Intro to antibiotics (2 lectures)

Question 1

Sites and their fluids that are considered sterile

Answer 1

Bloodstream	Blood
Subarachnoid space	CSF
Pleural space	Pleural fluid
Peritoneum	Peritoneal fluid
Pericardium	Pericardial fluid
Synovium	Synovial fluid
Urinary tract	Urine (directly from bladder)

anything that grows in these sites is a pathogen/infection

Question 2

Contamination

Answer 2

organism inadvertently introduced during specimen collection or processing

ex. Coag-neg staph in blood of patient

Question 3

Colonization

Answer 3

organism is present at body site but is not invading host tissue or inducing signs and symptoms of infection

ex. Pseudomonas aeruginosa in the sputum of a patient who is not displaying signs of pneumonia
Infection

Question 4

Infection

Answer 4

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

ex. Streptococcus pneumoniae in sputum of patient with fever, productive cough, and shortness of breath

Question 5

Antibiotics may be chosen before results of the cultures are available based on what information

Answer 5

Site of infection and likely causative organism
Gram-stain result (does result correlate with most likely suspected organism?) (in the CSF gram stain may give an exact diagnosis)

Question 6

empiric therapy vs. directed or targeted therapy

Answer 6

empiric therapy - Antibiotics are chosen that have activity against likely pathogens

direct therapy - Antibiotic regimen is modified once culture and susceptibility results are available

Question 7

Spectrum 
of Activity

Answer 7

Spectrum of activity is the general list of bacteria that are killed or inhibited by an antibiotic

• Established during early clinical trials of the antibiotic
• Recent local, regional and national susceptibility patterns of each bacteria should be continuously evaluated because antibiotic activity may change over time due to emergence of resistance

Question 8

narrow vs broad spectrums of activity

Answer 8

Narrow Spectrum = antibiotic has activity against a limited group of bacteria

Broad Spectrum = antibiotic has activity against a wide variety of bacteria

Question 9

Appropriate antimicrobial therapy depends on

Answer 9

knowledge of:
 the potential site of infection; 
the infecting pathogen(s); 
the expected activity of the antibiotic(s) against the infecting pathogen(s); 
and host characteristics. 

Therefore, appropriate diagnosis is crucial.

Question 10

Minimum Inhibitory Concentration or MIC

Answer 10

lowest concentration of an antibiotic that inhibits visible bacterial growth (unaided eye) of a bacteria after 18
to 24 hours of incubation

Question 11

Minimum Bactericidal Concentration or MBC

Answer 11

the lowest concentration of an antibiotic that results in a decrease of > 99.9% of the bacterial inoculum
(MIC

Question 12

Susceptibility Breakpoints

Answer 12

based on the MIC of an antibiotic for a specific bacteria

Susceptible - organism will most likely be eradicated with normal doses of the antibiotic
Intermediate - treatment may be successful using maximal doses of the antibiotic
Resistant - MIC exceeds usual serum concentrations of the antibiotic, so less than optimal results are expected

Question 13

Susceptibility Breakpoints are based on

Answer 13

• Pharmacokinetics of the drug - achievable serum and tissue concentrations
• General activity of the antibiotic
• Site of infection
• Data from clinical efficacy trials

In general, MIC values should not be compared between different antibiotics

Interpretive guidelines for S, I and R for each antibiotic and each bacteria are different

Question 14

how is the MIC determined

Answer 14

Macrodilution, microdilution, Disk Diffusion – Kirby Bauer, E-Test®, and PCR

Question 15

Macrodilution

Answer 15

Macrodilution - two-fold serial dilutions of an antibiotic are incubated with a standard inoculum of the infecting bacteria in test tubes
• MIC = first tube without visible growth
• Results yield the EXACT MIC of the antibiotic against the infecting organism
• Not routinely performed because it is labor and resource intensive

Question 16

MBC from macrodilution

Answer 16

(not routine) – extension of macrodilution test; MBC represents the antibiotic concentration that kills bacteria (no colonies are present on agar plates)

Question 17

Microdilution

Answer 17

microtiter plates or cassettes with (already prepared) serial dilutions of several antibiotics tested at the same time
• Automated; most common method
•Due to size restraints of cassettes, not all concentrations of an antibiotic can be tested for susceptibility
• Result may be reported as an MIC range, such as ≤ 8 µg/ml Susceptible
• Vitek,Microscan

Question 18

Disk Diffusion – Kirby Bauer

Answer 18

QUALITATIVE test of the in vitro activity of an antibiotic; MIC is not determined
• Filter paper disks impregnated with a fixed concentration of an antibiotic are placed on agar plates inoculated with a standard concentration of the infecting bacteria
• Clear zone of inhibition is observed around the disk - bacteria only grows where concentrations of the antibiotic are below those required to inhibit bacterial growth
• Zone diameters (in mm) correlate with S, I, R

Question 19

E-Test®

Answer 19

Combines quantitative benefits of broth dilution with the ease of disk diffusion; yields an MIC
• Plastic strip impregnated with known, predefined concentration gradient of antibiotic is placed on agar plates • inoculated with standard concentration of infecting bacteria
• Clear elliptical zone of inhibition is observed around strip - bacteria only grow where concentrations are below those required to inhibit bacterial growth
• MIC = where ellipse crosses the strip

Question 20

Hospital Antibiograms

Answer 20

• Susceptibility data for most common bacteria isolated in hospital annually
• Helps guide choice for empiric antibiotic therapy before bacteria has been identified

Question 21

Empiric therapy

Answer 21

Antibiotics are administered that have activity against the predicted or most likely pathogens causing a patient’s infection based on the signs and symptoms of infection. The site of infection may or may not be known, and the culture results are pending, negative, or unobtainable.

• Therapy chosen based on DOC for most likely organism and regional susceptibility patterns
• Given until culture and susceptibility results for infecting bacteria are available

Question 22

Directed or targeted therapy

Answer 22

antibiotics are selected to treat documented/established infections
• Antibiotic selected based on results of susceptibility studies → change to a more narrow spectrum agent
• Given for predefined duration of therapy

Question 23

Prophylactic therapy

Answer 23

antibiotics given to prevent the development of infection
• Therapy chosen based on DOC for most likely organism and regional susceptibility patterns
• Administered for as long as patient is at risk

examples: antibiotics given during surgery or dental procedures,

Question 24

Combination therapy

Answer 24

Broaden bacterial coverage to cover all organisms causing infection
Decrease the emergence of resistance
Take advantage of synergy when antibiotics are used together

Question 25

drug Synergy

Answer 25

activity of the antimicrobial combination is greater than expected from the additive activity of the individual antibiotics

(A + B) > A + B

Question 26

Additive

Answer 26

activity of the combination is no greater than the sum of the effects of each individual agent
(A + B) = A + B

Question 27

Antagonism

Answer 27

activity of the combination is less than expected from the additive activity of the individual agents
(A + B)

Question 28

Bacteriostatic

Answer 28

antibiotic that inhibits bacterial growth; killing depends on host defense mechanisms
• Disadvantages: in setting of inadequate host defenses, any partially inhibited organisms may survive, replicate, and produce recurrent infection when the antibiotic is discontinued (or serum concentrations fall below MIC)
• Examples include macrolides, ketolides, tetracyclines, glycylcyclines, sulfonamides, clindamycin, Synercid, linezolid

Question 29

Bactericidal

Answer 29

antibiotic kills bacteria; less dependent on host defense mechanisms
• Preferable if the host immune system is compromised or does not function well (e.g. neutropenic, immunosuppressed)
• Required for the treatment of meningitis, endocarditis, osteomyelitis, febrile neutropenia
• Examples: penicillins, cephalosporins, carbapenems, aztreonam, fluoroquinolones, aminoglycosides, vancomycin, daptomycin, Bactrim®, metronidazole

Question 30

Cell wall synthesis inhibitors are bactericidal or bacteriostatic

Answer 30

bacteriocidal

Question 31

Protein synthesis inhibitors are bactericidal or bacteriostatic

Answer 31

bacteriostatic

except aminoglycosides- bacteriocidal

Question 32

nucleic acid synthesis inhibitors are bactericidal or bacteriostatic

Answer 32

bacteriocidal

Question 33

metabolic inhibitors are bactericidal or bacteriostatic

Answer 33

bacteriocidal

Question 34

Pharmacodynamics (PD)

Answer 34

is the study of the time course or rate of bacterial killing relative to serum concentrations. The study of pharmacodynamic provides a rational basis for optimizing dosing regimens by describing the relationship between drug, host, and antimicrobial effect by integrating both pharmacokinetic and MIC data.

Question 35

Concentration-dependent

Answer 35

the higher the serum concentration of the antibiotic, the more rapid and extensive the degree of bacterial killing.
• Concentration-dependent agents also appear to have prolonged persistent effects (post antibiotic effects or PAE) that allow for infrequent dosing.

Question 36

Post-Antibiotic Effect

Answer 36

PAE is the time it takes for a bacteria to regrow once serum concentrations of the antibiotic have dropped below the MIC
• Duration of PAE is drug and organism specific
• Agents with long PAEs can be dosed to allow serum concentrations to fall below the MIC
• Gram-positive bacteria - all antibiotics have some PAE; 2 hours for β-lactams
• Gram-negative bacteria - prolonged PAEs with protein or nucleic acid synthesis inhibitors such as aminoglycosides and fluoroquinolones

Question 37

Time Dependent Killing

Answer 37

Killing depends on time of exposure above the MIC (Not higher serum concentrations)

must give the next dose right away when it is predicted that the concentration will fall below MIC

Question 38

Renal and hepatic function

Answer 38

diminished function may lead to drug accumulation and undue toxicity - adjust doses!
• Antibiotics primarily eliminated by kidneys include most β-lactams, vancomycin, the aminoglycosides, some FQs, Bactrim, daptomycin, tetracycline
• Antibiotics primarily eliminated by the liver include the macrolides, Synercid, linezolid, clindamycin, metronidazole, some FQs, Bactrim, doxycycline, tigecycline