Intro to Antibiotics Flashcards
What is the definition of MIC?
- Minimum Inhibitory Concentration: lowest concentration of an antibiotic that inhibits visible bacterial growth
- THINK: “the first clear tube” in series of dilutions
What is a Hospital Antibiogram, and what is its purpose?
- The susceptibility data for the most common bacteria isolated in a hospital annually
- Helps guide the choice for empiric antibiotic therapy before bacteria has been identified
Describe the Micro Broth Dilution method of MIC determination.
-Quantitative test: Micro-titer plates or cassettes with serial dilutions of several antibiotics tested at same time
**Most common method: automated, convenient pre-loaded plates
-Results reported as a MIC range (i.e. less than or equal to 8)
What are the most common pathogens of the following sites: 1. Lung 2. Abdomen 3. Meninges
- Streptococcus pneumoniae (general) and MRSA (hospital acquired)
- E. coli
- Strep. pneumoniae
What are the sterile body sites and their sterile specimens?
- Bloodstream: Blood
- Subarachnoid space: CSF
- Pleural space: Pleural fluid
- Peritoneum: Peritoneal fluid
- Pericardium: Pericardial fluid
- Synovium: synovial fluid
- Urinary Tract: Urine (*debatable: directly from bladder)
Describe the Disk Diffusion-Kirby Bauer method of MIC determination
- Qualitative test: MIC is not determined, but susceptibility characteristics can be attained from results
- Filter paper disks with fixed concentration of antibiotic are placed onto agar plates inoculated with infecting bacteria
- Clear zone of inhibition is observed around the disks where concentrations of antibiotic were greater than/equal to those required to inhibit the bacterial growth
- Zone diameters (in mm) correlate with S, I, R
As a rule, when do you obtain cultures from suspected sites of infection?
BEFORE antibiotics are initiated
What is the definition of infection?
-Pathogenic organism is damaging host tissue and eliciting signs and symptoms of infection
What is the purpose of susceptibility breakpoints?
To categorize specific antibiotic MIC values for a given bacteria
What is the definition of contamination?
- THINK: “from the lab”
- The organism is inadvertently introduced during specimen collection or processing (i.e. coagulase negative Staph from the skin ends up in the blood sample of a patient)
- This does not serve as an indication of infection
What happens in antagonistic combination therapy?
Activity of combination is less than expected from the additive activity of individual agents: (A+B) is less than A+B
What happens in synergistic combination therapy?
Activity of combination is greater than expected from additive activity of individual antibiotics: (A+B) > A +B
Describe the Empiric therapy form of antibiotic use?
- Antibiotics given with activity against MOST LIKELY infecting bacteria
- Chosen based on drug of choice for most likely organism and regional susceptibility patterns
- Given until culture and susceptibility results for infecting bacteria are available
What is the definition of colonization?
- THINK: “there, but no infection”
- The organism is present at the body site, but is not invading host tissue or inducing signs/symptoms of infection (i.e. P. aeruginosa present in sputum of patient in hospital who is not displaying signs of pneumonia)
What type of bacteria do hospitalized patients become colonized with within 48 hours of admission?
Gram negative aerobes
Where are the likely sites of infection for the following pathogens: 1. E. coli 2. S. aureus 3. Group A strep
- Urine (causes UTIs)
- Skin (skin abscess)
- Throat (“strep throat”)
Why are the interpretive guidelines for S, I and R different for each antibiotic and each bacteria?
Because they are based on:
- Pharmacokinetics of the drug
- General activity of the drug
- Site of infection
- Data from clinical efficacy trials
What is the definition of MBC?
- Minimum Bactericidal Concentration: lowest concentration of an antibiotic that kills bacteria (decreases inoculum by 99.9%)
- Typically MIC less than MBC
What does narrow spectrum of antimicrobial activity signify ?
Antibiotic has activity against a limited group of bacteria (i.e. NAF penicillinase-resistant penicillins)
Describe two examples of prophylactic antibiotic therapy.
- Peri-operative surgical prophylaxis
- Decreases chances of wound infection from normal skin flora - Endocarditis prophylaxis:
- Given before dental procedure
- Directed against oral flora
- Decreases heart valve infection
What does broad spectrum of antimicrobial activity mean?
Antibiotic has activity against a wide variety of bacteria (i.e. Carbapenem beta-lactam antibiotics)
What are the three categories for susceptibility breakpoints?
- Susceptible: organism will most likely be eradicated with normal doses of antibiotic
- Intermediate: treatment may be successful using maximal doses of antibiotic
- Resistant: MIC exceeds usual serum concentrations of antibiotic, so less than optimal results are expected
Describe the Macro Broth Dilution method of MIC determination.
- Quantitative test: Two-fold serial dilutions of an antibiotic, incubated with a standard inoculum of the infecting bacteria in test tubes
- THINK: Make varying dilutions of an antibiotic in test tubes, add the bacteria, and see at which concentration the bacterial growth isn’t visible = exact MIC
- Not routinely used: labor and resource intensive
Describe the E-Test method of MIC determination.
- Combines quantitative benefits of broth dilution with ease of disk diffusion: yields a MIC
- Plastic strip with known concentration gradient of antibiotic is placed onto agar plates inoculated with infecting bacteria
- Clear elliptical zone of inhibition is observed around the strip
- Bacteria only grow where concentrations are below those required to inhibit bacterial growth -MIC = where ellipse crosses the strip
What is the purpose of combination antibiotic therapy?
- Broaden bacterial coverage (make sure you get all the organisms causing infection)
- Decrease emergence of resistance
- Synergistic possibilities
Describe the Directed or Targeted therapy form of antibiotic use?
- Antibiotics selected to treat DOCUMENTED/KNOWN infections
- Based on results of susceptibility studies (more narrow spectrum agent)
- Predefined duration of therapy
What are some characteristics of normal flora?
- Usually harmless bacteria
- Occur on skin, respiratory, genitourinary, and gastrointestinal tracts
- Suppress growth of pathogens
- Antibiotic use can alter normal flora
- May become pathogenic if present outside of normal site in the body
