Antimicrobial Resistance and Susceptibilities

Question 1

idescribe the clinical importance of antimicrobial resistance

Answer 1

kills people

Question 2

describe how bacterial susceptibility can be used to reduce/prevent antimicrobial resistance

Answer 2

1. can use phenotypic, molecular based, and mass spectrometry methods
   -phenotypic: dilution, diffusion, gradient test
   -molecular: PCR, qPCR, cycle sequencing
2. susceptibility breakpoint/breakpoint MIC: the concentration that separates sensitive and resistant strains (S, I, R)

-not always straightforward! (the I)

-determined using in vitro microbiological data, animal and human PK/PD data, clinical/biological outcome data from clinical studies

3. limitation of MIC tests: artificial conditions! grown overnight with aerobic incubation of a standard inoculum of bacteria in low protein, liquid medium at pH of 7.2 (aka NOT the body or conditions of life)
   -site of infection: often acidic, anaerobic, tissue protein may bind up drug, bacteria densities may be much higher, and bacteria likely not in exponential phase of growth (cell wall inhibitors may not work)

Question 3

describe the existing antibiotic classifications systems according to medical importance in humans (WHO classification)

Answer 3

authorized for use in humans only

authorized for use in both humans and animals

not authorized in humans/not medically important

Question 4

define MIC, MIC50, MIC90

Answer 4

MIC: the lowest concentration that prevents visible growth of bacteria in culture
-each MIC is specific to ONE bacteria and ONE antibiotic
-you canNOT compare MIC between different bacteria OR between antibiotics

MIC50: concentration of antibiotic required to prevent growth of 50% of bacteria isolates

MIC90: concentration of antibiotics required to prevent growth of 90% of bacterial isolates

MIC50 and MIC90 imply that a POPULATION OF ISOLATES has been tested

Question 5

describe the basic laboratory basis for determination of MIC using the microbroth dilution method

Answer 5

1. preparation of a test inoculum (mueller hinton broth and test bacteria)
   -isolate an individual colony, put it in a broth, and let it grow (takes a while)
2. preparation of different dilutions of antimicrobial agent
3. determine the minimum inhibitory concentration

Question 6

describe factors that are used to determine MIC in human and veterinary species

Answer 6

determined using

1. in vitro microbiological data
2. animal and human PK/PD data
3. clinical/biological outcome data from clinical studies

Question 7

describe and list examples of time dependent and concentration dependent antimicrobials

Answer 7

concentration dependent:
-aminoglycosides: amikacin, gentamicin
-fluoroquinolones: enrofloxacin
-utilize Cmax/MIC, AUC/MIC
-prolonged post Abx effect; can get away with once daily dosing but need to give higher dosage

time dependent: everything else
-beta lactams: penicillin
-glycopeptides: vancomycin
-oxazolidinones: linezolid
-macrolides, tetracyclines, sulfonamides
-utilize: T>MIC
-want T>MIC for 50-80% of dosing interval, or 100% if immunocompromised (even better if 4-5 fold greater than MIC)
-no to minimal post-Abx effect = need to give more frequently

Question 8

define post-antibiotic effect and describe its application

Answer 8

bacterial growth is inhibited AFTER exposure to the antibiotic

-ex. aminoglycosides, fluoroquinolones: PAE occurs for both gram + and gram - organisms
-partially explains why once daily dosing works

-ex. cephalosporins: PAE for gram + but not for gram -

just understand the idea of it for exam!

Question 9

what other factors affect antibiotic efficacy?

Answer 9

1. protein binding: only free/unbound antibiotic concentration in interstitial fluid of antibiotic site is in charge of the antibacterial activity; highly protein bound = less effective
2. lipophilicity
3. degree of ionization
4. size of molecule

Question 10

describe how veterinarians can apply the veterinary CLSO (or equivalent) recommendations to determine if a particular antibiotic is an appropriate choice for treatment of bacterial infections

Answer 10

need to know:

1. tissue concentrations of antibiotics at the site of your infection in species of interest
   -ideally in animals with disease but rare to find in vet med
2. type of antibiotic: time or concentration dependent
3. MIC breakpoint (CLSI or other)
   -veterinary CLSI ideal
   -some extrapolations can be made