bacteriology

Question 1

what is syphilis and associated symptom

Answer 1

STI with skin rash

Question 2

how does endocarditis present

Answer 2

fever and weight loss

Question 3

what type of bacteria is toxoplasma

Answer 3

opportunistic

Question 4

how does bacterial tuberculosis present

Answer 4

weight loss, fever, co-infection with HIV

Question 5

common diagnostic techniques

Answer 5

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)

Question 6

what to do with specific clinical details (e.g. history, travel)

Answer 6

look for specific bacteria matching symptoms

Question 7

where are normal bacteria

Answer 7

nasopharynx, skin, upper bowel, lower bowel, vagina

Question 8

blood culture sterile sites: steps

Answer 8

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

Question 9

when to give blood culture to lab, and affect of sample

Answer 9

give before treating with antibiotics (skew results); where plated pus/sample, can inhibit growth, giving false negative

Question 10

gram +ve vs gram -ve; staining

Answer 10

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

Question 11

what else is observed after gram type

Answer 11

shape and division (clumps or chains)

Question 12

how to separate Staphylococci types

Answer 12

divide in clumps; coagulase test (+ve and -ve): ability of bacteria to produce clot in plasma

Question 13

S. aureus complex: infections, gram and coagulase test

Answer 13

major pathogen (includes MRSA) responsible for severe infections e.g. skin/soft tissue, endocarditis, osteomyelitis; gram +ve and coagulase -ve

Question 14

type and division of Streptococci, and how it is separated into groups

Answer 14

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

Question 15

what is S. pnueomoniae

Answer 15

gram -ve bacilli

Question 16

non-sterile site: diarrhoea - what bacteria to look for after request and why

Answer 16

common after food poisoning; Salmonella (incl. S. typhi), Shigella, Campylobacter, E. coli O157, C. dificile (if recent antibiotic use), Cholera

Question 17

non-sterile site: diarrhoea - what else to look for after request because of travel/other history

Answer 17

parasites: Amoeba, Giardia, Cryptosporidium; viruses e.g. norovirus

Question 18

bacterial investigations of stool sample

Answer 18

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

Question 19

parasitic investigations of stool sample

Answer 19

don’t grow, so concentration in faeces then special strains (for different parasites)

Question 20

why are there different agar

Answer 20

optimal for different bacteria

Question 21

what must be thought of before requesting test

Answer 21

pre-test probability (based on clinical judgement), as if this is low, then low positive predictive value (sensitivity)

Question 22

what does sensitivity testing do

Answer 22

tests bacteria against antibiotics to determine resistance and MIC (minimum inhibitory concentrations; minimal amount of antibiotic needed to inhibit growth in vivo)

Question 23

how is antibiotic senstivity or resistance determined

Answer 23

based on MIC and pharmacodynamics and pharmacokinetics by usual dose of drug

Question 24

how is MIC, and therefore sensitivity or resistance, calculated

Answer 24

gradient, disc diffusion

Question 25

why would you genotype entire genome

Answer 25

to determine genotype and associated phenotype, preventing need of culture for and sensitivity testing

Question 26

gold standard for serology

Answer 26

seroconversion (seronegative to seropositive when specific antibody develops and becomes detectable in blood)