Bacteriology lab Flashcards
What are the 4 main bacterial lab investigation techniques?
1) Cultures- (sterile site (blood/CSF) or not (anywhere else/faeces (only report important)- main use is then test drug resistance (molecular would need to check genes)
2) Serology-Abx (evidence of infection)-either cant grow or molecular too hard
3) Molecular techniques-very sensitive to organisms
4) Antimicrobial susceptibility technique
Is Skin a sterile site? Should you be worried about pathogens there?
No-very much no
lot of commensals on
and tons of pathogens too but doesnt mean theyll cause any issue
An in hospital you weaken the patient immune system (catheters, etc)
How many bottle of blood would you ususally take for a blood culture?
2-one aerobic and oe anaerobic
for childen-can be less (or use peads bottles)
some people just do one to proper fill the aerobic (when suspectic a specific pathogens)
How are blood cultures treated when they arrive in the lab? should you start the patient on Abx before?
Warmed and kept at ideal temperatures to grow
limit/ dilute abx to increase growth
The growing bacteria will produce toxic metabolites which changes an indicator-machine checks that an tell u if there is growth -if there is=POSITIVE (takes about 16-20h)
If they flag positive-rapid test (1h-but doesnt give sensitivity)
then grow to agar plates to do the sensitivity testing
BE CAREFUL GIVING PATIENTS ABX-can give fasle negatives
What informations are immediatly tested for after a positive blood culture?
GRAM TEST-Gram+ (thick peptido glycan) or Gram - (double layer thin wall + LPS)
Matters because Abx treatment change
Then microscope-shape, color, clumps or not, (small purple clumbs-staphilococci)-again specific Abx for
Staphillococci coagulase test-positive or negative result gives 2 class of Bacteria-test if can form a clot Coag + (staph aureus)-major pathogens (MRSA, endocarditis) Coagulase -=more likely to be commensals and not be main cause of infection (but can be dangerous with prostethics)
Sensitivity
about 2 days for full test
When cleaning skin with alcohol, what is the more important step?
Its the drying part that ensures death-and kills the commensals you dont want in your sample-so ensure it dries
NOT THE RUBBING
What do streptococci look like? What groups of them are there?
Longer, chains, gramm +
groups heamolytic sterptococci-B-heamolysis -group A-sterp pylogenis-sore throat/quinsy/necrotisis fascitis
Group B- strep agalactae-infections in neonates
Also group C and G-bit like group A but less virulent (like cellulities)
alpha and gamma also heamolitic -streptomococcus pneumoniae
then use specific Abx
Does the gram result indicate how Abx resistant the organsism is?
Gram - (pink stain-like sausages)-usually much more Abx resistant to amoxicillen-
How do you decide which organisms to report for non sterile sites?
Decide on “most likely pathogens” - Shigella, salmonelle, E/coli 0157, Campylobacter in diarhoae for ex
thats where the history of the patient is important and should be told to the lab-travelling, etc (makes them check for more/specific stuff
if tell em-look for parasite/viruses/etc
check the report-what theyve checked + what theyve grown
how do you work with sample from non sterile samples?
Culture on agar plates to then do sensibility test
BUT-most food poisoning dont need Abx-so labs switching to PCR (and then sensitivity to if yes)
In faeces-try and supress as much growth of unneed bacteria-only pathogens grow
Salmonella, Campylobacter and shigella checked routine (E.Coli in UK labs). C.diff needs PCR
AGARS SELECTS THE PATHOGENS-Just checks grwoth on the specific agar
What is an MIC (in terms of ABX resistance)? How is it interpreted
Minimum inhbitory concentration-minimum ABx conc requires to inhbit the growth of that organism in vitro - but its more of a gradient in reality
Then modified (EUCAST GUIDLINES)-look at any data they can gather-and decide on a cutoff at which its decided to be resistant or not (using clinical outcomes after)
What are the 2 main lab techniques to check resistance?
Gradient MIC-strip which has the dilutions eatch notches-check when bacteria can grow/cutoff-labour intesive
Other one is the disc diffusion-place several Abx (and different conc) in a plate, and then check if bacteria can grow close (zone of inhbition-size)-able to check several Abx at a time (big size doesnt always mean sensitive-depends on organism and Abx
Why cant Molecular tests replace cultures yet?
Too many genes that code for resistance
ALso PCR need primers-need to know what youre looking for-
With the rise of Whole genome sequencing, it should be possible soon to get genotype->phenotype easier
