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Difficult or Tricky AntibioticResistance Phenotypes to
Recognize
Dr Koh Tse Hsien
Department of PathologySingapore General Hospital
Contents
Gram-positive
Staphylococcus aureuswith penicillinase production
Vancomycin Resistant Enterococci (VRE)
Staphylococcus aureuswith reduced susceptibility toglycopeptides
(hVISA, VISA, VRSA)
Gram-negative
ampC Cephalosporinases ( AmpC and pAmpC)
Carbapenemases (IMP, VIM, KPC, and OXA-48)
Mechanisms of Resistance
Target modification
Antibiotic sequestrationEnzymatic inactivation
Penicillinase in S. aureus
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Penicillinase production
Vancomycin susceptibility in S.aureus
Vancomycin-intermediate S.aureus(VISA)
First described in Japan in 1997
Intermediate resistant (MIC 4-8 mg/L)
Hetero-intermediate resistant (MIC
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Vancomycin intermediateStaphylococcus aureus
Lab detection of VISA
Not detected by routine disk or automated
methods
Etest (consistently one twofold dilutionhigher)
Broth microdilution
EUCASTMIC breakpoint (mg/L) Zone diameter breakpoint (mm)
S R > S R 1000
Resistant
4-1024
Resistant
2-32
Intermediate
Teic MIC
Mg/L
16-512
Resistant
0.5
Susceptible
0.5
Susceptible
Species E. FaeciumE. faecalis
E. Faecium
E. faecalis
E. Gallinarum
E. Casseiflavus
E. flavescens
Genetic
determinant
Acquired Acquired Intrinsic
Transferable Yes Yes No
Van A VRE
VSE Van B VRE
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VRE
VRE Lab Detection
Disk diffusion-incubate full 24 h, view withtransmitted light
for hazy zones
MIC methods
Screening plates (vancomycin 6 mg/L)
Multiplex PCR to detect vanA, vanB
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Identify all VRE to species level vanC motile E. gallinarum
Mechanisms of Resistance
inner membrane
outer membrane
periplasmic space
Loss of membrane permeability
Efflux
Enzymatic inactivation
EUCAST
MIC breakpoint (mg/L) Zone diameter breakpoint (mm)
S R > S R 32 mg/L-
-1.5 mg/L1mg/L susceptible-
pAmpC DHA-1DHA in Klebsiella pneumoniae
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CLSI(8) Enterobacter, Citrobacter, and Serratia may develop
resistance during
prolonged therapy with third-generation cephalosporins as a
result of
derepression of AmpC - lactamase. Therefore, isolates that are
initially
susceptible may become resistant within three to four days after
initiation oftherapy. Testing of repeat isolates may be
warranted.
Enterobacterspp., Citrobacter freundii, Serratia spp. and
Morganella
morganii. If susceptible in- vitro, the use of monotherapy of
cefotaxime
should be discouraged, owing to the risk of selection of
resistance, or
suppress the susceptibility testing result for this agent.
BSAC (EUCAST?)
Inducible plasmid AmpC (DHA) in K. pneumoniae?
CDS method
E. cloacae, E. aerogenes, C. freundii, inducible pAmpC High
frequency of derepressedmutants (10-5 to 10-6)
Report R to all cephalosporins Test cefepime and carbapenems Do
not attempt to test other -lactams
S. marcescens No derepressedmutants with ceftazidime, tazocin
and
aztreonam
Derepressedmutants with other -lactams includingcefotaxime.
H. alvei, P. stuartii, P. rettgeri and M. morganii
Very low mutation rate (10-8) Test and report accordingly
http://web.med.unsw.edu.au/cdstest/
Carbapenemases in Gram-
negative bacilli
Carbapenemases
Class A
KPC
Class B
IMP, VIM, NDM
Class C
Class D
OXA-48
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EUCAST
MIC breakpoint (mg/L) Zone diameter breakpoint (mm)
S R > S R
