bacteriology lab Flashcards
what are common diagnostic techniques in the bacteriology lab *
culture - sterile sites (blood/CSF) or non sterile sites
serology
molecualr techniques
antimicrobial susceptibility testing
summarise culturing *
on agar plates
difficult
used to determine AB sensitivity
from non-sterile sites - loads of commencal bacteria - need to have an idea of what you afre looking for
summarise serology *
look for body response
eg syphilis - difficult to grow and dont get many circulating organisms
summarise molecular testing *
whole genome
wide range of genes for enzymes - so at minute cant tell you sensitivity
can be used for screening
used when name of organism iss present - eg we know c diff is resistant -and therefore if present already knwo the treatment
summarise antimicrobial suseptibility testing *
use impregnated disks or grading strips
correlate with suseptible or resistant
examples of things in hospital that make people more suseptible to infection
flushing action of urinary flow removes organisms
pathogenic bacteria can reside in the plastic
cannular - bypass the skin and the bacteria can enter
describe how blood cultures are stored *
in bottles
machine warms them and agitates them - so they get supply of the broth
bottles have beads that absorb Ab - do everything possible to help bacteria grow
how can you see if a blood culture is positive *
there is indicator on the bottom of bottles - yellow is +ve
changes becasue of the metabolites produced - takes 16-20 hrs
go onto agar plates - cam identify bacteria but not its suseptibility
what is the structure of gram +ve bacteria
thick peptidoglycan cell wall
structure of gram -ve bacteria *
LPS polysaccharide layer
thin peptidoglycan cell wall
what can be seen when you look at gram +ve bacteria - stained *
purple - dye gets stuck in the cell wall
can see the shape
eg staphylococci - circle and in bunches
describe the coagulase test *
rare
ability of bacteria to form a coagulase in horse/rabbit plasma
either coagulase +ve or -ve
describe staphylococci
staph aures - MRSA - coag +ve
cause skin/soft tissue infection, osteomyelitis, endocarditis
coag -ve - skin commensals, can infect prosthetic material causing line, pacemaker infections and endocarditis
how do you investigate haemolytic streptococci *
B haemolysis = pathogenic: gp A (step pyogenes - sorre throat, skin and soft tissue infection) gp B (strep agalactiae - neonatal sepsis) gp C and G (skin adn soft tissue infections)
a haemolysis - strep pneumonia
suseptible to penicillin or cephalexin
what is the appearance of gram -ve bacteria
light pink
what is the importance of determining gram staining *
-ve are usually more resistant
what is best guess diagnosis *
the things that are looked for by the lab depend on the present clinical symptoms and past history
lab will only look for things that relate to the history/clinical symptoms
what would be looked for if diarrhoea was the presenting symptom *
look for salmonella, shigella, campylobacter, e coli always
look for c difficile if previous AB use, and cholera if asked to
look for amoeba, giardia and cryptosporidum (when abroad)
what investigations are done on stool samples for bacteria *
culture on agar - with AB and not all nutrients at 42 degrees eg - temp where campylobacter grow - just allow pathogenic bacteria to grow
different pathogens have different culture requirements
C difficile - difficult to grow - so do PCR
food poisening becoming PCR because sensitivity not important because slef limiting - just need to identify it
what investigations are done for parasites when there is diarrhoea *
concentration.
special stains
diff parasites have different stains
describe campylobactor testing *
when present grey
done at high temps on agar
describe the appearance of vibro cholerae on agar *
yellow
discribe the appearance of salmonella on agar *
red
produce hydrogen sulfide - black circles
why is it important to request tests based on clinical judgement *
otherwise the positive predictive value is low
PPV depends on prevalence ie the pretest probability
if pretest probability is low - so will PPV = increased risk of false positives
describe sensitivity testing *
use either minimym inhib conc or disc diameters
see whether the bacteria is resistant or sensitive to the AB
MIC is the least amount of AB to inhibit growth in vitro
describe gradient MICs *
gradient of antibacterial on strip
where bacteria crosses strip = MIC
if doesnt cross strip - resistant
why would you use disc diffusion over MIC *
MIC is labour intensive
how do you interpret disc diffusion *
there is a zone of inhibition
- interpret according to the size of zone - size varies on AB and organism
how would you diagnose subaccute bacterial endocarditis *
multiple blood cultures
have continuous bacteraemia
how woulod you diagnose symphilis *
serum for Ab
how would you diagnose toxoplassma *
serum for Ab
how would you diagnose TB *
culture
INF- gamma - bodies response - can be -ve because immunocomp
how can seroconversion be used for diagnosis *
initial blood sample -ve for IgG maybe +ve for IgM
but second sample +ve illustrate that you have developed an infection
not really used because ave to wait 14 days
would have treated anyway
no use PCR
