Susceptibility Testing

Question 1

what is MIC

Answer 1

lowest concentration of drug that completely inhibits growth of bacteria

on the plate it is the first well without growth

Question 2

what is a breakpoint MIC

Answer 2

the maximum bacterial MIC that predicts successful therapy
specific to the source species, drug, bug, and site

Question 3

define a result of “susceptible” and what that means for the MIC

Answer 3

treatment of this bacteria with this antibiotic has high likelihood of therapeutic success
bacterial MIC is less than or equal to breakpoint MIC

Question 4

define a result of “intermediate”

Answer 4

therapeutic effect is uncertain
may have efficacy in body sites where the drugs are physiologically concentrated

Question 5

define a result of “resistant” and what that means for the MIC

Answer 5

treatment of this bacteria with this antibiotic has high likelihood of therapeutic failure
bacterial MIC is greater than or equal to breakpoint MIC

Question 6

what is the result if bacterial MIC is greater than or equal to breakpoint MIC

Answer 6

resistant

Question 7

what is the result if bacterial MIC is less than or equal to breakpoint MIC

Answer 7

susceptible

Question 8

when do susceptibility tests underestimate susceptibility in vivo

Answer 8

surface infections treated with topicals and infections in the urinary bladder

Question 9

when do susceptibility tests overestimate susceptibility in vivo

Answer 9

intracellular bacteria (ex: R. equi), infections inside the eye, in the prostate, or in the CNS

Question 10

what drug factors are needed to access protected sites

Answer 10

lipophilic and low protein binding

Question 11

how can we increase chance of success when using an intermediate susceptibility drug

Answer 11

increase dose if concentration dependent
decrease dosing interval if time dependent

Question 12

what susceptibility pattern should make you highly suspicious of an ESBL

Answer 12

resistant to all beta-lactams, but susceptible to amoxi-clav

Question 13

what susceptibility pattern is definitive of ESBL

Answer 13

resistant to cefpodoxime or ceftazidine

Question 14

what susceptibility pattern is seen with carbapenemase producing bacteria

Answer 14

resistant to imipenem
BAD

Question 15

describe the susceptibility pattern of pseudomonas

Answer 15

intrinsically resistant to beta-lactams, most cephalosporins, tetracyclines, chloramphenicol, TMS.
easily develops resistance to fluoroquinolones during tx.
may be susceptible to fluoroquinolones, amikacin, imipenem

Question 16

describe the susceptibility pattern for enterococci

Answer 16

intrinsic resistance to aminoglycosides, cephalosporins, clindamycin, TMS
may be able to use ampicillin plus an aminoglycoside, although resistant to each individually

Question 17

what other antibiotic has the exact same susceptibility as the one listed below:
ampicillin

Answer 17

amoxicillin

Question 18

what other antibiotic has the exact same susceptibility as the one listed below:
amoxi-clav

Answer 18

ampicillin-sulbactam

Question 19

what other antibiotic has the exact same susceptibility as the one listed below:
oxacillin

Answer 19

methicillin

Question 20

what other antibiotic has the exact same susceptibility as the one listed below:
imipenem

Answer 20

meropenem

Question 21

what other antibiotic has the exact same susceptibility as the one listed below? what is not the same?
cephalothin

Answer 21

cephalexin

but not the same as cefazolin

Question 22

what other antibiotic has the exact same susceptibility as the one listed below? what is not the same?
ceftazidine

Answer 22

cefpodoxime (except for pseudomonas)
not the same as other 3rd generations

Question 23

what other antibiotic has the exact same susceptibility as the one listed below:
clindamycin

Answer 23

lincomycin

Question 24

what other antibiotic has the exact same susceptibility as the one listed below:
TMS

Answer 24

all potentiated sulfonamides

(except for EPM, then need sulfadiazine for CNS penetration and protozoal activity)

Question 25

if a bacteria is susceptible to tetracycline, what does that tell you about the others in the class

Answer 25

likely susceptible to doxy and minocycline

tetracycline is the least active in the class

Question 26

if a bacteria is not susceptible to tetracycline, what does that tell you about the others in the class

Answer 26

may still be susceptible to doxy/minocycline, but not guaranteed

tetracycline is the least active one

Question 27

if a bacteria is susceptible to erythromycin, what does that tell you about the others in the class

Answer 27

likely susceptible to azithro and clarithromycin

erythromycin is the oldest and least active in the class

Question 28

if a bacteria is not susceptible to erythromycin, what does that tell you about the others in the class

Answer 28

may still be susceptible to azithro and clarithromycin, but not guaranteed

erythromycin is the least active

Question 29

what does combination of a beta-lactam and an aminoglyocoside do to the MIC

Answer 29

may lower MIC by 1 dilution
synergism

Question 30

T/F: on the susceptibility report, the drug with the lowest MIC is the best to use

Answer 30

false. each one has a different breakpoint

Question 31

what does NI mean

Answer 31

no breakpoint available or intrinsic resistance
may be able to interpret MIC based on breakpoints of similar bacteria

Question 32

what should you ask the lab if an organism is found but no sensitivity report provided

Answer 32

is it a contaminant or are they unable to do the test due to slow growth pattern