Broth microdilution reference methodology

Broth microdilution
reference methodology
Erika Matuschek, PhD
EUCAST Development Laboratory (EDL)
Växjö, Sweden
ICARS - ILRI webinar series
19 April 2022

Standardisation of AST
• Results change with changed parameters.
– It is crucial to adhere to the methodology to get
reproducible and reliable results!
• Standardisation of:
– Potency of antimicrobial agent/disk potency
– Media
• Type of media, supplements, pH, agar depth etc.
– Inoculum
– Incubation
– Reading of results

MIC – the gold standard
Minimum Inhibitory Concentration
• “The lowest concentration that, under defined in
vitro conditions, prevents visible growth of
bacteria within a defined period of time” (ISO
20776-1).
• The MIC recorded as the lowest concentration of
the agent that completely inhibits visible growth.

Broth microdilution
Microtiter plates (≤200 µL)

Broth microdilution methodology
Inoculum 5 x 105 CFU/mL
Media MH broth (non-fastidious organisms)
MH-F broth (Fastidious organisms)
Incubation 35°C in air, 16-20 h (sealed panels)
Reading Read the MIC as the lowest
concentration of antimicrobial agent that
completely inhibits growth of the
organism as detected by the unaided eye
ISO 20776-1, 2019
EUCAST media for fastidious organisms

Broth microdilution reference methodology

Broth microdilution (BMD)
ISO 20776-2, 2019
• Rapidly growing aerobic bacteria
• Fresh or frozen panels (<-60°C)
• Microdilution: ≤200 µL per well
• Standardisation of
– Antimicrobial agents
• Solvents, diluents, 2-fold dilutions
– Inoculum
– Incubation time and temperature
• Referral to EUCAST and CLSI documents for
– Testing of fastidious organisms
– Reading of MIC endpoints

EUCAST recommendations for BMD
• Perform BMD according to the ISO standard
– Final inoculum of 5×105 CFU/mL
– Sealed panels
– Incubation in air, at 35°C for 16-20 h
• 24 h incubation for glycopeptides
• Commercial panels with freeze-dried antimicrobials
are accepted by EUCAST if they produce expected
results
– BUT many commercial products include few dilutions,
resulting in:
• Truncated MICs in both the lower and upper end
• Products difficult to verify and control
Non-fastidious and
fastidious organisms

EUCAST media for BMD
• Broth media
–Non-fastidious organisms
• Un-supplemented Mueller-Hinton broth
(ISO 20776-1)
–Fastidious organisms
• MH-F broth: Mueller-Hinton broth with 5%
lysed horse blood and 20 mg/L β-NAD

Preparation of MH-F broth
• Prepare and autoclave cation-adjusted MHB according to the
manufacturer’s instructions, but with 100 mL less deionized
water per litre to allow for the addition of lysed horse blood.
• Cool medium to 42-45°C.
• Aseptically add 100 mL 50% lysed horse blood and 1 mL β-
NAD stock solution per litre medium and mix well.
• Dispense MH-F broth in sterile containers with screw caps.
• Store MH-F broth at 4-8°C.
– Storage conditions and shelf life should be determined as part of
the laboratory quality assurance programme. A shelf life of 3
months can be expected.
https://www.eucast.org/ast_of_bacteria/media_preparation

Preparation of 50% lysed horse
blood for MH-F broth
• Aseptically dilute mechanically defibrinated horse blood with
an equal amount of sterile deionized water.
• Freeze the blood at -20°C overnight and thaw. Repeat the
cycle until the cells are completely lysed (three cycles is
usually sufficient but the ISO standard 20776-1 suggests that
up to seven cycles may be required).
• Clarify the 50% lysed horse blood by centrifugation and
discard the pellet. A clear solution is essential for reading.
Repeating the centrifugation may improve the clarity of the
solution.
• The stock solution may be stored at -20°C in aliquots and
defrosted as required. Do not refreeze unused solution.
https://www.eucast.org/ast_of_bacteria/media_preparation

Preparation of inoculum
• Final inoculum of 5×105 CFU/mL
• Start with McF 0.5 (~ 1-2 x 108 CFU/mL for
E. coli ATCC 25922)
– Dilutions depend on:
• Volume of broth in wells (Fresh or freeze-dried
panels)
• Volume to be dispensed in each well

Preparation of inoculum
• Example for freeze-dried Sensititre panels with
100 µl dispensing (reconstitution) volume
Organism McF Transfer volume to 11 mL
broth
Non-fastidious Gram-positive
and Gram-negative bacteria
0.5 50 µL
Proteus, Morganella and
Providencia spp.
0.5 10 µL (to reduce problems
with swarming)
Haemophilus influenzae 0.5 50 µL
Streptococcus spp. and other
fastidious Gram-positive cocci
0.5 100 µL
Check manufacturer’s recommendations for commercial plates!

Control of inoculum / Viable counts
• Remove 10 μL from the growth control well
immediately after inoculation
• Dilute in 10 mL of broth or saline
• Spread 100 μL of this dilution over the
surface of a suitable agar plate, which is then
incubated overnight
• 20-80 colonies are expected from an
acceptable test suspension
ISO 20776-1, 2019

Inoculation of BMD plates
• Manually, preferably with a multi-channel
pipette
• Using a dispensing robot
AIM: part of the Sensititre system
from Thermo Scientific

Incubation
• Seal panels with a tight lid or adhesive seal to avoid
evaporation
• To avoid uneven heating, micro-dilution trays should not be
stacked more than four high
– By placing an empty tray at the top of each stack, condensation
on the inside of the adhesive cover is reduced, which facilitates
reading.
• Incubate at 35°C in air for 16-20 h
– Glycopeptides should be read after 24 h incubation
– Prolonged incubation is only allowed when specified in the
EUCAST Breakpoint Table
• Corynebacterium, Aerococcus, Kingella kingae and Campylobacter

Reading BMD endpoints
• Results should be read manually. The use of a mirror
may facilitate reading.
– If an automated reader or camera system is used, it must be
calibrated to manual reading.

Reading BMD endpoints
• Read the MIC as the lowest concentration of
antimicrobial agent that completely inhibits
growth of the organism as detected by the
unaided eye.
• Read MICs only when there is sufficient growth,
i.e. obvious button or definite turbidity, in the
positive growth control.

Growth appearance
• Growth appears as turbidity or as a deposit of cells at the
bottom of the well (i.e. as a pellet). The appearance of
growth differs depending on the microorganism and the
antimicrobial agent tested.
• For round-bottom wells, growth will most often appear as
a button/pellet centered in the middle. For flat-bottom
wells, growth may be scattered.
• Growth in antibiotic-containing wells may differ from
growth seen in the positive growth control, even for pure
cultures.

Trailing endpoints
• Most antimicrobial agent-organism combinations give
distinct endpoints.
• Some agent-organism combinations may give trailing
endpoints with a gradual fading of growth over 2 to 3
wells.
• Unless otherwise stated, endpoints should be read at
complete inhibition of growth.

Reading of BMD panels
Positive
control
Antimicrobial concentration
Unless otherwise stated, endpoints should
be read at complete inhibition of growth.

Turbidity without pellet
• Haze or turbidity without a pellet is often seen for
Pseudomonas spp. and Acinetobacter spp. This should
be regarded as growth and the endpoint read at the first
well with complete inhibition (clear broth).

Haemolysis
• For fastidious organisms tested in MH-F broth,
haemolysis of the blood can be seen. This is often
accompanied by turbidity or a deposit of growth (pellet).
• Haemolysis with turbidity or pellet should be regarded as
growth when determining endpoints.

Skipped wells
• Occasionally a skip may be seen, i.e. a well showing no
growth bordered by wells showing growth. There are
several possible explanations including incorrect
inoculation, contaminations, heterogenous resistance
etc.
• When a single skipped well occurs, retest the isolate or
read the highest MIC value to avoid reporting isolates as
false susceptible.
• Do not report results for antimicrobial agents for which
there is more than one skipped well.

Examples skipped wells
Retest or read the
highest MIC value!
Results invalid!

Specific reading instructions
• The following antimicrobial agents require specific
reading instructions:
• Bacteriostatic antimicrobial agents, both with Gram-
positive and Gram-negative organisms
• Trimethoprim and trimethoprim-sulfamethoxazole
• Cefiderocol

Gram-positive cocci with bacteriostatic
antimicrobial agents
• Disregard pinpoint growth (tiny buttons) when trailing
growth occurs.
Doxycyline
Doxycyline
Fusidic acid
Tetracycline
Linezolid

Gram-negative organisms with
bacteriostatic antimicrobial agents
• Disregard pinpoint growth (tiny buttons) when trailing
growth occurs.
Tigecycline
Tigecycline
Eravacycline
Eravacycline

Trimethoprim and
trimethoprim-sulfamethoxazole
Read the MIC at the lowest concentration that inhibits ≥80 %
of growth as compared to the growth control.
Growth
control

Summary of BMD methodology
Inoculum 5 x 105 CFU/mL
Media MH broth (non-fastidious organisms)
MH-F broth (Fastidious organisms)
Incubation 35°C in air, 16-20 h (sealed panels)
Reading Read the MIC as the lowest
concentration of antimicrobial agent that
completely inhibits growth of the
organism as detected by the unaided eye

MIC determination (broth microdilution according to ISO standard 20776-1 except for fosfomycin where
agar dilution is used)
Medium: Mueller-Hinton broth
Inoculum: 5x10
5
CFU/mL
Incubation: Sealed panels, air, 35±1ºC, 18±2h
Reading: Unless otherwise stated, read MICs at the lowest concentration of the agent that completely inhibits
visible growth. See ”EUCAST Reading Guide for broth microdilution” for further information.
Quality control: Staphylococcus aureus ATCC 29213. For agents not covered by this strain, see EUCAST QC
Tables.
MIC determination (broth microdilution according to ISO standard 20776-1)
Medium: Mueller-Hinton broth + 5% lysed horse blood and 20 mg/L β-NAD (MH-F broth)
Inoculum: 5x105
CFU/mL
Incubation: Sealed panels, air, 35±1ºC, 18±2h
Reading: Unless otherwise stated, read MICs at the lowest concentration of the agent that completely inhibits
visible growth. See ”EUCAST Reading Guide for broth microdilution” for further information.
Quality control: Streptococcus pneumoniae ATCC 49619. For agents not covered by this strain, see
EUCAST QC Tables.
Information in EUCAST Breakpoint Tables
Staphylococcus spp.
Streptococcus pneumoniae

Quality control
• Reference methods don’t automatically give
the correct results just because they are
reference methods.
• For reproducible and reliable results,
methodology must be strictly adhered to.
• Quality control must be performed for all
materials used (BMD panels, broth etc).
– Commercial BMD panels must often be tested
with several strains to cover all agents

Example of QC of BMD panels
On target
Upper limit
Lower limit
Out of range
QC strain AMP FEP CTX CAZ CIP COL ERT GEN IMI LEV MER NIT PTZ
E. coli ATCC 25922 4 ≤0.06 0.12 0.25 ≤0.015 ≤0.25 0.008 0.5 0.12 ≤0.03 ≤0.015 8 4
E. coli ATCC 25922 4 ≤0.06 0.06 0.5 ≤0.015 ≤0.25 0.008 0.5 0.12 ≤0.03 ≤0.015 8 4
E. coli ATCC 25922 8 ≤0.06 0.06 0.25 ≤0.015 0.5 0.008 0.5 0.12 ≤0.03 ≤0.015 16 4
E. coli ATCC 25922 8 ≤0.06 0.12 0.25 ≤0.015 0.5 0.008 0.5 0.12 ≤0.03 ≤0.015 8 2
E. coli ATCC 25922 4 ≤0.06 0.06 0.25 ≤0.015 0.5 0.008 0.5 0.12 ≤0.03 ≤0.015 8 2
E. coli ATCC 25922 4 ≤0.06 0.06 0.25 ≤0.015 0.5 0.008 0.5 0.12 ≤0.03 ≤0.015 8 2
E. coli ATCC 25922 4 ≤0.06 0.12 0.25 ≤0.015 0.5 0.008 0.5 0.12 ≤0.03 ≤0.015 16 2
E. coli ATCC 25922 4 ≤0.06 0.06 0.25 ≤0.015 0.5 0.008 0.5 0.25 ≤0.03 0.03 8 4
E. coli ATCC 25922 4 ≤0.06 0.12 0.25 ≤0.015 0.5 0.008 0.5 0.12 ≤0.03 0.03 8 4
E. coli ATCC 25922 4 ≤0.06 0.12 0.25 ≤0.015 0.5 - 0.5 0.25 ≤0.03 0.03 8 4
Target 4 0.03-0.06 0.06 0.12-0.25 0.008 0.5-1 0.008 0.5 0.125-0.250.015-0.030.015-0.03 8 2
Range 2-8 0.015-0.12 0.03-0.12 0.06-0.5 0.004-0.015 0.25-2 0.004-0.015 0.25-1 0.06-0.5 0.008-0.060.008-0.06 4-16 1-4
P. aeruginosa ATCC 27853 >32 1 8 1 0.25 2 4 0.5 2 1 0.25 >128 2
P. aeruginosa ATCC 27853 >32 2 >8 1 0.25 2 2 0.5 1 1 0.25 >128 2
P. aeruginosa ATCC 27853 >32 1 >8 2 0.25 2 4 1 2 1 0.25 >128 4
P. aeruginosa ATCC 27853 >32 - 8 1 0.25 2 2 1 2 1 0.25 >128 2
P. aeruginosa ATCC 27853 >32 2 8 2 0.25 2 2 1 2 1 0.25 >128 2
P. aeruginosa ATCC 27853 >32 2 >8 - 0.25 1 4 1 4 1 0.5 >128 -
P. aeruginosa ATCC 27853 >32 1 >8 2 0.25 1 4 1 1 1 0.25 >128 -
P. aeruginosa ATCC 27853 >32 2 >8 2 0.25 1 >4 1 2 1 0.5 >128 2
Target 1-2 2 0.25-0.5 1-2 1 2 1-2 0.25-0.5 2-4
Range 0.5-4 1-4 0.125-1 0.5-4 0.5-2 1-4 0.5-4 0.125-1 1-8
Enterobacterales panel

Example of QC of BMD panels
On target
Upper limit
Lower limit
Out of range
K. pneumoniae ATCC 700603 >32 1 4 >16 0.25 2 0.06 8 0.12/0.5 1 0.06 64 16
K. pneumoniae ATCC 700603 >32 0.5 2 16 0.5 1 0.06 4 0.25 1 0.03 128 8
K. pneumoniae ATCC 700603 >32 0.5 2 >16 0.5 1 0.06 8 0.12/0.5 0.5 0.03 64 8
K. pneumoniae ATCC 700603 >32 0.5/2 2 16 0.5 1 0.06 4 0.25 0.5 0.03 128 8
K. pneumoniae ATCC 700603 >32 1 2 >16 0.5 0.5 0.03 8 0.12/0.5 1 0.03 64 8
K. pneumoniae ATCC 700603 >32 1 2 16 0.5 1 0.06 4 0.25 1 0.03 64 8
K. pneumoniae ATCC 700603 >32 0.5 2 >16 0.5 1 0.06 4 0.25 1 0.03 64 8
K. pneumoniae ATCC 700603 >32 1 2 >16 0.5 1 0.06 4 0.12 1 0.03 64 8
K. pneumoniae ATCC 700603 >32 1 2 >16 0.5 ≤0.25 0.06 4 0.12 1 0.03 128 16
K. pneumoniae ATCC 700603 >32 0.5 2 >16 0.5 0.5 0.06 4 0.25 1 0.03 64 16
Target 16
Range 8-32
E. coli NCTC 13846 >32 0.25 0.25 0.5 >4 8 0.03 0.5 0.25 >4 0.03 16 4
E. coli NCTC 13846 >32 0.25 0.25 0.5 >4 4 0.03 1 0.25 >4 0.03 16 4
E. coli NCTC 13846 >32 0.25 0.25 0.5 >4 4 0.03 0.5 0.12 >4 0.03 16 4
E. coli NCTC 13846 >32 0.12 0.25 0.25 >4 4 0.03 0.5 0.12 >4 0.03 16 4
E. coli NCTC 13846 >32 0.25 0.5 0.5 >4 4 0.03 0.5 0.25 >4 0.03 16 4
E. coli NCTC 13846 >32 0.25 0.5 0.5 >4 4 0.03 0.5 0.25 >4 0.03 16 4
E. coli NCTC 13846 >32 0.25 0.25 0.5 >4 4 0.03 0.5 0.12 >4 ≤0.015 16 4
E. coli NCTC 13846 >32 0.25 0.25 0.5 >4 2 0.03 ≤0.25 0.12 >4 0.03 16 8
E. coli NCTC 13846 >32 0.25 0.25 0.5 >4 4 0.03 0.5 0.12 >4 0.03 16 8
E. coli NCTC 13846 >32 0.25 0.25 0.5 >4 4 0.03 0.5 0.12 >4 0.03 16 4
Target 4
Range (2-8)
Enterobacterales panel

Disk diffusion as control method
• Advantages of performing disk diffusion in
parallel for at least some agents
representing relevant antimicrobials:
– Purity test
– Control of the inoculum
– A second method that can be used to interpret
difficult BMD tests

Caveats for BMD
• BMD can only be used for organisms that
grow sufficiently well in broth (MH or MH-F
broth)
– Agar dilution is recommended for anaerobic
bacteria and N. gonorrhoeae and will be
recommended for Nocardia spp.
• Some agents cannot be tested with BMD due
to poor reproducibility
– Mecillinam and fosfomycin should be tested with
agar dilution (ISO 20776-1)

Implementation of BMD
• ISO 20776-2 (2021): To guide manufacturers
in the conduct of performance evaluation
studies
• Testing scheme:
– QC each day of testing
• ≥95% within range
– Reproducibility (10 isolates x 3)
• MICs within ± one two-fold dilution of the mode/median
for ≥ 95 % of the results
– Clinical isolates (n=300)
• Essential agreement and bias

ISO 20776-2:
Data analysis clinical isolates
• Essential agreement (EA)
– MIC within ± one two-fold dilution step from
the reference MIC (≥ 90%)
• Bias
– Whether the results that differ from the
reference method are significantly skewed or
predominantly in one direction (less than ±
30% bias)

Example of EA and bias analysis
EA = 93%
57.3% below, 4.4% above
Bias = - 52.9%
EA = 78 %
71.9% below, 2.6% above
Bias = - 69.3%
0.016 0.03 0.06 0.125 0.25 0.5 1 2 4 No %
0.008 >2 dilutions lower 1 0.9
0.016 7 1 2 dilutions lower 9 7.9
0.03 1 2 2 3 1 dilution lower 46 40.4
0.06 1 10 9 Identical 53 46.5
0.125 2 2 1 dilution higher 5 4.4
0.25 11 6 2 dilutions higher
0.5 1 4 4 4 1 >2 dilutions higher
1 2 8 14 2
2 6 8
4 3
PCG BMD
PCG
Etest
0.016 0.03 0.06 0.125 0.25 0.5 1 2 4 No %
0.008 3 1 >2 dilutions lower
0.016 5 1 2 dilutions lower 25 21.9
0.03 1 4 5 1 dilution lower 57 50.0
0.06 1 8 9 Identical 29 25.4
0.125 8 5 1 dilution higher 3 2.6
0.25 6 2 1 2 dilutions higher
0.5 3 7 9 >2 dilutions higher
1 2 4 15 4
2 8
4 2
PCG
MTS
PCG BMD

Implementation of an MIC method
in a clinical laboratory
• No official EUCAST criteria
• Guidance based on EDL experience:
– Relevant QC strains
• At least three repeats per QC strain
– Strains representing target organism
– Beta-lactamase producing strains must be added
for inhibitor-combination agents
– Relevant resistant QC strains if available (e.g. E.
coli NCTC 13846 for colistin)
• Analyse vs. target and range in EUCAST QC
tables
Use QC strains to practise reading of results!

Implementation of an MIC method
in a clinical laboratory
• Guidance based on EDL experience:
– Clinical isolates with known MICs
• 25-100 isolates representing target organisms
(depending on product and agents)
– Wild-type isolates and isolates with elevated MICs
– EUCAST/CCUG reference collections for
S. pneumoniae and P. aeruginosa
– UKNEQAS specimens
– Resistant QC strains

EUCAST/CCUG reference collections
• To be used to evaluate or implement MIC methods in
laboratories
• Can be ordered from CCUG, https://ccug.se/
• S. pneumoniae
– 10 strains with varying levels of susceptibility to beta-lactam
agents
– Reference MIC values for beta-lactams and agents from other
classes
• P. aeruginosa
– 9 strains with varying levels of susceptibility to agents from
different classes
– Reference MIC values and genomic resistance profile (WGS)
https://www.eucast.org/ast_of_bacteria/strains_with_defined_susceptibility

CCUG 74415
Streptococcus pneumoniae from sputum
Animicrobial agent
MIC
(mg/L)
Benzylpenicillin 0.5
Ampicillin 0.5
Amoxicillin 0.5
Amoxicillin-clavulanic acid1
0.25-0.5
Piperacillin-tazobactam2
0.5-1
Cefotaxime 1
Ceftriaxone 1
Cefuroxime 4
Imipenem 0.03
Meropenem 0.06
Levofloxacin 1
Moxifloxacin 0.125
Vancomycin 0.5
Azithromycin >2
Clarithromycin >2
Erythromycin >2
Clindamycin >2
Doxycycline 8
Tetracycline >8
Linezolid 1
Rifampicin 0.03-0.06
Trimethoprim-sulfamethoxazole3
1
1
Fixed 2 mg/L clavulanic acid
2
Fixed 4 mg/L tazobactam
3
Trimethoprim:sulfamethoxazole in the ratio 1:19
Example S. pneumoniae

CCUG 75455
Pseudomonas aeruginosa (ST244)
Animicrobial agent MIC
(mg/L)
Main resistance mechanismsa,d Strength of
resistance
genotypeb,d
Correlation with
phenotypec,d
Piperacillin-tazobactam1 >32 AmpC overexpression (AmpD aa162InsPERIQGHCDIA), MexXY
overpexpression (mexZnt343∆1), MexAB-OprM overexpression (NalD deleted),
PBP3 mutation (V465L)
High Full correlation
Cefepime 32 AmpC overexpression (AmpD aa162InsPERIQGHCDIA), MexXY
Ceftazidime >16 AmpC overexpression (AmpD aa162InsPERIQGHCDIA), MexXY
Ceftazidime-avibactam2 8-16 AmpC overexpression (AmpD aa162InsPERIQGHCDIA), MexXY
Moderate Full correlation
Ceftolozane-tazobactam1 4 AmpC overexpression (AmpD aa162InsPERIQGHCDIA), MexXY
Imipenem 32 Inactivation of OprD (K407X), AmpC overexpression (AmpD
aa162InsPERIQGHCDIA)
Meropenem 32 Inactivation of OprD (K407X), AmpC overexpression (AmpD
aa162InsPERIQGHCDIA), MexXY overpexpression (mexZnt343∆1), MexAB-
OprM overexpression (NalD deleted), PBP3 mutation (V465L)
Aztreonam 64 AmpC overexpression (AmpD aa162InsPERIQGHCDIA), MexXY
Ciprofloxacin 1 MexXY overpexpression (mexZnt343∆1), MexAB-OprM overexpression (NalD
deleted)
Levofloxacin 4 MexXY overpexpression (mexZnt343∆1), MexAB-OprM overexpression (NalD
deleted)
Amikacin 16 MexXY overpexpression (mexZnt343∆1) Moderate Full correlation
Tobramycin 1 MexXY overpexpression (mexZnt343∆1) Weak Full correlation
Colistin Note3 LPS colistin resistance mutation (ParR E214K) High Full correlation
1
Fixed 4 mg/L tazobactam
2
Fixed 4 mg/L avibactam
3
A reference MIC value could not be established due to poor reproducibility in repeated tests.
a
List of resistance mechanisms/mutations
b
Expected effect of the summatory of mutations (weak, moderate, high)
c
Correlation between genotype and phenotype (full correlation, partial correlation, no correlation)
d
Adapted from S. Cortes-lara et al Clin Microbiol Infect 2021; 27: 1631-1637
Example P. aeruginosa

Etest MTS Etest MTS Etest MTS Etest MTS Etest MTS Etest MTS
2 dilutions lower 1 1 2 3 2 9 4
1 dilution lower 3 5 10 10 3 12 7 5 11 12 78 33
Identical 17 15 9 9 16 6 13 15 4 4 16 14 138 58
1 dilution higher 1 2 2 4 6 15 6
2 dilutions higher
Total 20 20 20 20 20 20 20 20 20 20 20 20 240 100
%
Beta-lactam
MIC values
Benzyl-
penicilli
Ampicillin
Total
Amoxicillin Cefotaxime Ceftriaxone Meropenem
Example of an evaluation of gradient
tests using the S. pneumoniae collection
• Gradient tests for 6 beta-lactam agents from two
manufacturers (Etest and MTS) were tested on MH-F
agar from two manufacturers (BBL and Oxoid).

UKNEQAS specimens for MIC
method implementation

Example of implementation of BMD
using UKNEQAS isolates
Escherichia coli specimen 3253
Antimicrobial agent
Reference
MIC
(mg/L)
MIC
(mg/L)
Reference
categorisation
(SIR)
Categorisation
(SIR)
Comments
Amikacin 8 16 S I Results close to breakpoint. MIC within ± 1 dilution.
Amoxicillin ≥128 >32 R R
Amoxicillin-clavulanic acid 16 16 R R
Ampicillin ≥128 >32 R R
Cefotaxime 32 >8 R R
Ceftazidime 64 >16 R R
Ceftriaxone 32 >4 R R
Cefuroxime ≥128 >16 R R
Ciprofloxacin 0.25 0.25 S S
Ertapenem 0.06 0.06 S S
Gentamicin 0,5-1 1 S S
Imipenem 0.25 0.25 S S
Meropenem 0.03 0.03 S S
Piperacillin-tazobactam 4 8 S S
Tobramycin 8-16 8 R R

Broth microdilution reference methodology

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