L26 - O'Keefe - Intro to Antibiotics Part 1 Flashcards
To diagnose infection, what 4 topics are beneficial to know?
- site of infection
- infecting pathogen
- spectrum of activity of antibiotic
- host characteristics
If you isolate an organism, does that mean that there’s an infection?
No
What are normal flora?
Harmless bacteria that are naturally found and suppress growth of pathogenic bacteria
Can normal flora become pathogenic?
Yes, if host defenses are impaired or disrupted
Define Contamination
When a organism is accidentally introduced during collection or processing of a specimen
Give an example of contamination
Coag-neg staph in blood of patient
Define colonization
when an organism is present but not invading the host or inducing signs/symptoms of infection
Give an example of colonization
Psudomonas aeruginosa in patient’s sputum but they aren’t displaying signs of pneumonia
Define infection
pathogen is damaging host tissue and eliciting signs/symptoms of an infection
Give an example of infection
Strep pneumoniae in sputum of patient who has fever, cough, and shortness of breath
What bacteria are associated with throat infections?
Group A Strep
What bacteria are associated with infections in urine?
E. Coli
What bacteria are associated with skin abscesses?
S. aureus (or group a strep)
What bacteria are associated with lung infections?
pneumococcus (aka strep pneumoniae)
Define empirical antibiotic therapy
when antibiotics are chosen to be given based on preliminary info (before culture results are available)
- antibiotics chosen have activity against likely pathogens
- *this is done to try to avoid morbidity/mortality due to a delay in treatment
What preliminary info is used to determine empirical antibiotic therapy?
Site of Infection
Likely causative organism
Gram-stain result
Is normal flora found everywhere in the host?
No, some body sites are sterile (see lecture for a list)
What is the purpose of susceptibility testing?
To see if the proper antibiotic was chosen for empiric therapy
What is directed or targeted therapy?
When the antibiotic regiment is modified once culture and susceptibility results are available
*antibiotics are selected to treat documented infections (organism and susceptibility are known)
Define antimicrobial spectrum of activity. What are the 2 possible spectra?
General list of bacteria killed or inhibited by an antibiotic
*established during early clinical/pre-clinical trials and could change with the emergence of resistance
Narrow or Broad spectrum
What is Narrow spectrum?
antibiotic has activity against a LIMITED group of bacteria
Give an example of a narrow spectrum antibiotic used to treat Methacillin resistant bacteria
Nafcillin
What is broad spectrum?
antibiotic with activity against a WIDE variety of bacteria
Give an example of a broad spectrum antibiotic class that is used for anaerobes, all gram positive bacteria, etc.
Carbapenem
What is MIC?
Minimum inhibitory concentration
lowest concentration of antibiotic that inhibits visible bacterial growth (ex: in broth culture system where we can see growth in tubes)
What is MBC?
minimum bactericidal concentration
lowest concentration that kills bacteria
*decreases bacterial inoculum by 99.9% (3 logs)
What is the relationship between MIC and MBC?
MIC is less than or equal to MBC
*so it takes more antibiotic to kill an organism than to inhibit it’s growth
Define susceptibility breakpoint
interpretive guidelines by CLSI that categorize antibiotic MIC values as susceptible, intermediate, or resistant (SIR)
Why are the interpretive guidelines for SIR different for each antibiotic and bacteria?
they are based on PK (achievable serum/tissue conc)
depends on site of infection
depends on antibiotic activity
*thus…MIC values should NOT be compared for diff antibiotics
Define Susceptible
organism is most likely eradicated with normal doses of antibiotic –> is “sensitive”
Define Intermediate
treatment MAY b successful using max doses of antibiotic
ex: urine infection where antibiotic gets concentrated in that one region
Define Resistant
MIC exceeds usual serum concentration of antibiotic, so less than optimal results are expected
What are methods that can be used to determine the in-vitro activity of an antibiotic?
Quantitative determination of MIC: Broth dilution (macro and micro) and E test
Qualitative: Kirby Bauer Disk Diffusion
(*note: zone diameters are correlated to S and R but you can’t determine the exact MIC)
Describe macrodilution (process)
2-fold serial dilutions of antibiotic are incubated with standard inoculum of bacteria in test tubes
MIC = 1st tube without visible growth)
Is macrodilution used commonly?
No because it’s labor and resource intensive
What is not typically determined by macrodilution?
MBC (if you were to determine it, it would be via growing colonies on agar plates and finding the one where there’s no growth)
Describe the microdilution process
microtiter plates/cassettes with serial dilutions of several antibiotics are tested simultaneously (1st clear well = mic)
- is an automated process and the most common!
- note: size restraints limit # of concentrations that can be tested…MIC may be given as a range
How does the Kirby Bauer Disk Diffusion test work?
filter paper disks with a known concentration of antibiotic are put on agar plates with a standard conc/log (or lag??) phase growth of bacteria
clear zone around disk is seen where antibiotic was effective in inhibiting bacterial growth
How does the E test work?
plastic strip containing a gradient of antibiotic concentrations is put on an agar plate that has a standard concentration of bacteria
MIC = where the clear ellipse shaped zone crosses the strip
What is PCR used for in relation to bacteria?
45 minute result from blood culture gives bacteria identification as well as tests probes for specific antibiotic resistance genes (ex: mecA, KPC, vanA/B)
What are susceptibility reports?
Gives antibiotic tested, MIC, and SIR for individual bacteria
*are used for antibiotic selection (directed therapy) but are only an INDICATION of antibiotic activity
What is a hospital antibiogram?
Report with susceptibility data for most common bacteria that are seen in hospitals
*helps guide choices for empiric antibiotic therapy (prior to identification of bacteria)
