bacteriology Flashcards
microbiological diagnosis: explain the concept of best-guess diagnosis and the contribution of the laboratory to it, recall how a microbiology laboratory works and what investigations may be performed
what is syphilis and associated symptom
STI with skin rash
how does endocarditis present
fever and weight loss
what type of bacteria is toxoplasma
opportunistic
how does bacterial tuberculosis present
weight loss, fever, co-infection with HIV
common diagnostic techniques
culture (sterile sites e.g. blood, CSF - no bacteria should be present; non-sterile sites - commensal bacteria present), serology, molecular techniques (screening, identify resistance genes), antimicrobial susceptibility testing (main reason persist with culture; likelihood to respond to antibiotics)
what to do with specific clinical details (e.g. history, travel)
look for specific bacteria matching symptoms
where are normal bacteria
nasopharynx, skin, upper bowel, lower bowel, vagina
blood culture sterile sites: steps
placed into plastic bottles with growth media (anaerobic and aerobic; paeds normally optimised for aerobic as not much blood) -> machine warms it, agitates so bathe in broth for optimal conditions to grow -> toxic metabolites produced which interact with indictor at bottom to change colour -> machine detects colour change and notifies of positive growth (usually 16-20 hours) -> removed for rapid identification testing -> placed on agar plates for susceptibility testing to antibiotics
when to give blood culture to lab, and affect of sample
give before treating with antibiotics (skew results); where plated pus/sample, can inhibit growth, giving false negative
gram +ve vs gram -ve; staining
gram +ve: thick peptidoglycan cell wall, gram -ve: thin peptidoglycan cell wall, LPS polysaccharide membrane outside this; gram +ve stain more purple as stains peptidoglycan cell wall
what else is observed after gram type
shape and division (clumps or chains)
how to separate Staphylococci types
divide in clumps; coagulase test (+ve and -ve): ability of bacteria to produce clot in plasma
S. aureus complex: infections, gram and coagulase test
major pathogen (includes MRSA) responsible for severe infections e.g. skin/soft tissue, endocarditis, osteomyelitis; gram +ve and coagulase -ve
type and division of Streptococci, and how it is separated into groups
gram +ve and divide chain to chain; haemolytic (B is main pathogens; within which is group A which is Strep pathogens); group C and G like group A but less virelent
what is S. pnueomoniae
gram -ve bacilli
non-sterile site: diarrhoea - what bacteria to look for after request and why
common after food poisoning; Salmonella (incl. S. typhi), Shigella, Campylobacter, E. coli O157, C. dificile (if recent antibiotic use), Cholera
non-sterile site: diarrhoea - what else to look for after request because of travel/other history
parasites: Amoeba, Giardia, Cryptosporidium; viruses e.g. norovirus
bacterial investigations of stool sample
culture on agar plate, only Salmonella, Shigella, Campylobacter looked for routinely; PCR initially as less sensitive (if positive, then do sensitivity test); different culture requirements to suppress growth (only grow pathogens to kill other bacteria); C. difficile detected by toxin/PCR toxin gene as difficult to grow
parasitic investigations of stool sample
don’t grow, so concentration in faeces then special strains (for different parasites)
why are there different agar
optimal for different bacteria
what must be thought of before requesting test
pre-test probability (based on clinical judgement), as if this is low, then low positive predictive value (sensitivity)
what does sensitivity testing do
tests bacteria against antibiotics to determine resistance and MIC (minimum inhibitory concentrations; minimal amount of antibiotic needed to inhibit growth in vivo)
how is antibiotic senstivity or resistance determined
based on MIC and pharmacodynamics and pharmacokinetics by usual dose of drug
how is MIC, and therefore sensitivity or resistance, calculated
gradient, disc diffusion
