2. Antibiotics: Choosing the Best Antibiotic

When deciding which antibiotic to use in each situation, what is the first question you should ask yourself?

is this a bacterial infection that needs antibiotics or is this a viral infection that will not respond to antibiotics?

What acronym can be used to determine the best antibiotic in a bacterial infection?

SPACED

What if it is an uncomplicated viral infection? What can be done? What is important to know about viral infections?

there are very few therapeutic options that will directly kill the virus; supportive therapy which may include anti-inflammatories; some viral infections set the stage for secondary bacterial infections

What does the S stand for in SPACED? What question does it help us to answer?

spectrum; will the infectious agent be harmed by the antibiotic that I have chosen?

Some antibiotics have a ________ spectrum while others have a ________ spectrum.

wide; narrow

What is spectrum sometimes dictated by? What else can it also be influenced by?

the bacterial molecule(s) that is/are targeted by the antibiotic; the ability of the antibiotic to penetrate the bacteria

What is spectrum related to in terms of the actual bacteria?

Gram status and oxygen tolerance

What is a consequence of the overuse of broad-spectrum antibiotics?

can inappropriately harm the gut microbiome

What are broad-spectrum drugs more likely to foster?

antibiotic resistance

What does the P stand for in SPACED? What questions can it help us to answer?

W
I
W

pharmacokinetic/pharmacodynamic (PK/PD)

• will the antibiotic reach the site of infection?

• if it does reach the site of infection, will the drug concentration at the site of infection be greater than the minimum inhibitory concentration (MIC) for the bacteria?

• what is the relative volume of distribution of the antibiotic?

Antibiotics with what type of V_d will not be able to reach privileged sites?

a low V_d

What are the privileged sites?

C
P
E
T
J
A

• CNS/brain

• prostate

• eye

• testes/ovaries

• joints/bones

• abscesses

What are the bactericidal antimicrobials?

B
A
N
F
F

• b-lactams

• aminoglycosides

• nitrofurans/imidazoles

• fluoroquinolones

Bacteriostatic antibiotics ________ the growth of the bacteria, and thus the immune system must ________ the bacteria before the bacteria ________ and ________.

inhibit; clear; recover; regrow

What does the A in SPACED stand for? What does it tell about?

adverse; adverse events associated with the antibiotic chosen and the species in which it will be used

What does the C stand for in SPACED? What are some questions it answers?

W
A

compliance

• will the owners be able to comply with the at-home/at-farm administration of the drug?

• am i complying with all regulatory rules regarding this antibiotic?

Which antimicrobial is illegal for extralabel use in food animals? Why?

chloramphenicol; causes aplastic anemia in humans

Which drug has cardiotoxic effects in humans on accidental injection?

tilmicosin

What does the E stand for in SPACED? What question does it help to answer?

environment; what is the environment of the infection?

Which drugs should not be used in anaerobic environments? Why?

A
A
F
S

• aminoglycoside

• aminocyclitols

• fluoroquinolones

• sulfonamides

they need O2

Which drugs should not be used in aerobic environments? Why?

nitroimidazoles; inhibited by O2

Which drugs should not be used in pus and debris?

S
A

• sulfa drugs

• AGs

What drugs should not be used in an environment high in cations (Ca, Al, Mg)? Why?

Q
T

• quinolones

• tetracyclines

they are inhibited by milk and antacids

What are the drugs of choice for privileged sites?

Q
M
C
F

• quinolones

• macrolides

• CHPC

• florfenicol

What does the D in SPACED stand for? What question does it help to answer?

diagnostics; what does the laboratory data show?

What is the purpose of a kirby-bauer sensitivity disk?

shows sensitive vs. resistant bacteria

The clinical laboratory will provide you with ________ versus ________ information that is predicted based on the ________ of the pathogen isolated from your sample and CLSI-established ________ values.

sensitive; resistant; MIC; breakpoint

concentration of a drug selected to predict clinical outcome for a specific pathogen in a specific disease, in a specific species, given a specific regimen

breakpoint

If the MIC is greater than the breakpoint (breakpoint is to the left/lower of the MIC), what does this mean for the pathogen?

it is resistant

If the MIC is less than the breakpoint (breakpoint is to the right/higher of the MIC), what does this mean for the pathogen?

it is sensitive

Where is the MIC of a pathogen determined?

in the lab

True or false: The breakpoint uses the laboratory MIC to predict efficacy out in the field.

true

group of clinical laboratory people who examine clinical outcomes and MIC data to determine breakpoints and periodically updates the breakpoints based on new outcome data

CLSI