Antibiotic Susceptibility Testing Flashcards by Shelly Underwood

What does AST stand for?

Antimicrobial Susceptibility Testing

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What does AST do?

Determines activity of antimicrobial against bacteria

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What can you use AST on?

Bacteria, virus, parasite and fungi

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What are some reasons an antibiotic may not be on your panel?

-Not approved for vet med
-Not approved for species
-Not useful for infection in that location
-Not effective against that bacteria (intrinsic resistance)

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What are the methods of AST?

Phenotypic, Molecular and Mass Spectrometry

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What is key to remember about breakpoints?

They change based on new studies and information

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Describe the phenotypic method of dilution/automated?

-Make a broth dilution of the bacteria
-Add bacteria to plate with different concentration antimicrobials (may be preassem-bled)
-Incubate 18-24 hours
-Read growth
-Lowest concentration that inhibits bacterial growth is minimum inhibitory concentration (MIC)

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Describe the phenotypic method of diffusion/gradient method?

-bacteria are made into standardized concentration into liquid and added to agar plate
-E-test strip of kirby bauer disc added
-incubate 18-24 hours
-No growth on plate (clear area) - zone of inhibition (MM)
-Read where bacteria growth meets the strip

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What is a bacteriostatic drug?

-Inhibit bacterial growth and reproduction
-Inhibit RNA synthesis
-Delay bacteria until host immunity can kill

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What is a bactericidal drug?

-Directly kill bacteria
-inhibit cell wall or DNA synthesis
-Not dependent on host response (rapid)

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What is MIC?

Minimum Inhibitory Concentration
-lowest concentration of antimicrobial that inhibits growth and does not kill microbes

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What is MBC?

-Minimum bactericidal concentration
-Lowest concentration needed to kill the microbe
-Bacteriostatic MBC to MIC >4
-Bactericidal MBC to MIC <4

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What is a breakpoint?

Maximum MIC that predicts successful therapy

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Who makes the MIC?

CLSI

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What is considered when creating break points?

-Normal range of MIC in particular bacteria species
-PK/PD of drug in species of interest
-Site of infection
-Pharmaceutical company data
-Challenging cause cost money to get data to back these things up

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What do ideal breakpoints combine?

Species, Drug, Bacterial Species and Site

What do breakpoints tell you?

If the organism is suspectable (S), Intermediate (I) or Resistant (R)

What does it mean if the organism is susceptible?

Concentration associated with high likelihood of therapeutic success

What does intermediate mean?

Concentration associated with uncertain therapeutic effect
-efficacy in certain body sites
-buffer zone to prevent error
-Consider site of action, if local it may be ok

What does it mean if it is resistant?

Should not use that drug
-concentration associate with high likelihood of therapeutic failure

What do we do if there is no breakpoint?

Depends on the lab.
-Some will not report on that drug
-Some will use breakpoints from another species of animal or similar organism
-Some default to human breakpoints
-Some substitute similar organism in same genus
-NI or NN (inconclusive)

When do breakpoints change?

When there is new data or when they are checked every 2 years

So what if you plan to apply the antimicrobial topically? Can you trust the breakpoint?

No, breakpoints are based on systemic concentrations

-Much higher concentrations than through oral
-Can be R in systemic but work locally

If there is a lack of response to your treatment dose that mean there is resistance? If it is not resistant then what could it mean?

No
-Poor penetration at that site
-Underlying disease or immunocompromised
-BIOFILM

How do you know if a organism is resistant then?

-Phenotypic - AST and standard breakpoints -Genotypic - gene ID, mecA staph, ESBL enterobacterales -Intrinsic - target not available in that species -Multidrug resistant - MDR- resistant to >3 antimicrobial classes

When should I perform AST?

-Sample appropriately collected -Appropriately transported to lab -Evidence Inflammation -Signiant growth isolated -Want to use systemic antibiotics

You have a case where you predict that strep equi is the causative agent. Do you need susceptibility? will you get results back?

No, and you would not because they are very predictable in susceptibility

When is susceptibility testing not needed?

-Predictable susceptibility of pathogen -No pathogen has been isolated -If you suspect a fastidious organism -when topical is indicated

What are some bacteria with predictable suceptibilities?

Beta hemolytic streptococcus, clostridium, erytsipelothrix rhusiopathiae, listeria monocytogenes, bacillus anthracis, actinomyces, actinobacillus, haemonphilus, pasturella (except bovine), fusobacterium *May still be hard to treat*

A dog is having increased accidents, a UA is analyzed (Increase rbc, wbc and protien and gram + cocci), owner want culture and sensitivity, what kind of test do you order?

Aerobic culture and susceptibility (Anaerobes rare for a UTI) -Start on enrofloxacin while waiting

If the empiric antibiotic you are treating with comes back with an I on sensitivity what do you do?

-If patient is doing well don't need to change but may want to increase dose if possible -If not doing well switch to an S antibiotic (Or old and has other issues)

You have a cocker spaniel with recurrent otitis enter the clinic. You send it off for culture and find staph pseudointermidius. Do you need sensitivity?

Yes! MDR?

What are some MRSP treatment options?

-Skin and ear infection common -No rifampin by itself or become resistant way fast -Topical therapy -Mupirocin - last resort (top dog) -Human meds (rare)

If a patient returns like 3 months after treatment for the same thing do you need to culture and sensitivity it again?

Yes, things may have changed

Can 2 organisms be causing and infection? How do you know which sensitivity to use?

Yes Combine them and pick a drug that is S to both

Are there any antibiotics to help with BIOFILMS currently?

No, doesnt take into account bacterial lifestyle Can use Tris-EDTA, NAC, H2O2 and DMSO to try to break it up