Antimicrobial Introduction, Bacteria, Susceptibility Testing Flashcards
What two bacteria are of greatest public health concern relative to food poisoning as it relates to
veterinary use of antimicrobials?
Salmonella and Campylobacter
What is the connection between veterinary antimicrobial use and
resistance in these bacteria?
Resistance development by vertical transmission
- fecal matter from infected animal gets in the food product –> consumer becomes infected due to improper cooking
Explain what the term “four quadrant coverage” implies relative to antimicrobial utility for a patient.
Gram positive
- aerobes/facultative anaerobes: strep, staph, corynebacteria, enterococci
- obligate anaerobes: clostridia
Gram-negative
- aerobes/facultative anaerobes: pasteurella, E coli, klebsiella, proteus
- obligate anaerobes: bacteroides, fusobacter
There are no _____ of importance in vet med
Gram-negative cocci
Explain the differences between constitutive versus acquired resistance.
Constitutive resistance: resistance contained in the DNA that can be passed on to daughter cells, cannot be transmitted to other species
Acquired resistance
- inactivating enzyme
- change in target site
- development of active efflux pump
- impermeability of bacteria
- development of bypass mechanisms in metabolic pathways
- increasing production of competitive metabolites
Vertical transmission
Mutation occurs and clone reproduces
Horizontal transmission
Predominate mechanism, transfer of genetic material between bacteria
- same species: plasmid
- across species: transposon
Genes for multiple antimicrobial resistance often occur in
Packets
- much greater impact
_____ has a huge problem with antimicrobial resistance
Pseudomonas
- gram-neg rod, extracellular
- aggressive in tissue destruction
Which pathogens have relatively predictable susceptibilities to certain
antibiotics such that in vitro susceptibility testing is often not performed:
- rickettsia
- leptospirosis
- streptococci
- most anaerobes
- mycoplasma
Regardless of gram stain, most anaerobes are similar as to which antimicrobials they are susceptible to except:
- fusobacteria: very sensitive to a wide range of antibiotics
- bacteriodes fragilis: resistant to many antimicrobials that other obligate anaerobs are sensitive to
Which organism is often deemed as one of the most aggressive relative to tissue destruction?It
produces a collagenase
Pseudomonas
Which one cannot be removed by filtration for sterilization of liquids?
Mycoplasma
Which gram‐negative obligate anaerobe tends to be the most susceptible to antimicrobials?
Fusobacterium
Where do enterococci normally reside in the body?
Commonly found in feces
How do enterococci compare to beta‐hemolytic streptococci relative to their antibiotic
susceptibility?
Enterococci have a huge problem with antimicrobial resistance compared to sensitive beta-hemolytic strep
What is enterococci relative pathogenicity in surface wounds?
Not aggressively pathogenic in surface wounds
- problem in septicemia, endocarditis, UTIs, peritonitis
Of the two major species of enterococci, which is the most common and which is the most
likely to have multiple drug resistance
- enterococcus faecalis: most commonly isolated
- enterococcus faecium: more prone to resistance problems (need in vitro susceptibility results)
What are the primary staphylococcal species causing infection in man, dog, cow, horse, and pig?
Staphylococcus aureus
Goal of antimicrobial susceptiblity testing
To find an in vitro test that will predict in vivo efficacy
- uses MBC and MIC
MBC
Minimum bactericidal concentration
- amount of antibiotic necessary to kill 99.9% of all organisms
- used when treating immunocompromised patients
MIC
Minimum inhibitory concentration
- amount of antibiotic necessary to inhibit growth of an organism
- most commonly used in vitro technique
Rule of thumb
- for bactericidal antimicrobials, the MBC is within 1 or 2 dilutions of the MIC
- for bacteriostatic antimicrobials the MBC is often > 100X the MIC
Tube dilution MIC
Broth inoculated with bacteria
- turbidity = growth
- looking for lowest concentration that shows no growth
MIcrotiter panel
- column: different dilutions
- row: different antibiotics
- looking for MIC
Etest
Contains more dilutions than microtiter, leading to a more precise MIC
- where the ellipse stops is the MIC
Kirby Bauer Diffusion
Looking for zone of inhibition
- diameter does not determine resistance/susceptibility!! Is based on molecular weight, etc
What are the 4 interpretations of MICs
- S: susceptible
- I: intermediate
- R: resistant
- NI: no interpretation
Susceptible
>
Resistant
Breakpoint panel
Does not test the whole range
- only looking at 3 dilutions (R, S, I)
- similar to Kirby Bauer
- does not determine how resistant or susceptible something is (qualitative)
MIC has advantages in the following:
- tracking resistance patterns over time
- when the PK-PD relationship is known
______ is quantitative, while _____ and ______ are qualitative
MIC; KB and BP
______ and ______ suffice for most clinical situations
KB and BP
What are the 3 pharmacodynamic of bacterial killing
- concentration-dependent killing
- area under the inhibitory curve (AUIC)
- time-dependent killing
Concentration dependent killing
You want a high peak concentration, looking at how high a drug goes above the MIC
- aminoglycosides
Time dependent dynamic
Looking at time above MIC
- beta lactams
AUC/MIC dynamic
Fluoroquinolones
Enteric gram neg bacteria
- E coli
- Klebsiella
- Proteus
- Salmonella
- Enterobacter
- Citrobacter
- Serratia
Nonenteric gram neg bacteria
- Pasteurella
- Mannheimia
- Haemophilus
- Bordetella
