bacteriology lab

Question 1

what are common diagnostic techniques in the bacteriology lab *

Answer 1

culture - sterile sites (blood/CSF) or non sterile sites

serology

molecualr techniques

antimicrobial susceptibility testing

Question 2

summarise culturing *

Answer 2

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

Question 3

summarise serology *

Answer 3

look for body response

eg syphilis - difficult to grow and dont get many circulating organisms

Question 4

summarise molecular testing *

Answer 4

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

Question 5

summarise antimicrobial suseptibility testing *

Answer 5

use impregnated disks or grading strips

correlate with suseptible or resistant

Question 6

examples of things in hospital that make people more suseptible to infection

Answer 6

flushing action of urinary flow removes organisms

pathogenic bacteria can reside in the plastic

cannular - bypass the skin and the bacteria can enter

Question 7

describe how blood cultures are stored *

Answer 7

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

Question 8

how can you see if a blood culture is positive *

Answer 8

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

Question 9

what is the structure of gram +ve bacteria

Answer 9

thick peptidoglycan cell wall

Question 10

structure of gram -ve bacteria *

Answer 10

LPS polysaccharide layer

thin peptidoglycan cell wall

Question 11

what can be seen when you look at gram +ve bacteria - stained *

Answer 11

purple - dye gets stuck in the cell wall

can see the shape

eg staphylococci - circle and in bunches

Question 12

describe the coagulase test *

Answer 12

rare

ability of bacteria to form a coagulase in horse/rabbit plasma

either coagulase +ve or -ve

Question 13

describe staphylococci

Answer 13

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

Question 14

how do you investigate haemolytic streptococci *

Answer 14

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

Question 15

what is the appearance of gram -ve bacteria

Answer 15

light pink

Question 16

what is the importance of determining gram staining *

Answer 16

-ve are usually more resistant

Question 17

what is best guess diagnosis *

Answer 17

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

Question 18

what would be looked for if diarrhoea was the presenting symptom *

Answer 18

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)

Question 19

what investigations are done on stool samples for bacteria *

Answer 19

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

Question 20

what investigations are done for parasites when there is diarrhoea *

Answer 20

concentration.

special stains

diff parasites have different stains

Question 21

describe campylobactor testing *

Answer 21

when present grey

done at high temps on agar

Question 22

describe the appearance of vibro cholerae on agar *

Answer 22

yellow

Question 23

discribe the appearance of salmonella on agar *

Answer 23

red

produce hydrogen sulfide - black circles

Question 24

why is it important to request tests based on clinical judgement *

Answer 24

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

Question 25

describe sensitivity testing *

Answer 25

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

Question 26

describe gradient MICs *

Answer 26

gradient of antibacterial on strip

where bacteria crosses strip = MIC

if doesnt cross strip - resistant

Question 27

why would you use disc diffusion over MIC *

Answer 27

MIC is labour intensive

Question 28

how do you interpret disc diffusion *

Answer 28

there is a zone of inhibition

• interpret according to the size of zone - size varies on AB and organism

Question 29

how would you diagnose subaccute bacterial endocarditis *

Answer 29

multiple blood cultures

have continuous bacteraemia

Question 30

how woulod you diagnose symphilis *

Answer 30

serum for Ab

Question 31

how would you diagnose toxoplassma *

Answer 31

serum for Ab

Question 32

how would you diagnose TB *

Answer 32

culture

INF- gamma - bodies response - can be -ve because immunocomp

Question 33

how can seroconversion be used for diagnosis *

Answer 33

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