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Micro - Intro to Antibiotics + their Classification (i) (Intro (Antibiotic…
Micro - Intro to Antibiotics + their Classification (i)
Intro
Antibiotic prescribing affects both the patient and the wider community
resistant pathogens could spread + become endemic
antibiotic = naturally occurring substance produced by a microbe that inhibits another
antibacterials can be synthetic, semi-synthetic or naturally occurring
antimicrobials include antibacterials, antivirals + antifungals
Uses
tx
empiric 1st
before lab results are available
broad
can often disrupt normal flora, allowing superinfection (colonisation of thrush or C diff -> diarrhoea, megacolon, perforation, death)
then targeted - narrow based on pathogen ID + susceptibility tests
prophylaxis
Ways to Classify
family
beta-lactams
glycopeptides
aminoglycosides
quinolones
macrolides
tetracyclines
MOA
cell wall synthesis inhibitors
b-lactams
vancomycin
cycloserine
bacitracin
polymyxin
DNA rep inhibitors
quinolones
metronidazole
RNA synthesis inhibitors (rifampin aka rifampicin)
protein synthesis inhibitors
50S
chloramphenicol
macrolides
clindamycin
linezolid
quinupristin
dalfopristin
30S
aminoglycosides
tetracyclines
antimetabolites
dapsone
sulphonamides
trimethoprim
spectrum
broad
good for polymicrobial/unknown infections
can cause superinfection
e.g. piptazobactam against gram +ve cocci + gram -ve bacilli, incl pseudomonas + anaerobes, ie.g. gut flora)
narrow
min disruption to normal flora
unsuitable as empiric Tx
e.g. Penicillin G against gram +ve cocci + some gram -ve cocci
activity
bacteriostatic
prevent bacteria multiplying (inhibits growth)
host immune defences still needed
tetracyclines
bactericidal
kill bacteria
indicated for meningitis, endocarditis, BSI
good when patient immunosuppressed
penicillin
susceptibility testing
in vitro test to predict the likely success/failure of an antibiotic in vivo
disc diffusion
disc has defined amount of antibiotic on agar which is inoculated with bacteria
zone of inhibition = circle around antibiotic with no bacterial growth
diameter measure in mm, then compared to a standard interpretation chart (EUCAST in Europe)
qualitative: bacteria either, susceptible, intermediately susceptible or resistant
MIC can't be quantitatively measured
easy + cheap
4-6 antibiotics cn be tested @ once on 1 disc
e.g. GAS = BACITRACIN SUSCEPTIBLE, S PNEUMONIAE = OPTOCHIN SUSCEPTIBLE
MIC
= lowest conc of an antibiotic required to inhibit bacterial growth
quantitative prediction of clinical response
esp NB in ceratin infections (e.g. endocarditis)
bacteria + antibiotic placed in a set of test tubes (increasing amount on antibiotic per tube)
result = test tube containing lowest antibiotic conc with a clear solution (no turbidity - no bacterial growth)
now there's an agar-based E-test
plastic strip impregnated with a decreasing conc of antibiotic going down (visible on a numerical scale)
separate strip needed for each antibiotic
elliptical zone of clearing
result = no. above where bacteria touches strip
Genotypic method
test for specific genes conferring resistant
however just because a gene is present doesn't meant tx will fail (also depends on mode + level of expression)
NAAT: rapid + sensitive