Transcript MRSA, ESBLs and Carbapenem Resitance

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Methicillin-resistant Staph
MRSA, ORSA,
 mecA gene
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 Codes for a supplemental penicillin-binding
protein, PBP2a
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Homogeneous expression
 Easily detected with standard methods
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Heterogeneous expression
 More difficult to detect
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Methods of Detection
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Disk diffusion
Broth dilution MIC
Agar dilution MIC
MRSA chromogenic Agar
Etest
Oxacillin agar screen
Cefoxitin disk diffusion
PBP 2a detection
mecA gene detection
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Oxacillin Disk Interpretation
Interpretive Criteria (in mm) for Oxacillin Disk Diffusion Tests
S.aureus
Susceptible
≥ 13mm
Intermediate Resistant
11-12mm
≤10
Co NS
≥18
N/A
≤17mm
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Cefoxitin Disk for mecA-mediated
Oxacillin-Resistance Staph. species
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Cefoxitin current recommendation
(CX,30)
 Replaces oxacillin disk to detect mecA-
mediated resistance
 Cefoxitin easier to read; better inducer
 Read with reflected light
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Testing Cefoxitin to Predict the
Presence of mecA
S. aureus
S. lugdunensis
CoNS
Cefoxitin zone (mm)
Res
Susc
21*
22**
24*
25**
* Report as oxacillin resistant
** Report as oxacillin susceptible
CoNS, coagulase-negative staphylococci
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Performance of Cefoxitin and Oxacillin
Disk Tests
Sensitivity
S. aureus
fox
ox
CN-S
fox
ox
Specificity
98%
98%
100%
99%
99%
99%
97%
89%
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Oxacillin-salt agar Screen for S.
aureus
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Medium
 Mueller Hinton Agar + 4% NaCl + 6 ug/ml
Oxacillin
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Inoculum
 Direct Suspension; 0.5 McFarland standard
 Use 1 ul loop or swab
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Incubation
 35C for full 24 hours
 Presence of either interpreted as RESISTANT
NOT TO BE USED FOR CoNS
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Gold Standards
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Detection of mecA
gene by PCR
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Detection of PBP2a
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Vancomycin Resistant S.aureus
T here is only few report of VRSA
There are some reports reduced
susceptiblity of S.aureus to vancomycin.
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Extended-Spectrum-β
Lactamase
(ESBLs)
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 ESBL-producing
isolates can cause
nosocomial outbreaks
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Which microorganisms are ESBls positive
K.pneumoniae and K.oxytoca
 E.coli
 Enterobacter spp
 Salmonella spp
 Morganell morganii,Proteus mirabilis
 Serratia marcescens
 Pseudomonas aeruginosa
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ESBL Reporting Rule
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The rule
 “Strains of Klebsiella spp. E. coli, and Proteus
mirabilis that produce ESBLs may be clinically
resistant to therapy with penicillins,
cephalosporins, or aztreonam, despite apparent in
vitro susceptibility to some of these agents.”
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The message…
 Report “confirmed” ESBL-producing strains as R
to all penicillins, cephalosporins, and aztreonam
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ESBL
Phenotypic Confirmatory Test
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Test:
 cefotaxime
 cefotaxime/clavulanic acid
 ceftazidime
 ceftazidime/clavulanic acid
Results:
 clavulanic acid restores activity of cefotaxime or
ceftazidime or both
QC
 E. coli ATCC 25922; K. pneumoniae ATCC 700603
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For disk diffusion testing, a > 5 mm increase in a
zone diameter for either antimicrobial agent tested in
combination with clavulanic acid versus its zone
when tested alone confirms an ESBL-producing
organism.
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K. pneumoniae ATCC 700603 (positive control)
and E. coli ATCC 25922 (negative control)
should be used for quality control of ESBL
tests
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If an ESBL is detected, all penicillins,
cephalosporins, and aztreonam should be
reported as resistant, even if in vitro test
results indicate susceptibility.
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AmpC beta-lactamases
AmpC beta-lactamases differ from ESBL’s in that they are
cephalosporinases >50 Enzyme and are resistant to beta-lactamase
inhibitors
They hydrolyze the cephamycins (eg. cefoxitin), but not the 4th
generation cephalosporins (eg. cefepime)
High-level production of AmpC usually causes resistance to all betalactams & monobactam (aztreonam ) except carbapenems and 4th
generation cephalosporins (eg. cefepime)
Plasmid-mediated AmpC’s have been detected in organisms such as
E .coli, Klebsiella sp, Proteus sp and Salmonella sp
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KPC
Carbapenems are a class of β-lactam antibiotics with a
broad spectrum of antibacterial activity.
The following drugs belong to the carbapenem class and are
approved for use by health authorities:
Imipenem (FDA approval 1985)
Meropenem (FDA approval 1996)
Ertapenem (FDA approval 2001, since approved for
multiple indications)
Doripenem (FDA Approval 2007)
Panipenem/betamipron (Japanese approval 1993)
Biapenem (Japanese approval 2001)
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Laboratory Approach to KPC Identification
Reagents
1. 5 ml Mueller Hinton broth (MHB) or
0.85% physiological saline
2. Mueller Hinton agar (MHA)
3. 10 μg meropenem or ertapenem
susceptibility disk
4. E. coli ATCC 25922: 18–24hr
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Carbapenemase production is detected by
the MHT when the test isolate produces the
enzyme and allows growth of a carbapenem
susceptible strain (E.coli ATCC 25922)
towards a carbapenem disk.
The result is a characteristic clover leaflike indentation.
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MHT Positive Klebsiella pneumoniae ATCC BAA-1705
•MHT Negative Klebsiella pneumoniae
ATCC BAA-1706
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Inducible Clindamycin Resistance
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Selecting Antimicrobial Agents for Testing
andReporting
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Test/Report Groups
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The following antimicrobial agents should
not be routinely reported for
bacteria isolated from CSF
Agents administered by oral route only
1st- and 2nd-generation cephalosporins
(except cefuroxime parenteral) and
cephamycins
Clindamycin
Macrolides
Tetracyclines
Fluoroquinolones
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