Clinical Interpretation of MICs in
the Era of Multidrug Bacterial
Resistance
Wael E Shams, M.D.
Division of Infectious Diseases
Department of Internal Medicine
James H Quillen College of Medicine
East Tennessee State University
Conflict of Interest & Disclosure

I received a research grant from Cubist
Pharmaceuticals for an investigator
initiated study on the use of daptomycin
for preoperative antibiotic prophylaxis.
Antimicrobial Susceptibility
Testing (AST)



AST measures the ability of a specific organism
to grow in the presence of a particular drug in
vitro.
AST is usually performed using guidelines
established by the Clinical and Laboratory
Standards Institute (CLSI), formerly called NCCLS
AST helps to predict success or failure of an
antimicrobial agent when used to treat an
infection caused by the organism tested.
Leekha et al. Mayo Clin Proc. 2011 Feb;86(2):156-67.
Minimum Inhibitory
Concentration (MIC)


MIC is the lowest concentration of an
antibiotic that inhibits visible growth (broth
turbidity) upon in vitro testing of a
particular organism.
MIC breakpoint is a discriminating
concentration used in the interpretation of
results to define isolates as susceptible,
intermediate or resistant.
MacGowan et al. Antimicrob Agents Chemother. 2009 Dec;53(12):5181-4.
Hessen and Kaye. Infect Dis Clin North Am. 2004 Sep;18(3):435-50
TESTING METHODS TO
DETERMINE MIC’S


The gold standard in vitro method to
determine MIC is broth dilution where a
standard inoculum of bacteria (105-6 CFU/ml)
in broth is exposed to serial twofold dilutions
of antimicrobial agent for 18-24 hours.
Agar dilution follows similar principles except
that bacteria are inoculated onto agar plates
containing serially diluted concentrations of
the antimicrobial agent.
TESTING METHODS TO
DETERMINE MIC’S (Cont.)



Disk diffusion (Bauer): implies placement of antibiotic
impregnated disks on a freshly plated “lawn” of a
particular organism and assessment of the zone of
growth inhibition around the discs after incubation.
E-test uses the same principle as the disks. However,
instead, a test strip impregnated lengthwise with
graded concentrations of an antibiotic is placed on a
lawn of bacteria.
Automated systems (e.g. Vitek, Microscan and
Phoenix) uses broth microdilution plates.
MIC Interpretation




AST data are reported as MICs that are interpreted
as susceptible, intermediate or resistant according to
CLSI criteria.
Susceptible: implies that infection due to bacteria
tested will probably respond to that antibiotic
provided in standard doses
Intermediate: implies an uncertain response to that
antibiotic provided in standard doses
Resistant: implies that infection will probably not
respond to this antibiotic tested.
MacGowan et al. Antimicrob Agents Chemother. 2009 Dec;53(12):5181-4.
Leekha et al. Mayo Clin Proc. 2011 Feb;86(2):156-67.
Case 1



75 y male patient with h/o arrhythmia s/p pacer
placement, PVD s/p b/l AKA with residual
stump wound infection and recurrent Foley
associated UTI’s developed a new fever.
Urine had pyuria and both urine and blood
cultures grew MDR Pseudomonas aeroginosa.
Echo showed endocarditis/ pacer lead
vegitation
Case 1 (Continued)

Pseudomonas aeroginosa Susceptibility
mcg/ml:
AMIKACIN
<=2 S
AZTREONAM
> 32 R
CEFTAZIDIME
=16 I
GENTAMICIN >=16 R
PIPER/TAZO
= 64 S
IMIPENEM
>= 16 R
CEFEPIME
= 16 I
LEVOFLOXACIN >=8 R
Automated
Pseudomonas
Susceptibility May
Be Wrong!!
Sader et al. J Clin Microbiol. 2006
Mar;44(3):1101-4
Case 1 (Continued)


Manual susceptibility testing showed
resistance for pipercillin and full
susceptibility to cefepime.
Patient was treated successfully with
cefepime + amikacin
Case # 2

67 y female NH resident with h/o DM
developed urinary symptoms suggestive
of recurrent UTI’s. She was started on
levofloxacin without improvement. She
then started running fevers and left
flank pain. A urine culture was
submitted.
Case #2 (Continued)

E coli susceptibility mcg/ml:
AMOXICILLIN/CA
AZTREONAM
CEFAZOLIN
CEFUROXIME
CEFTAZIDIME
CEFTRIAXONE
CEFEPIME
TOBRAMYCIN
LEVOFLOXACIN
>32 R
2S
>64 R
16 I
16 I
4 S
16 I
<1 S
>8 R
Case #2 (Continued)

Patient was treated in the NH with 3
doses of IV Tobramycin. On the 3rd day
she became more sick and started
vomiting. She was then admitted to ICU
with E coli urosepsis and ARF. Blood
cultures grew E coli.
Case # 2 (Continued)

E coli susceptibility mcg/ml:
AZTREONAM
CEFAZOLIN
CEFOXITIN
CEFTAZIDIME
CEFTRIAXONE
PIPER/TAZO
CEFEPIME
MEROPENEM
TOBRAMYCIN
ESBL
LEVOFLOXACIN
>32 R
>64 R
>64 R
16 I
4 S
32 S
4 S
2 S
<1 S
Neg >8 R
Case # 2 (Continued)


Patient was switched to meropenem,
cleared her bacteremia and recovered
well.
E coli isolate was retested manually and
proved positive for ESBL production.
What is ESBL?



Extended-spectrum ß-lactamase (ESBL)-is an enzyme that
confers resistance to most ß-lactam antibiotics (including
pipercillin and all cephalosporins) except carbapenems.
The gene that encodes this enzyme is carried on a plasmid
that can be transferred promiscuously among most Gram
negative bacteria particularly Escherichia coli, Klebsiella
pneumoniae and Enterobacter.
ESBL may be partially inhibited in vitro by ß-lactamase
inhibitors and complex side chain of advanced generation
cephalosporins resulting in false susceptibility to ß-lactam/ßlactamase combination antibiotics, and advanced generation
cephalosporins (e.g. Zosyn and Cefepime) on automated
testing. If these antibiotics are used in vivo the gene will be
induced, the ESBL will be produced and the patient will fail
antibiotic therapy.
Paterson and Bonomo. Clin Microbiol Rev. 2005
Look For ESBL!



Automated susceptibility testing machine MAY
only label E Coli and Klebsiella as ESBL
Other ESBL + Enterobacteriacae, particularly
Enterbacter species will be missed by
automated machines
Resistance or intermediate susceptibility to
Astreonam, Ceftazidime, or Ceftriaxone raise
the red flag>>>> Ask the lab to perform
manual phenotypic testing for ESBL production.
Phenotypic Testing For ESBL
Production
Luzzaro et al., 101st General Meeting of the American Society for
Microbiology, Orlando, Florida, 2001.
Antibiotics to Treat Infections
Caused by ESBL + Organisms



Carbapenems e.g. imipenem,
ertapenem, meropenem, and
doripenem
Tigecycline (For non-septic, nonbacteremic patient WITHOUT ESBL +
Organism UTI)
Aminoglycosides e.g. Tobramycin and
Amikacin
Consumption of Imipenem and not Pipercillin
Correlated well with β-Lactam Resistance in
Pseudomonas aeruginosa
WHAT IF I GIVE
EVERYBODY
CARBAPENEMS?
Lepper et al. Antimicrob Agents Chemother. 2002 September; 46(9): 2920–2925
Crabapenem Restriction to Control a Hospital
Outbreak of Multiresistant Acinetobacter baumannii
Corbella et al. J Clin Microbiol. 2000 Nov;38(11):4086-95.
Local Antimicrobial Resistance Data
Trend Down of MDR Gram – ve bacteria
with Carbapenem Restriction
7
6
5
Total
4
3
2
1
0
Infection Control- James H Quillen VAMC
Linear (Total)
Case # 3


73 y male with h/o indwelling Foley’s catheter and
recurrent Foley associated UTIs was about to
complete another course of Ertapenem for ESBL+
Klebsiella UTI and started spiking fevers. Repeat
blood cultures drawn thru PICC line came back
positive for GNR 3 hours earlier than peripherally
drawn set. PICC was discontinued and cath tip is also
growing GNR.
Patient was started on Meropenem + Tobramycin
pending susceptibility results.
Case 3 (Continued)

ENTEROBACTER CLOACAE SUSCEPT. mcg/ml:
AMIKACIN
<2 S
TOBRAMYCIN >16 R
AZTREONAM
>64 R
CEFTAZIDIME >64 R
CEFTRIAXONE >64 R
CEFEPIME
> 32 R
PIPER/TAZO
>128 R
TIGECYCLINE = 2 S
IMIPENEM
= 0.25 S
MEROPENEM = 4
I
ERTAPENEM
> 8 R
LEVOFLOXACIN >=8 R
WHAT IS KPC?



Klebsiella pneumoniae carbapenemases (KPC 1-7)
are class A β-lactamases that can hydrolyze
penicillins, cephalosporins, monobactams, and
carbapenems.
KPC β- lactamases are partially inhibited by
clavulanic acid , often encoded by genes that are
plasmid mediated.
KPC + bacteria often carry other genes encoding
resistance for other antimicrobial agents including
aminoglycosides, fluoroquinolones, and
trimethoprim/sulfamethoxazole.
www.thelancet.com/infection Vol 9 April 2009
KPC β- lactamases

Initially described with Klebsiella
pneumoniae.


Now known to occur with other species
from Enterobacteriaceae, such as
Escherichia coli, and Enterobacter.
Also isolated from Pseudomonas
aeruginosa.
www.thelancet.com/infection Vol 9 April 2009
LOOK FOR KPC!


Most automated testing systems machine will
NOT detect, flag or alert the clinician to KPC
production and will falsely show KPC + isolates
as susceptible to imipenem and meropenem.
Resistance or intermediate susceptibility to
Ertapenem and meropenem with suscpetibility
to imipenem >>>> Ask the lab to perform
manual phenotypic testing for KPC production.
Phenotypic Testing for KPC
Production: Modified Hodge Test
Local KPC + Enterobacter
Cloacae isolate tested at
ETSU Microbiology Lab,
Courtesy picture by
Dr. Donald Ferguson
2/2010
Anderson et al. J Clin Microbiol 2007; 45: 2723–25.
Antibiotics to Treat Infections
Caused by KPC + Organisms



Tigecycline (For non-septic, nonbacteremic patient WITHOUT KPC +
Organism UTI)
Aminoglycosides e.g. Tobramycin and
Amikacin if susceptible
Colistin
Case #4



A 57 y male with dilated cardiomyopathy had
fever 3 weeks after implantation of LVAD as a
bridge to heart transplant. Cultures of blood and
of material from the LVAD driveline site grew
MRSA.
He was treated with vancomycin but he
continued to have fevers with persistently
positive blood cultures.
The LVAD could not be removed because no
donor heart was available.
Case 4 (Continued)

Blood cultures persistently
CEFAZOLIN
OXACILLIN
TRIMETHOPRIM/Sulfa
VANCOMYCIN
LINEZOLID
QUINOPRISTIN/DALFO
grew MRSA
>=32 R
>=8
R Automated
<2/38 S Susceptibility
in mcg/ml
=1 S
=1 S
<0.25 S
Case # 4 (Continued)




Vancomycin susceptibility using E-test came back
as 3 mcg/ ml and 2mcg/ ml using Broth Dilution
Patient also failed linezolid and
quinopristin/dalfopristin with persistent
bacteremia.
He was then treated with iv trimethoprimsulfamethoxazole with clearance of MRSA
bacteremia.
LVAD was then removed and he had a successful
heart transplant.
Reduced Vancomycin Efficacy against
MRSA Isolates with MIC's in the
Higher Range of Susceptibility


Vancomycin treatment failure is more often
encountered when MRSA isolates demonstrate
vancomycin MIC’s of 1-2 µg/ml, which is within the
susceptible range of ≤2 μg /ml as defined by the
current Clinical Laboratory Standards Institute (CLSI)
guidelines.
Mortality associated with MRSA bacteremia was
significantly higher when vancomycin was used for
treatment of infection with strains with a high
vancomycin MIC (>1 μg /mL).
Sakoulas et al. J Clin Microbiol 2004; 42: 2398-402.
Howden et al. Clin Infect Dis 2004; 38: 521-8.
Soriano et al. Clin Infect Dis. 2008 Jan 15;46(2):193-200.
Look for MRSA with high
Vancomycin MIC (>1 μg /mL)!



Automated systems will fail to identify
MRSA isolates with MIC's in the higher
range of susceptibility
E-tests may give false higher
vancomycin MIC results
Broth dilution remains the gold
standard to identify these isolates.
Bland et al. South Med J. 2010 Nov;103(11):1124-8.
Shams, Walker, and Sarubbi. Eighth Annual ICAAC/IDSA 46th Annual Meeting. Washington,
DC. October 24-28, 2008.
Antibiotics to treat infections caused
by MRSA with high Vancomycin MIC
(>1 μg /mL)!






Tigecycline (For non-septic, non-bacteremic patient
WITHOUT MRSA UTI).
Linezolid (For non-septic, non-bacteremic patient
WITHOUT MRSA UTI).
Daptomycin (For patient WITHOUT MRSA Pneumonia).
Telavancin (For non-septic, non-bacteremic patient).
Ceftaroline (For non-septic, non-bacteremic patient).
Trimethoprim/Sulfamethoxazole (caution with renal
failure).
Case # 5




72 y male presented with fever and right diabetic
foot/leg cellulitis complicating known right great toe
chronic ulcer with underlying osteomyelitis.
He was started on vancomycin + zosyn and developped
widespread erythematous rash, his blood pressure
started dropping, and he became tachypneac,
tachycardic and lethrgic.
Infectious disease was consulted for possible
vancomycin allergy, and vancomycin was stopped.
Patient was moved to ICU and started on fluids and
vasopressors. Blood cultures remain negative but toe
ulcer culture grew MSSA.
Case # 5 (Continued)

Foot wound drainage and Blood cultures grew MRSA
CEFAZOLIN
<2 S
CIPROFLOXACIN
=4 I
CLINDAMYCIN
>8 R
Automated Susceptibility
in mcg/ml
ERYTHROMYCIN
>8 R
OXACILLIN
< 0.25 S
ICR
+ Positive
TRIMETHOPRIM/S <=10 S
VANCOMYCIN
1 S
LEVOFLOXACIN
=2 S
Case # 5 (Continued)



Patient had Toxic Shock Syndrome
Antibiotics were switched to clindamycin +
ceftriaxone
Patient recovered well and had skin sloughing
towards the end of his illness.
Case # 6





42 y diabetic female presents with foot
infection.
Primary care physician prescribes oral
ciprofloxacin for outpatient therapy.
She develops fever, spreading leg cellulitis
and gets admitted to hospital.
IV clindamycin is added as she has history of
anaphylaxis on PCN.
She continues to have fevers.
Case # 6 (Continued)

Foot wound drainage and Blood cultures grew MRSA
CEFAZOLIN
>32 R
CIPROFLOXACIN
=4 I
CLINDAMYCIN
<0.5 S
Automated Susceptibility
in mcg/ml
ERYTHROMYCIN
>8 R
OXACILLIN
>8 R
ICR
+ Positive
TRIMETHOPRIM/S <10 S
VANCOMYCIN
2 S
BETA LACTAMASE POS +
LEVOFLOXACIN
=2 S
What is ICR?

Inducible clindamycin resistance is
conferred by erm genes that encode
enzymes which may confer inducible or
constitutive resistance to macrolide,
linocosamide and streptogramin (MLSB)
antibiotics via methylation of the 23S
rRNA, reducing binding by MLS agents
to the ribosome.
Fiebelcorn et al., Journal of Clinical Microbiology, Oct. 2003, p. 4740–4744
LOOK FOR ICR!


Look for ICR on automated susceptibility panel
before using clindamycin. It may be +ve and
the machine will still falsely show clindamycin as
susceptible
If ICR is not declared, and Staphlyococcus
isolate is resistant to erythromicin ask for D-test
if you still want to use clindamycin.
D-test to detect ICR
Fiebelcorn et al., Journal of Clinical Microbiology, Oct. 2003, p. 4740–4744
CLINDAMYCIN USE AGAINST
MIC RESULTS!!!


CLINDAMYCIN WAS USED AS ADJUNCT
ANTIBIOTIC IN CASE # 5 TO HALT
STAPHYLOCOCCUS TOXIN PRODUCTION
DESPITE MIC of 8 mcg/ml CONSISTENT
WITH RESISTANCE.
CLINDAMYCIN CUOLD NOT BE USED ALONE
TO TREAT MRSA INFECTION IN CASE # 6
DESPITE MIC OF 2 CONSISTENET WITH
SUSCEPTIBILITY AS MRSA ISOLATE HAD
POSITIVE ICR.
IN SUMMARY!!




MIC is a gold standard for antibiotic potency but is
also a crude measure with limitations.
If MICs and their implied susceptibility patterns
appear unusual, a clinician needs to communicate
with microbiology lab and request further clarification
and manual testing.
MICs of different agents for a particular organism are
not directly comparable.
Pharmacokinetics and pharmacodynamics of a
particular antibiotic are essential to know prior to its
prescription.
Remember: If it doesn’t make
sense, call the lab!!
She is a doctor!
Thank You