Introduction to Antibiotics

Question 1

What is the most important thing to do before the administration of antibiotic therapy?

Answer 1

Bacterial infection MUST be confirmed

Question 2

What is the normal flora?

Answer 2

The human body harbors a number of microorganisms that colonize certain body systems called “normal flora”, which are normally harmless bacteria that occur naturally on the skin, and in the respiratory, gastrointestinal, and genitourinary tracts.

Question 3

What can happen to the normal flora of patients hospitalized for more than 48 hours?

Answer 3

Patients who are hospitalized for more than 48 hours can have their usual normal flora replaced by the “normal flora” of the hospital, which tend to be gram-negative aerobes.

Question 4

What are the sites of normal flora colonization?

Answer 4

Skin
GI Tract
GU Tract
Respiratory Tract

Question 5

What are the normal sterile sites?

Answer 5

Blood
CSF
Pleura 
Synovium
Bone
Urine

Question 6

What is contamination?

Answer 6

Contamination – an organism is introduced into the clinical specimen during the sample acquisition process

Question 7

What is colonization?

Answer 7

Colonization – an organism is present at a body site but is not invading host tissue or eliciting host responses.

Question 8

What is infection?

Answer 8

Infection – a pathogenic organism is present at a body site and is damaging host tissues and eliciting host responses and symptoms consistent with infection.

Question 9

What bacteria is common as a skin pathogen?

Answer 9

Staphylococcus aureus

Question 10

What bacteria is common as a respiratory tract infection?

Answer 10

Streptococcus pneumoniae

Question 11

What bacteria is common as a urinary tract infection?

Answer 11

Escherichia coli

Question 12

Narrow Spectrum

Answer 12

Narrow Spectrum: the antibiotic has activity against a limited group of bacteria (e.g., penicillin has activity against some gram-positive and gram- negative cocci, but not gram-negative bacilli).

Question 13

Broad Spectrum

Answer 13

Broad Spectrum: the antibiotic has activity against a wide variety of bacteria, such as gram-positive and gram-negative bacteria (e.g., imipenem has activity against gram-positive and gram-negative aerobes and anaerobes).

Question 14

Minimum Inhibitory Concentration

Answer 14

MIC– the lowest concentration of an antibiotic that prevents visible growth (unaided eye) of a bacteria after 18 to 24 hours of incubation

Question 15

Minimum Bacteriacidal Concentration

Answer 15

MBC – the lowest concentration of an antibiotic that results in a decrease of > 99.9% of the bacterial inoculum (MIC less than MBC)

Question 16

Susceptibility Breakpoints

Answer 16

Susceptibility Breakpoints – interpretive guidelines established by the Clinical and Laboratory Standards Institute (CLSI) that categorize the MIC values or zone sizes for each antibiotics against each bacteria

Question 17

Susceptible (S)

Answer 17

Susceptible (S) – organism will most likely be eradicated during treatment of infection using normal doses of the specified antibiotic; concentrations of the antibiotic represented by the MIC are easily achieved in patient’s serum with usual doses.

Question 18

Indeterminate (I)

Answer 18

Intermediate (I) – results are considered equivocal or indeterminate; MICs are higher, and treatment may be successful when maximum doses are used or if the drug concentrates at the site of infection.

Question 19

Resistant (R)

Answer 19

Resistant (R) – indicates less than optimal results are anticipated if the particular antibiotic is used; the MIC exceeds usual serum concentrations (even if maximal doses are used).

Question 20

Can MIC values be compared between drug classes and why?

Answer 20

Susceptibility breakpoints differ for each antimicrobial drug class and even between antibiotics within the same drug class – therefore, MIC values often cannot be compared between antibiotics.

Question 21

What is Broth Dilution used for?

Answer 21

Broth Dilution (macrodilution with test tubes, microdilution with automated microtiter plates or cassettes) – a quantitative determination of the in vitro activity of an antibiotic since an exact MIC or MIC range can be determined

Question 22

What are macrodilutions?

Answer 22

Macrodilution testing employs two-fold serial dilutions of an antibiotic (based on achievable serum concentrations after usual doses) incubated in test tubes with a standard inoculum of the patient’s infecting bacteria; the exact MIC of the antibiotic is the first tube without visible growth; labor and resource intensive.

Question 23

What are microdilutions?

Answer 23

Microdilution methods employ microtiter plates or cassettes that contain wells with serial dilutions of several antibiotics that can be tested for susceptibility simultaneously in an automated system.

Question 24

How can MBC be determined using macrodilutions?

Answer 24

Among the clear vials, they will be plated to see what the amount of bacteria remaining in each was to see where the concentration of antibiotic was able to kill 99.9% of the bacteria.

Question 25

What is the Disk Diffusion (Kirby Bauer) method used for?

Answer 25

Disk Diffusion (Kirby Bauer Method) – a qualitative determination of the in vitro activity of an antibiotic where:

a. Filter paper disks impregnated with a fixed concentration of an antibiotic are placed on agar plates inoculated with a standardized inoculum of the patient’s infecting bacteria.
b. Bacteria multiply on the plate while antibiotic diffuses out of the disk; bacterial growth occurs only in areas where drug concentrations are below those required to cause inhibition of bacterial growth.
c. A clear zone of inhibition is then observed around the disk - the larger the diameter, the more active the drug against the bacteria.

Question 26

What is the E Test?

Answer 26

It can combine the quantitative effects of the broth dilution method with the qualitative method of disk diffusion.

A plastic strip impregnated with a known, prefixed concentration gradient of antibiotic is placed on an agar plate with a standardized inoculum of the patient’s infecting bacteria. A zone where no bacteria will grow will be formed starting at some level of the strip and increase in diameter going up from there.

Question 27

What are hospital antibiograms?

Answer 27

The susceptibility data in an antibiogram is typically used to help guide the choice of empiric antibiotic therapy before the infecting organism has been identified in the lab. Clinicians use the antibiogram to determine the most active antibiotic against specific organisms at that specific institution.

Question 28

What is empiric therapy?

Answer 28

Empiric Therapy – 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.

Question 29

What is targeted therapy?

Answer 29

Directed or targeted therapy – antibiotics are used to treat an established infection where the site of infection, causative pathogen, and antibiotic susceptibilities are known.

Question 30

What is prophylactic therapy?

Answer 30

Prophylactic Therapy – antibiotics are given to prevent the development of infection during a procedure or immunocompromised state when there is a considerable risk of infection

Question 31

What are some of the various reasons to use combination therapy?

Answer 31

a. To provide coverage against all organisms in a mixed, polymicrobial infection where a single antibiotic does not cover all of the infecting organisms – used to broaden bacterial coverage.
b. To take advantage of synergistic properties when the antibiotics are used together.
c. To decrease the emergence of resistance – only for tuberculosis.

Question 32

Synergy

Answer 32

Synergy – the activity of the antimicrobial combination is greater than that expected from the additive activity of the individual antimicrobials

Question 33

Additive

Answer 33

Additive – the activity of the antimicrobial combination is no greater than the sum of the effects of each individual component

Question 34

Antagonistic

Answer 34

Antagonism – the activity of the antimicrobial combination is less than that expected from the additive activity of the individual antimicrobials

Question 35

Bacteriostatic

Answer 35

Bacteriostatic – antimicrobial agents that inhibit the growth of susceptible bacteria and rely on host defenses to help kill the bacteria and subsequently
eradicate the infection

Question 36

Bactericidal

Answer 36

Bactericidal – antimicrobial agents that kill susceptible bacteria in the absence of host defenses

Question 37

Concentration-Dependent Activity

Answer 37

Concentration-dependent – 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 38

Time-Dependent Activity

Answer 38

Concentration-independent (time-dependent) – higher serum concentrations of the antimicrobial do not produce enhanced bacterial killing. The extent of bacterial killing is largely dependent on the time of exposure. These agents are not rapidly bactericidal, and typically have a short or nonexistent PAE.

Question 39

Post-Antibiotic Effect

Answer 39

Post-Antibiotic Effect (PAE) – the time it takes for a bacteria to recover after exposure to an antibiotic, or the time it takes for bacteria to recover and begin regrowth after an antibiotic has been removed.