bacteriology

Question 1

when is blood culture used? what are the sites?

Answer 1

• used to determine antimicrobial resistance
• sterile sites e.g. blood and CSF
• non-sterile sites

Question 2

what is the problem with non-sterile sites?

Answer 2

• usually a lot of bacteria here so culturing is difficult

Question 3

what is serology used to determine?

Answer 3

the body’s response to an infection

so measuring the body’s response by doing a blood at beginning of chickenpox and at end

Question 4

what do molecular techniques do?

Answer 4

detect resistance genes

Question 5

what does antimicrobial susceptibility testing do?

Answer 5

used to test AB resistance

but takes a long time

Question 6

what are the different types of agar?

Answer 6

• chocolate agar: cooked blood, let’s bacteria use blood nutrients to grow
• MacConkey agar: designed to grow gram -ve bacteria

Question 7

describe the cell walls of gram +ve and -ve

Answer 7

gram +ve = thick wall, purple stain, retains dye

gram -ve =thin wall, pink stain, loses dye

Question 8

how can different forms of staphylococci be distinguished?

Answer 8

coagulase test to test between coagulase +/- staphylococci

• coagulase + = staphylococci aureus
• coagulase - = common skin microbes

Question 9

what is the normal presentation of staphylocci?

Answer 9

form clumps

look like bunches of grapes as they bud divide

Question 10

what is the normal presentation of streptococci?

Answer 10

• generally forms chains in the gram stain

Question 11

on blood agar, what are th 2 groups streptococci separate into?

Answer 11

• alpha haemolysis: incomplete haemolysis, turns green e.g. Strep pneumoniae
• beta haemolysis: complete haemolysis, clears agar
  e. g. Group A - Strep Pyogenes, group B - strep agalactiae

Question 12

what is important to remember about gram negative bacilli?

Answer 12

do not take up gram stain
so appear pink
e.g. E coli

Question 13

what are the possible causes of diarrhoea?

Answer 13

• bacteria: salmonella, shigella, campylobacter, E. coli, C. difficle
• parasites: amoeba, giardia, cryptosporidium
• viruses

Question 14

what does salmonella, campylobacter and virbrio cholerae look like on agar?

Answer 14

• salmonella: colonies are black due to hydrogen sulphide production
• campylobacter: can survive at 48 degrees so heat to kill other bacteria
• virbrio cholerae: cholera makes agar turn green

Question 15

what is PPV and what does it depend on?

Answer 15

• depends on pre-test probability of the sample being positive
• more likely a pt is to have disease, more likely a positive test represents a true positive

Question 16

what is MIC? what is used clinically instead?

Answer 16

MIC = lowest amount of AB required to inhibit growth of bacteria in vitro
- use breakpoints clinically which correlate MIC with clinical success as an AM

Question 17

what does it mean when a bacterium has an MIC below the breakpoint?

Answer 17

good chance of success with that AB

Question 18

what is a bacterium with a MIC above breakpoint?

Answer 18

resistant