Antimicrobial Resistance & Susceptibility

Question 1

antimicrobial resistance

Answer 1

the ability of a bacterium to survive and/or multiply when faced with a drug used to inhibit its growth and/or kill it

Question 2

what are the outcomes of antimicrobial resistance

Answer 2

• drug is no longer effective
• increased animal disease and spread
• drug eliminates good pathogens but not the targeted pathogen
• use in animals can lead to resistance in humans

Question 3

selective pressure

Answer 3

driving cause of antimicrobial resistance

antimicrobial kills off the bacteria that are susceptible and leave behind the ones that are resistant –> resistant population survives and grows

Question 4

what are 4 mechanisms of bacteria use to become resistant

Answer 4

1. pumping drug out of the bacteria
2. preventing the entry of the drug into the bacteria
3. inactivating the drug once inside
4. changing the target site for the drug

Question 5

intrinsic resistance

Answer 5

drug resistance that is naturally present in the microorganism

drug never has and never will work against the pathogen type

can NOT change this resistance - does not develop over time

Question 6

acquired resistance

Answer 6

ability of a microorganism to survive and multiply in the presence of an antimicrobial that would normally inhibit/kill that organism

drug used to work but no longer does - develops over time

Question 7

what are two methods of acquired resistance

Answer 7

1. mutation
2. acquisition

Question 8

mutation

Answer 8

bacteria mutates its genetic code

rate of mutation increases with stress

Question 9

acquisition

Answer 9

most common method

horizontal transfer of genes encoding resistance
- conjugation of plasmid
- transduction via bacteriophages
- transformation by up taking naked DNA

Question 10

can acquisition of resistance occur between bacterial species

Answer 10

yes

Question 11

how many resistant organisms does a normal population of bacteria have

Answer 11

very few - carrying around extra DNA is heavy

antimicrobial treatment will eliminate the susceptible bacteria and leave behind the resistant ones that will then proliferate to form a resistant population

Question 12

antimicrobial susceptibility testing

Answer 12

method for determining if a bacterial isolate is susceptible to a range of antimicrobials that may be used to treat an infection with this bacteria

only a PREDICTION - not a guarantee

Question 13

contraindications for susceptibility testing

Answer 13

1. strain is predictably susceptible to certain antimicrobials
2. infection type has high level of efficacy for empiric therapy (ex. first time UTIs)
3. infection is highly mixed (will not provide useful information)

Question 14

considerations for susceptibility testing

Answer 14

• location of infection
• breakpoints
• MIC
• zone of inhibition

Question 15

how does location of infection impact susceptibility test result interpretation

Answer 15

test will predict if a particular drug will reach the tissue in concentrations high enough to inhibit bacteria

requires PK data to establish breakpoints specific for that location

Question 16

breakpoints

Answer 16

cutoff values that determine the interpretation of MIC/zone of inhibition values as either susceptible, intermediate, or resistant

drug, host, and bacteria specific values

Question 17

do all species have established breakpoints

Answer 17

no - must extrapolate from other species or body sites

not always appropriate/accurate

Question 18

what drug concentration are breakpoints based off of

Answer 18

the concentration in the SERUM

Question 19

MIC

Answer 19

minimum inhibitory concentration

lowest concentration of antimicrobial drug that inhibits growth of a bacterium (ug/mL)

Question 20

zone of inhibition

Answer 20

diameter of bacterial growth that is inhibited around an antimicrobial disk

Question 21

what are the three methods of AST

Answer 21

1. broth micro dilution
2. E-test
3. disk diffusion

Question 22

broth micro dilution

Answer 22

measures MIC

culture gets isolated and inoculated into broth –> placed into wells –> variable concentration of different drugs added to wells (drugs based off of panel type) –> check for growth

Question 23

broth micro dilution results

Answer 23

1. no growth achieved at a specific concentration –> compare to breakpoint to determine if S, I, or R
   - MIC = lowest concentration that achieved no growth
2. growth achieved at all concentrations –> resistant
   - MIC > highest concentration
3. no growth achieved at any concentration –> susceptible
   - MIC < lowest concentration

Question 24

what does NA, NI, or NN mean on a susceptibility report

Answer 24

no information provided to categorize as S, I or R

Question 25

what are the pros of broth microdilution

Answer 25

- gets an MIC - has standardized breakpoints - works for some slow growing or fastidious organisms

Question 26

what are the cons of broth microdilution

Answer 26

- more expensive - need special equipment

Question 27

e-test

Answer 27

antimicrobial strip testing measures MIC

Question 28

disk diffusion

Answer 28

measures zone of inhibition standardized lawn of bacteria spread over plate --> filter paper disks with antimicrobial applied to lawn --> incubate --> check for zone diameter of inhibition

Question 29

disk diffusion result interpretations

Answer 29

S, I, or R depending on zone size, drug, and host species

Question 30

pros of disk diffusion

Answer 30

- easy - inexpensive - no special equipment - more flexibility with drugs used

Question 31

cons of disk diffusion

Answer 31

- no MIC measured - doesn't work for slow growing/fastidious organisms

Question 32

AST interpretation

Answer 32

categorization based on breakpoints established for host, drug, and organism based on the MIC or zone diameter used to determine whether a drug can be used to treat an infection

Question 33

susceptible

Answer 33

bacterial isolate is inhibited by usually attainable concentration of drug for infection site MIC

Question 34

intermediate

Answer 34

MIC approaches the usually attainable concentration levels for susceptible isolates clinical efficacy might be achieved in body sites where the drug concentrates MIC = intermediate breakpoint

Question 35

resistant

Answer 35

bacterial isolate is not inhibited by the usually achievable concentrations of antimicrobial MIC >/= resistant breakpoint

Question 36

effect of susceptibility report on drugs that reach greater concentration at the target site than in serum

Answer 36

susceptibility test will UNDERESTIMATE true susceptibility ex. topical administration for skin infections or UTIs

Question 37

effect of susceptibility report on drugs that reach lower concentration at the target site than in serum

Answer 37

susceptibility test will OVERESTIMATE true susceptibility ex. intracellular bacteria, eye, prostate

Question 38

does "no interpretation" mean that drug will not be effective

Answer 38

NO - only means that efficacy can't be predicted

Question 39

is an interpretation of resistance or susceptibility more likely to be correct

Answer 39

resistance R is more likely to correctly predict failure of the drug to work than S is likely to predict success of the drug