Bacteriology

Question 1

where are cultures extracted from to determine antimicrobial resistance?

Answer 1

sterile sites e.g. blood and CSF

non-sterile sites (difficult due to great numbers of bacteria)

Question 2

what is serology done for?

Answer 2

determine body’s response to an infection

Question 3

what are molecular techniques used for?

Answer 3

detect resistance genes

alternatively do antimicrobial susceptibility testing but this takes a long time

Question 4

what type of bacteria is found on skin and soft tissue culture usually?

Answer 4

gram +

e.g. Staph aureus

Question 5

what type of culture is found in the abdomen and urinary tract

Answer 5

gram -

Question 6

what causes a gram +ve stain?

Answer 6

due to thick peptidoglycan wall it retains the dye and shows up purple

Question 7

what causes a gram -ve stain?

Answer 7

due to thin wall (double membrane with pep membrane in-between)

loses the dye so looks pink

Question 8

what is the effect of gram -ve on antibiotics?

Answer 8

may not enter due to the other membrane

e.g. vancomycin only works on gram +ve

Question 9

what is the most common bacterium?

Answer 9

gram + cocci

staphylococci

Question 10

how do staphylococci look under the microscope?

Answer 10

they clump like a bunch of grapes as they divide

Question 11

how can you test to distinguish staphylococcus (aureus)?

Answer 11

coagulase test:
+ indicates s.aureus
- indicates common skin microbes

Question 12

how do streptococci look under the microscope?

Answer 12

form chains in gram stain

Question 13

what are the groups of streptococci on blood agar?

Answer 13

1) alpha haemolysis- due to incomplete haemolysis it turns green e.g. s.pneumoniae
2) beta haemolysis- due to complete haemolysis, clears the agar e.g. group A s.pyogenes and group B s.agalactiae

Question 14

how do bacilli look under the microscope

Answer 14

gram -ve so don’t take up the gram stain and appear pink

e.g. E.coli

Question 15

what are the possible causes of diarrhoea?

Answer 15

1) Bacteria – e.g. Salmonella, Shigella, Campylobacter, E. Coli, C. difficile, cholera.
2) Parasites – e.g. Amoeba, Giardia, Cryptosporidium.
3) Viruses.

Question 16

what bacteria cause diarrhoea? how can they be cultured?

Answer 16

Salmonella
– XLD agar – the salmonella colonies are black due to hydrogen sulphide produced.

Campylobacter
– 48hours to grow and can survive at 48 degrees so heat to kill other bacteria.

Vibrio cholerae
– TCBS agar – cholera makes the agar turn green.

Question 17

how can bacteria involved in diarrhoea be investigated?

Answer 17

• agar culture: salmonella, shigella and campylobacter looked for routinely as they are found in faeces
• PCR to detect toxic gene of c.difficile. This bacteria is difficult to culture

Question 18

how are parasites involved in diarrhoea investigated?

Answer 18

special stains

concentrations

Question 19

what is the positive predictive value dependent on?

what is the significance of this?

Answer 19

pre-test probability of the sample being positive

the more likely a patient is to have the disease, the more likely a positive test is a true positive

therefore there is no need to test for everything

Question 20

what is the MIC?

Answer 20

minimum inhibitory concentration

the lowest amount of AB needs to inhibit the growth of bacteria in vitro

Question 21

how is MIC used in evaluating the success of antibiotics?

Answer 21

As MICs aren’t very useful Breakpoints are used.
This correlates MIC with the clinical success of a AB

Breakpoint= chosen concentration (mg/L) of an antibiotic which defines whether a species of bacteria is susceptible or resistant to the antibiotic.

1) a bacterium with MIC < breakpoint means the AB has a chance of being successfully used
2) MIC > breakpoint indicates that the bacterium is resistant

Question 22

how can bacteria resistance also be evaluated?

Answer 22

disc diffusion where a set conc of AB on discs are incubated for 24hrs

a zone size is compared to breakpoints on an AB table

diameter is measured

Question 23

how long are most bacterial samples cultured for agar plates?

Answer 23

24-48 hours

• some need longer e.g. TB, brucella, actinomycetes
• a further 24hr needed for antibiotic sensitivities

Question 24

when is it optimal to collect the microbial specimen?

Answer 24

• in the acute phase of illness before starting antimicrobials
• at a proper site to avoid contamination with normal flora