Clinical Interpretation of MICs in the Era of Multidrug Bacterial Resistance
Wael E Shams, M.D.Division of Infectious Diseases
Department of Internal MedicineJames 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 >32 R
AZTREONAM 2 S
CEFAZOLIN >64 R
CEFUROXIME 16 I
CEFTAZIDIME 16 I
CEFTRIAXONE 4 S
CEFEPIME 16 I
TOBRAMYCIN <1 S
LEVOFLOXACIN >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 >32 R
CEFAZOLIN >64 R
CEFOXITIN >64 R
CEFTAZIDIME 16 I
CEFTRIAXONE 4 S
PIPER/TAZO 32 S
CEFEPIME 4 S
MEROPENEM 2 S
TOBRAMYCIN <1 S
ESBL Neg -
LEVOFLOXACIN >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/ß-lactamase 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, non-bacteremic 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
Lepper et al. Antimicrob Agents Chemother. 2002 September; 46(9): 2920–2925
WHAT IF I GIVE EVERYBODY CARBAPENEMS?
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 DataTrend Down of MDR Gram – ve bacteria with Carbapenem Restriction
Infection Control- James H Quillen VAMC
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
Anderson et al. J Clin Microbiol 2007; 45: 2723–25.
Local KPC + Enterobacter Cloacae isolate tested at ETSU Microbiology Lab, Courtesy picture by Dr. Donald Ferguson 2/2010
Antibiotics to Treat Infections Caused by KPC + Organisms
Tigecycline (For non-septic, non-bacteremic 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 grew MRSA
CEFAZOLIN >=32 R OXACILLIN >=8 R TRIMETHOPRIM/Sulfa <2/38 S VANCOMYCIN =1 S LINEZOLID =1 S
QUINOPRISTIN/DALFO <0.25 S
Automated Susceptibilityin mcg/ml
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 trimethoprim-sulfamethoxazole 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 ERYTHROMYCIN >8 R OXACILLIN < 0.25 S ICR + Positive TRIMETHOPRIM/S <=10 S VANCOMYCIN 1 S LEVOFLOXACIN = 2 S
Automated Susceptibilityin mcg/ml
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 ERYTHROMYCIN >8 R OXACILLIN >8 R ICR + Positive TRIMETHOPRIM/S <10 S VANCOMYCIN 2 S BETA LACTAMASE POS + LEVOFLOXACIN = 2 S
Automated Susceptibilityin mcg/ml
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
