PCR - antigen detection - serology to determine immunity virtually obsolete methods - electron microscopy - cell or tissue culture
how is parasite diagnosed
- microscopy of different life cycle stages
- culture rarely possible
- serology sometimes useful
- reference laboratories
what characteristics do bacterial cells have
- capsule
- cell wall
- single chromosome
- no nucleus
- flagellum
- fimbriae
- ribosomes
- plasma membrane
what is the outer membrane on a bacterial cell
component of the gram negative cell wall
is a gram positive cell wall thin or thick
thick
is a gram negative cell wall thin or thick
thin
what does a gram positive cell wall contain
- plasma membrane with proteins
- then periplasmic space
- then peptidoglycan
what does a gram negative cell wall contain
- plasma membrane with proteins
- then periplasmic space
- then peptidoglycan
- then outer membrane (which contains lipopolysaccharides and proteins)
what is gram stain colour of gram positive in light microscopy
purple
- think Purple for Positive
what is gram stain colour of gram negative in light microscopy
red
what shape is cocci
spherical
- cOcci like sphere
what shape is bacilli
rod shaped
- think bacilli = baton
where are lipopolysaccharides present
in outer membrane of gram negative bacteria
what is fimbriae on bacteria there for
adherence
what do lipopolysaccharides do
- protects peptidoglycan from bile salts
- blocks many antibiotics from getting into cell
- lipid A within LPS forms endotoxin which when in bloodstream may give rise to endotoxic shock (fever and low BP)
how do bacteria replicate
- binary fission
- asexual
- genetic material duplicates then cell divides into two
how does genetic variation in bacterial cell occur
- spontaneous mutation
- transfer of plasmids between bacteria
- conjugation is mechanism of gene transfer
how do you detect individual bacteria
- can only be seen with microscope using stain (e.g. gram, fluorescent dyes)
how do you detect colonies of bacteria
- cultured on solid medium (agar)
- can be seen with eye
what is a parasite
an organism that lives in or on another organism (host) and derives its nutrients at the expense of this host
what is a vector
- intermediate host
- transmitis parasite
what are protozoa
basically single celled organisms that can live both within the environment and the host
e.g. malaria, amoebae, flagellates
how are protozoa transmitted
protozoa that live in human’s intestine - faecal-oral route (amoebae)
protozoa that live in the blood or tissue of humans - insect vector
what is the malaria parasite called
plasmodium
what are helminth parasites
worms
what are nematodes
round worms (helminth)
what are cestodes
tapeworms
helminth
what are trematodes
flatworms/fluke
helminth
what is enterobiasis
thread worm (nematode - helminth)
what is amoebic dysentry
protozoa parasite
what is route of transmission for nematodes
faecal-oral route
what is ascaris lumbricoides
nematode (helminth)
what is the definitive host for tapeworms
humans
what is the route of transmission for tapeworms
eating raw/undercooked meat
where in the body can flatworms cause infection
blood, lung, liver, pancreas, intestines
what is schistosomiasis
trematode (helminth)
what is the route of transmission for schistosomiasis
contact with fresh water where the snail (intermediate host) is present
how to identify parasitic infection
- blood microscopy e.g. malaria
- stool microscopy for enteric pathogens
how to identify helminth infection
usually accompanied by eosinophilia and elevated IgE
what does a fungus form
spores
does a fungus have a cells wall
yes
- contains polysaccharide and it absorbs nutrients
what are the three groups of fungi
basidiomycetes (e.g. mushrooms)
ascomycetes (e.g. neurospora)
zygomycetes (e.g. bread moulds)
what is a dermatophyte’s main virulence factor
enzymes degrade and use keratin as a nutrient source
what diseases are caused by dermatophytes
‘ringworm’ infections
- name usually starts with tinea…
what are the main causative agents of dermatophytosis (fungal)
- epidermophyton
- microsporum
- trichophyton
what host factors contribute to pathogenicity of fungal infection
- favourable micro-environments (warm, moist) encourage growth on skin and mucous membranes
- broad-spectrum antibacterial agents reduce competition for colonisation sites in gut
- immunosuppression may create a window of opportunity for fungal invasion
what can cause immunosuppression of host defences
iatrogenic - steroids, chemotherapy, organ transplant
disease processes - AIDS, leukaemia, endocrinopathies
what is yeast
fungus
what is candida spp.
yeasts
what does candida auris (fungus) cause
UTI
urinary tract infections
what dos candida glabrata cause
skin infections
what are the diagnostic methods for fungus
direct detection - histopathology, high-res CT scan
detection of circulating fungal antigens
detection of circulating anitbodies to fungi
PCR for fungal DNA
culture of fungus from normally sterile site
MALDI-ToF biotyping
what are problems with anti-fungal drugs
- some have narrow spectrum of activity
- might be static (prevents multiplying) rather than cidal (kills)
- IV vs. oral
- toxicity
- resistance
- expensive
what are some antifungals
echinocandins - target wall
triazoles and allylamines - target sterols
polyenes - target membrane
flucytosine - targets DNA synthesis
what are the3
different types of structures of viruses
icosahedral - 20 faces, each an equilateral triangle
helical - protein binds around DNA/RNA in helical fashion
complex - neither icosahedral or helical
where can viruses replicate
can only replicate within host
what is a virion
- extracellular form of virus
- exists outside host and facilitates transmission
what is the structure of a virion
- lipid envelope
- protein capsid
- virion associated polymerase
- spike projections
- nucleic acid
how do viruses replicate
- attachment to cell
- uncoating of lipid envelope
- replication of nucleic acid (in nucleus mRNA synthesis)
- protein synthesis
- vision assembly
- budding and release
- maturation
what are the 3 possible outcomes of viral infection
- clearance of virus (with short, long term or no immunity) e.g. measles
- chronic infection e.g. HIV, hep B
latent infection e.g. herpes - transformation (long term infection with altered cellular gene expression) e.g. HPV
what is viral latency
- after infection virus lays dormant
- full genome retained in host cell but no expression, so few viral antigens and no viral particles produced
- reactivation can occur
viruses and cancer examples
EBV: Hodgkin’s lymphoma
human herpes 8: primary effusion lymphoma
human T cell lymphotropic virus (HTLV) leukaemia
HPV: cervical, anal cancers
hep B/C: hepatocellular carcinoma
what are the viral detection methods for the whole organism
- microscopy
- culture
what would be the viral detection methods for part of the organism
- antigen detection
- DNA/RNA detection (extraction of genetic material from sample)
- PCR
what would be the viral detection methods for immune response
- antibody detection (serology)
are antiviral therapies virustatic or virucidal
all are virustatic
what are antiviral therapies used for
prophylaxis - to prevent infection
pre-emptive therapy - when evidence of infection/replication detected but before symptoms are apparent
overt disease
suppressive therapy - to keep replication low
viruses with what properties can be eradicated
- no animal reservoir or ability to amplify in environment
- clearly identifiable
- no chronic carrier state
- efficient and practical intervention e.g. vaccine
- political/social support
e. g. measles, polio
what are some signs of clinical infection
inflammation, pain, pyrexia (fever), tachycardia (fast heart rate), rigors (temperature rises and shivers), increased white cell count, increased reactive protein (CRP)
what is a commensal
an organism which is part of normal flora
what is infectivity
ability to become established on or within a host
what are exotoxins
released extracellularly by the micro-organism
what are enterotoxins
exotoxins which act on the GI tract
what are endotoxins
structurally part of the gram negative cell wall
what are superantigens
certain exotoxins of strep progenies and staph aureus
- able to stimulate division of T cells in the absence of specific antigen
- overwhelming cytokine production causes ‘toxic shock’
