Lesson 12

Question 1

Organisms susceptible
to concentrations of antibiotic that are achievable in the blood of patients with normal dosing schedules

Answer 1

Susceptible

Question 2

Organisms that have an MIC higher
than the level of drug that is achievable in the blood of patients with a normal dosing regime.

Answer 2

Resistant

Question 3

Organisms that have a MIC to a
drug that is borderline between sensitive and resistant

Answer 3

Intermediate

Question 4


Mueller-Hinton broth with an antimicrobial

Determine the MIC: Absence of turbidity

Method: Serial dilutions

Standard inoculum size: 5 x 105 CFU/mL

MH Broth + 2%
NaCl:
Oxacillinresistant
staphylococci

MH Broth + 2-
5% Lysed Horse
Blood:
streptococci,
Neisseria
meningitides

Answer 4

Broth Dilution Method

Question 5

Reference test for anaerobes and gonorrheae

Only used in research

Media:
Aerobes: MHA
Anaerobes: Brucella-laked sheep blood
agar (Wadsworth method)
Fastidious bacteria: MHA + 5% Sheep’s
Blood
MRSA: MHA + NaCl

Standard inoculum size: 1 x 104 CFU/spot

Incubation: 35 ̊C for 48 hours

Shelf life: 1 week

Color of uninoculated agar:
MHA: Light yellow
MHA with sheep’s blood: Red

Answer 5

Agar Dilution Method

Question 6

Only for aerobic and
facultatively anaerobic
bacteria

Testing of 6-12
antimicrobial agents (90-
150 mm plates)

Zone of inhibition: Point
at which no growth is visible to the unaided eye
–
↑ Zone of Inhibition = ↓ MIC ■

Medium: Mueller Hinton Agar

Standard Inoculum size: 1.5 x 105 CFU/mL

Answer 6

Disk Diffusion Method (Kirby-Bauer Method)

Question 7


Best medium to use for routine susceptibility
testing of non-fastidious organisms because:
Shows acceptable batch-to-batch
reproducibility for susceptibility
testing
Low in sulfonamides, trimethoprim, &
tetracycline inhibitors
Supports growth of bacteria

Optimal thickness: 4 mm

Answer 7

Mueller Hinton Agar (MHA)

Question 8

Reportable Pathogens under the National Antimicrobial
Resistance Surveillance Program (ARSP)

Answer 8

Enterococcus faecium

Enterococcus faecalis

Staphylococcus aureus

MRSA

Streptococcus pneumoniae

Acinetobacter baumanii

Escherichia coli

Haemophilus influenzae

Klebsiella pneumoniae
Neisseria gonorrhoeae

Pseudomonas aeruginosa

Salmonella enterica

Non-typhoidal Salmonella

Shigella

Question 9

Checking for resistance to most beta-lactam antibiotics

Penicillin

Cephalosporins

Monobactam Aztreonams

Answer 9

ESBL Testing for Enterobacteriaceae

Question 10

Infecting organism respond to therapy with the antimicrobial agent

Answer 10

Susceptible

Question 11

• Microorganism
falls into a
range of
susceptibility
• MIC approaches
or exceeds the
level of
antimicrobial

Answer 11

Intermediate

Question 12

• Microorganism
is not inhibited
by the usually
achievable
concentrations
of the drug
• Antibiotic not
the appropriate

Answer 12

Resistant

Question 13

• Microorganism
is not inhibited
by the usually
achievable
concentrations
of the drug
• Antibiotic not
the appropriate

Answer 13

Resistant

Question 14

Recommended Incubation Requirements:

Answer 14

1. 35C for 16-18hours(mostbacteria)
2. 35 C for 20-24 hours (N. gonorrhoeae)
3. Increased CO2 (5-7%): N. meningitidis, S. pneumoniae

Question 15

Principle: Continuous concentration gradient of
an antimicrobial agent from a plastic strip into
an agar medium

Less time-consuming than broth dilution but more expensive

Reagent: Thin plastic strip impregnated under
surface with an antimicrobial gradient

Positive: Ellipse of growth inhibition

For Anaerobic bacteria and S. pneumoniae and H.
influenzae: Special enriched media

Answer 15

Epsilometer Test

Question 16

In vitro determination of the amount of antimicrobial agent required to kill as well as inhibit bacteria.
Used for: Treatment failure indicator as a result of an organism’s MBC exceeding the serum
level of an antimicrobial.

Standard Inoculum: 5x105 CFU/mL

Final Dilution: 1:1000 (0.01 mL from the original tube in 10 mL)

Volume spread per plate: 0.1 mL

Dilution Factor: 1:10,000 (104)

Answer 16

Minimum Bactericidal Concentration

Question 17

Determine the effectiveness of combined antimicrobials against a single bacterial isolate.

Answer 17

Synergy Test

Question 18

Principle: Turbidimetry

Susceptible: Lack of turbidity

Resistance: Increase in turbidity

Fluorometer: Also used for growth detection

Advantage: Shortened analysis (5-15 hrs)

Examples: BD Phoenix System, MicroScan WalkAway,
TREK Sensititre, VITEK 1 & 2

Answer 18

Automated Antimicrobial Susceptibility Test (AAST)

Question 19

What do you need to remember on a thawed Antimicrobial Solution?

Answer 19

Do not refreeze it!

Question 20

MIC is dependent on?

Answer 20

pH, Incubation temperature, amount of
inoculum, incubation time