Introduction to Antibiotics Flashcards
Normal flora
Usually harmless bacteria that occur naturall on the skin, respiratory, genitourinay, and GI tract
Suppress growth of pathogenic bacteria
May become pathogenic when host defenses are impaired/disrupted
Antibiotic use can alter the flora
Contamination
Organism inadvertently intorduced during specimen collection or processing
ie Coag-neg staph in blood of patient -> staph is commonly found on the skin and thus there is a chance it could have contaminated the blood during collection
Colonization
Organism is present at body site but not invading host issue or inducing signs of infection
Ie Pseudomonas aeruginosa in the sputum of a patient who is not displaying signs of pneumonia
Signs and Symptoms of infections
Pathogenic organism is damaging host tissue and eliciting signs and symptoms of infection
Likely pathogens based on site of infections
Certain bacterial will cause infection in particular body sites or fluids (ie E.colin in urine, s. aureus in skin wound)
Antibiotics may be chosen before culture results are back based on some preliminary information
Gram stain result can give some information
Susceptibility testing
Susceptiblility testing: laboratory tests organisms recoved in culture to determine if antibiotics will be effective
Minimum inhibitory concentration (MIC)
Lowest concentration of an antibiotic that inhibits visible bacterial growth
Minimum bactericidal concentration (MBC)
Lowest concentration of an antibiotic that kills bacteria; decreases bacterial inoculum by 99.9%
Susceptibility Breakpoints
Interpretive guidelines established by the CLSI, which categorize specific antibiotic MIC values for a given bacteria
SIR (susceptible, intermediate, resistant)
Based on Pharmacokinetics of drug (serum and tissue concentrations), general activity of antibiotic, site of infection, data from clinical efficacy trials
DO NOT COMPARE MIC VALUES BETWEEN DIFFERENT ANTIBIOTICS
Susceptible
Organism will most likely be eradicated with normal doses of the antibiotic
Intermediate
Treatment may be successful using maximal doses of antibiotic
Resistant
MIC exceeds usual serum concentrations of the antibiotic, so less than optimal results are expected
MIC Determination - macro broth dilution
Serial dilutions of an antibiotic are incubated with standard inoculum
MIC = first tube without visible growth
Results yield the exact MIC of the antibiotic against the infecting organism
MIC Determination - microdilution
Microtiter plates with serial dilutions of several antibiotics are tested at the same time
Automated
Most common method
Results are reported in ranges
MIC Determinations - Disk diffusion/Kirby Bauer
Qualitative test of the in vitro activity of an antibiotic
MIC is not determined
Paper disks with fixed concentration of antibiotic are placed on agar plate with infecting bacteria
Clear zone is observed around disk- bacteria grows where concentrations of antibiotic are below those quired to inhibit bacterial growth
Zone diameters may correlate with S, I, R
Methods for MIC Determination - E-Test
Combines quantitative benefits of broth dilution with ease of disk diffusion
Yields an MIC
Plastic strip covered in known concentration gradient of antibiotic
Clear elliptical zone of inhibtion is observed around strip
MIC = where ellipse crosses strip
Pharmacodynamics of anti-infectives
Pharmacokinetics: Concentration vs Time
Pharmacodynamics: Concentration vs effect
Pk/PD: Effect vs time
Post-antibiotic effect (PAE)
TIme it takes for a bacteria to regrow once serum concentrations of the antibiotic have dropped below the MIC
Drug and organism specific
Concentration dependent killing
Higher serum concentrations = more rapid and extensive killing Prolonged persistent (post-antibiotic) effect
Time dependent killing
Killing depends on time of exposure above the MIC, not higher serum concentrations
Drug can only kill above MIC
Have to give the next dose right away because there is usually no/very short PAE
This technique might be utilized in highly-resistant bacteria because by giving same concentrations over a longer period of time the antibiotic will stay in the blood stream and might treat the infection
Antibiotic regimen selection
Severity of infection, site of infection, infecting organism
Allergies, age, pregnancy, or nursing, disease states, other drug therapies
Bacteriostatic
Antibiotic that inhibits bacterial growth, killing depends on host defense
Ex: macrolides, ketolides, tertracyclines, glycylcyclines, sulfonamides, clindamycin, Synercid, linezolid
Bactericidal
Antibiotic that kills bacteria
Required for the treatment of meningitis, endocarditis, osteomyelitis, febrile neturopenia
Ex: penicillins, cephalosporins, carbapenems, aztreonam, fluoroquinolones, aminoglycosides, vancomycin, daptomycin, Bactrim, metronidazole
Abx primarily eliminated renally
B-lactams, naco, aminoglycosides, some FQs, Bactrim, daptomycin, tetracycline
Abx primarily eliminated by hepatically
Macrolides, Synercid, linezolid, clindamycin, metronidazole, some FQs, Bactrim, doxycycline, tigecycline
