Introduction to Antibiotics

Question 1

Normal flora

Answer 1

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

Question 2

Contamination

Answer 2

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

Question 3

Colonization

Answer 3

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

Question 4

Signs and Symptoms of infections

Answer 4

Pathogenic organism is damaging host tissue and eliciting signs and symptoms of infection

Question 5

Likely pathogens based on site of infections

Answer 5

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

Question 6

Susceptibility testing

Answer 6

Susceptiblility testing: laboratory tests organisms recoved in culture to determine if antibiotics will be effective

Question 7

Minimum inhibitory concentration (MIC)

Answer 7

Lowest concentration of an antibiotic that inhibits visible bacterial growth

Question 8

Minimum bactericidal concentration (MBC)

Answer 8

Lowest concentration of an antibiotic that kills bacteria; decreases bacterial inoculum by 99.9%

Question 9

Susceptibility Breakpoints

Answer 9

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

Question 10

Susceptible

Answer 10

Organism will most likely be eradicated with normal doses of the antibiotic

Question 11

Intermediate

Answer 11

Treatment may be successful using maximal doses of antibiotic

Question 12

Resistant

Answer 12

MIC exceeds usual serum concentrations of the antibiotic, so less than optimal results are expected

Question 13

MIC Determination - macro broth dilution

Answer 13

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

Question 14

MIC Determination - microdilution

Answer 14

Microtiter plates with serial dilutions of several antibiotics are tested at the same time
Automated
Most common method
Results are reported in ranges

Question 15

MIC Determinations - Disk diffusion/Kirby Bauer

Answer 15

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

Question 16

Methods for MIC Determination - E-Test

Answer 16

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

Question 17

Pharmacodynamics of anti-infectives

Answer 17

Pharmacokinetics: Concentration vs Time
Pharmacodynamics: Concentration vs effect
Pk/PD: Effect vs time

Question 18

Post-antibiotic effect (PAE)

Answer 18

TIme it takes for a bacteria to regrow once serum concentrations of the antibiotic have dropped below the MIC
Drug and organism specific

Question 19

Concentration dependent killing

Answer 19

Higher serum concentrations = more rapid and extensive killing
Prolonged persistent (post-antibiotic) effect

Question 20

Time dependent killing

Answer 20

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

Question 21

Antibiotic regimen selection

Answer 21

Severity of infection, site of infection, infecting organism

Allergies, age, pregnancy, or nursing, disease states, other drug therapies

Question 22

Bacteriostatic

Answer 22

Antibiotic that inhibits bacterial growth, killing depends on host defense
Ex: macrolides, ketolides, tertracyclines, glycylcyclines, sulfonamides, clindamycin, Synercid, linezolid

Question 23

Bactericidal

Answer 23

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

Question 24

Abx primarily eliminated renally

Answer 24

B-lactams, naco, aminoglycosides, some FQs, Bactrim, daptomycin, tetracycline

Question 25

Abx primarily eliminated by hepatically

Answer 25

Macrolides, Synercid, linezolid, clindamycin, metronidazole, some FQs, Bactrim, doxycycline, tigecycline