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Dr f.Rashedmarandi
Glossary I (Part 1). β -lactams: Class and Subclass Designation and Generic Name
a
b
c
d
e
Agents Included; Generic Names
Antimicrobial Subclass
Antimicrobial Class
penicillin
amoxicillin
ampicillin
azlocillin
mezlocillin
piperacillin
carbenicillin
ticarcillin
cloxacillin
dicloxacillin
methicillin
nafcillin
oxacillin
mecillinam
amoxicillin-clavulanic acid
ampicillin-sulbactam
piperacillin-tazobactam
ticarcillin-clavulanic acid
cefazolin
cephalothin
cephapirin
cephradine
cefamandole
cefonicid
cefuroxime (sodium)
cefoperazone
cefotaxime
ceftazidime
ceftizoxime
ceftriaxone
cefepime
cefmetazole
cefotetan
cefoxitin
moxalactam
cefaclor
cefadroxil
cefdinir
cefditoren
cefetamet
cefixime
cefpodoxime
cefprozil
ceftibuten
cefuroxime (axetil)
cephalexin
cephradine
loracarbef
aztreonam
doripenem
ertapenem
imipenem
meropenem
faropenem
penicillina
aminopenicillina
penicillins
ureidopencillina
carboxypenicillina
penicillinase-stable
penicillinsb
amidinopenicillin
β -lactam/β -lactamase
inhibitor combinations
cephalosporin Ic,e
cephems (parenteral)
cephalosporin IIc,e
cephalosporin IIIc,e
cephalosporin IVc,e
cephamycind
oxacephem
cephalosporine
cephems (oral)
carbacephem
carbapenem
monobactams
penems
penem
Penicillinase-labile; hydrolyzed by staphylococcal penicillinase.
Not hydrolyzed by staphylococcal penicillinase.
Cephalosporin I, II, III, and IV are sometimes referred to as 1st-, 2nd-, 3rd-, and 4th-generation cephalosporins, respectively.
Cephalosporin III and IV are also referred to as “extended-spectrum cephalosporins.” This does not imply activity against ESBL-producing gramnegative bacteria.
Although often referred to as a 2nd-generation cephalosporin, cephamycins are not included with the other cephalosporins with regard to reporting
of ESBL- producing strains.
For all confirmed ESBL-producing strains, the test interpretation should be reported as resistant for this antimicrobial class or subclass.
Penicillins
These penicillins are penicillinase-labile;hydrolyzed by staphylococcal
penicillinase
penicillin
penicillina
amoxicillin
aminopenicillina
azlocillin
ureidopencillina
mezlolin piperacillin
carbenicillin
ticarcillin
carboxypenicillina
Penicillins
Penicillinase-stable penicillin
Penicillinase-stable penicillin
Cloxacillin
Dicloxacillin
Meticillin
Nafcillin
oxacillin
ß- lactam /ß- lactamase inhibitor
combinations
ß- lactam /ß- lactamase inhibitor
combinations
amoxicillin-clavulanic acid
ampicillin-sulbactam piperacillintazobactam
ticarcillin-clavulanic acid
Cephems
cefazolin
cephalothin
cephapirin
cephradine
cefamandole
cefonicid
cefuroxime (sodium)
cefoperazone
cefotaxime
ceftazidime
ceftizoxime
ceftriaxone
cefepime
cefmetazole
cefotetan
cefoxitin
moxalactam
cefaclor
cefadroxil
cefdinir
cefditoren
cefetamet
cefixime
cefpodoxime
cefprozil
ceftibuten
cefuroxime (axetil)
cephalexin
cephradine
cephalosporin Ic,e
cephems (parenteral)
cephalosporin IIc,e
cephalosporin IIIc,e
cephalosporin IVc,e
cephamycind
oxacephem
cephalosporine
cephems (oral)
III and IV are also referred to as “extended-spectrum cephalosporins.” This does not imply
activity against ESBL-producing gram-negative bacteria.
Selecting Antimicrobial Agents for Testing
and Reporting
,criteria
• Considerations in the assignment of agents to
specific test/report groups include clinical efficacy,
prevalence of resistance, minimizing emergence of
resistance, cost, FDA clinical indications for usage,
in addition to the specific issues described.
Reporting Results
1.Susceptible (S)
The “susceptible” category implies that isolates are inhibited by the usually achievable concentrations of
antimicrobial agent when the recommended dosage is used for the site of infection.
2.Intermediate (I)
The “intermediate” category includes isolates with antimicrobial agent MICs that approach usually
attainable blood and tissue levels and for which response rates may be lower than for susceptible isolates. The
intermediate category implies clinical efficacy in body sites where the drugs are physiologically concentrated
(e.g., quinolones and β-lactams in urine) or when a higher than normal dosage of a drug can be used (e.g., βlactams). This category also includes a buffer zone, which should prevent small, uncontrolled, technical
factors from causing major discrepancies in interpretations, especially for drugs with narrow pharmacotoxicity
margins.
3.Resistant (R)
The “resistant” category implies that isolates are not inhibited by the usually achievable concentrations of the
agent with normal dosage schedules, and/or that demonstrate zone diameters that fall in the range where
specific microbial resistance mechanisms (e.g., beta- lactamases) are likely, and clinical efficacy of the agent
against the isolate has not been reliably shown in treatment studies.
Antibacterial Effect
&
Mechanism of action
Antibiotic class
Bacteria
Inhibition of cell wall synthesis
β lactams
penicillins
β lactamsβ lactamase
inhibitors
cephems(Cephalosporins,
cephamycin,oxacephem,
carbacephem)
Mechanism
non β Lactamase producing G+
Some fastidious aerobic Gsome anaerobic bacteria
β lactamase producing
bacteria
Aerobic & anaerobic g+ ,g-
Penems )Carbapenem,penem) Broad spectrum activity
against many aerobic g+,g-
Antibacterial Effect
&
Mechanism of action
Monobactams)Aztreonam) Only against aerobic gGlicopeptide(vancomycin,
teicoplanin)
Primarily aerobic g+
Aminoglicosides
Primarily g-rods,synergistic Inhibit protein synthesis at
combination with cell wall ribosomal level
active drugs(pen,amp,van)
against resistant g+
bacteria
Macrolides
fastidious g- .for G+ only
erythrmycin need to be tested.
Inhibition of cell wall
synthesis
Inhibit protein synthesis at
ribosomal level
Tetracyclines
G + , G-
Inhibit protein synthesis at
ribosomal level
Quinolones(quinolones,flu
oroquinolones)
G + , G-
Inhibiting DNA gyrase or
topoisomerase
Antibacterial Effect
&
Mechanism of action
Antibiotic class
Bacteria
Sulfonamide /trimetoprim
Folate pathway inhibitors
G+,G-
Lipopeptide
G-(polymyxin B,colistin)
G+ Daptomycin
Mechanism
Inhibition of folate pathway
Cell membrane
Single – Drug class
chloramphenicol
Inhibit protein synthesis
clindamycin
Inhibit protein synthesis
Linezolid
Inhibit protein synthesis
streptogramins
Inhibit protein synthesis
telitromycin
Inhibit protein synthesis
tigecycline
Inhibit protein synthesis
rifampin
RNA synthesis inhibitors
nitrofurantoin
Inhibit protein synthesis at
ribosomal level
“Warning”:
The following antimicrobial agents should not be routinely
reported for bacteria isolated from CSF .These antimicrobial
agents are not the drugs of choice and may not be effective for
treating CSF infections caused by these organisms
 agents administered by oral route only
1st- and 2nd-generation cephalosporins (except cefuroxime
sodium)
Clindamycin - macrolides - tetracyclines - fluoroquinolones
Notice!
“Warning”: The following antimicrobial agent/organism combinations may appear active in
vitro, but are not effective clinically and should not be reported as susceptible.
Antimicrobial Agents That Must Not
Be
Reported as Susceptible
Organism
penicillins, cephalosporins, and
aztreonam
ESBL-producing K. pneumoniae, K. oxytoca, E. coli, and P.
1st- and 2nd-generation
cephalosporins,
cephamycins, and aminoglycosides
penicillins, β-lactam/β-lactamase
inhibitor combinations, cephems, and
carbapenems
aminoglycosides (except high
concentrations),
cephalosporins,
clindamycin, and trimethoprimsulfamethoxazole
Salmonella spp., Shigella spp.
mirabilis
oxacillin-resistant Staphylococcus
spp.
Enterococcus spp.
Suggested panel for
routine testing & reporting
cefazolin
Enterobacteriaceae
Enterobacteriaceae from
Urine
cefepime
ampicillin gentamyci
n
Amoxicillinclavulanic acid
Ampicillinsulbactam
cefuroxime
Cefotaxim
Ceftizoxime
ceftriaxone
Ciprofloxacin
levofloxacin
Imipenem
meropenem
piperacilin
Amikacin
tetracycline
SXT
Chlor
amphenicol
Ciprpfloxacin
ofloxacin
Nitrofurantoin
Salmonella and Shigella
When fecal isolates of Salmonella and Shigella
spp. are tested, only ampicillin, a quinolone,
and trimethoprim-sulfamethoxazole should be
reported routinely. In addition,
chloramphenicol and a third- generation
cephalosporin should be tested and reported
for extraintestinal isolates of Salmonella spp.
Pseudomonas
aeroginosa
Suggested panel for
routine testing & reporting
Urine
Imipenem
meropenem
Ofloxacin
gentamycin
piperacilin
Amikacin
cefepime
ceftazidi
me
Ciprofloxacin
levofloxacin
Suggested panel for
routine testing & reporting
Staphylococcus
spp
Urine
Oxacilln
(cefoxitin
disk)
penicillin
clindamycin
Chlor
amphenicol
Ciprofloxacin
levofloxacin
Gentamycin
Ciproflox
acin
Nitrofurant
oin
SXT
linezolid SXT
tetracycline
vancom
ycin
staphylococcus
Penicillin-susceptible staphylococci are also susceptible to other
penicillins, cephems, and carbapenems approved for use by the FDA for
staphylococcal infections. Penicillin-resistant, oxacillin- susceptible strains
are resistant to penicillinase-labile penicillins, but susceptible to
other penicillinase-stable penicillins, β-lactam/β-lactamase inhibitor
combinations, relevant cephems, and carbapenems. Oxacillin-resistant
staphylococci are resistant to all currently available β-lactam antibiotics.
Thus, susceptibility or resistance to a wide array of β-lactam antibiotics
may be deduced from testing only penicillin and oxacillin.
Routine testing of other penicillins, β-lactamase inhibitor
combinations, cephems, and carbapenems is not advised.
cefoxitin disk test
The cefoxitin disk test is the preferred method
for testing S. aureus, S. lugdunensis, and
coagulase-negative staphylococci for
resistance to the penicillinase-stable
penicillins. Cefoxitin is used as a surrogate
for detecting oxacillin resistance; report
oxacillin as susceptible or resistant based on
the cefoxitin result.
Suggested panel for
routine testing & reporting
Penicillin
Ampicillin
linezolid
ciproflox
acin
Nitrofu
rantoin
Enterococcus
Urine
vancomycin
Gentamycin
chloramphenicol
strepto
mycin
. Warning:
For Enterococcus spp., cephalosporins,
aminoglycosides (except for high-level
resistance screening), clindamycin, and
trimethoprim-sulfamethoxazole may appear
active in vitro, but are not effective clinically,
and isolates should not be reported as
susceptible.
Enterococcus
ß lactamase - enterococcus :susceptible to
penicillin,ampicillin,amoxicillin,ampicillin
sulbactam,amoxicillin-clavulanic
acid,piperacillin,piperacillin tazobactam
Prediction with: penicillin disk test
ß lactamase + enterococcus : Resistant to
penicillin,ampicillin,amoxicillin,ampicillin sulbactam,amoxicillinclavulanic acid,piperacillin,piperacillin tazobactam
a β-lactamase test is the only reliable test
for detecting β-lactamase-producing Enterococcus spp.
Enterococcus
For blood and cerebrospinal fluid
isolates(Enterococci), a β-lactamase test is
also recommended.
Notice: combined therapy with high- dose
penicillin or high-dose ampicillin or
vancomycin or teicoplanin plus gentamicin
or streptomycin for bactericidal action
is usually indicated for serious enterococcal
infections, such as endocarditis.
Alternative drugs for VRE
Because of limited alternatives,
chloramphenicol, erythromycin, tetracycline
(or doxycycline or minocycline), and
rifampin may be tested for vancomycinresistant enterococci (VRE),
And
consultation with an infectious disease
practitioner is recommended.
Acinetobacter spp
Suggested panel for
routine testing & reporting
Urine
ceftazidime
Imipenem
meropenem
Amikacin
cefepime
ciprofloxacin
SXT
gentamycin
Piperacilin
ticarcilin
Cefotaxim
ceftriaxon
Burkholderia cepacia
Suggested panel for
routine testing & reporting
ceftazidime
meropenem
SXT
Stenotrophomonas
maltophilia
Suggested panel for
routine testing & reporting
SXT
levofloxacin
Minocyclin
Suggested panel for
routine testing & reporting
S.pneumonia
Penicillin
(oxacilin)
linezolid
vancomycin
clindamycin
ofloxacin
tetracycline
Erythromycin
chloramphenicol
SXT
Pneumococcus from CSF =MIC
penicillin and cefotaxime or ceftriaxone or meropenem
Vancomycin (MIC or Disk)
Pneumococcus from other sites:
Oxacillin Disk Screening Test
zone ≤ 19 mm =» MIC of
penicillin,cefotaxime,ceftriaxone
. Susceptibility testing of penicillins and other β-lactams
approved by FDA for treatment of Streptococcus pyogenes
or Streptococcus agalactiae is not necessary for clinical
purposes and need not be done routinely, since as with
vancomycin, resistant strains have not been recognized.
Interpretive criteria are provided for pharmaceutical
development, epidemiology, or monitoring for emerging
resistance. Any strain found to be nonsusceptible should
be referred to a reference laboratory for confirmation.
Streptococcus spp.other than
S.pneumonia
Suggested panel for
routine testing & reporting
Penicillin
or
ampicillin
Linezolid
vancomycin
clindamycin
ofloxacin
Cefepime
or
cefotaxime
Or
ceftriaxone
Erythromycin
chloramphenicol
Group B streptococci
Recommendations for intrapartum prophylaxis for Group B
streptococci are penicillin or ampicillin. While cefazolin is
recommended for penicillin-allergic women at low risk for
anaphylaxis, those at high risk for anaphylaxis may receive
clindamycin or erythromycin. Group B streptococci are
susceptible to ampicillin, penicillin, and cefazolin, but may
be resistant to clindamycin and/or erythromycin. When a
Group B streptococcus is isolated from a pregnant woman
with severe penicillin allergy (high risk for anaphylaxis),
clindamycin and erythromycin should be tested and reported.
ESBLs
Strains of Klebsiella spp. E. coli, and P.
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.
• Some of these strains will show zones of inhibition
below the normal susceptible population, but above
the standard breakpoints for certain extendedspectrum cephalosporins or aztreonam; such strains
may be screened for potential ESBL production by
using the screening breakpoints .Other strains may
test intermediate or resistant by standard
breakpoints to one or more of these agents.
• In all strains with ESBLs, the zone diameters
for one or more of the extended-spectrum
cephalosporins should increase in the
presence of clavulanic acid.
Method
Initial Screen Test
Phenotypic Confirmatory Test
Medium
Mueller-Hinton Agar
Mueller-Hinton Agar
Antimicrobial Disk
Concentration
For K. pneumoniae, K. oxytoca, and
E.
coli:
Cefpodoxime
10
µµg
or
ceftazidime
30 µµg or aztreonam
30 µµg or cefotaxime
30 µµg or ceftriaxone
30 µµg
For P. mirabilisa: Cefpodoxime 10µ
µg or ceftazidime
30
µµg or cefotaxime
30µ
µg
(The use of more than one
antimicrobial agent for screening
improves the sensitivity of detection.)
Ceftazidime
30 µµg
Ceftazidime-clavulanic acidb 30/10µ
µg and
Cefotaxime
30µ µg
Cefotaxime-clavulanic acidb 30/10
µµg
(Confirmatory testing requires use of
both cefotaxime and ceftazidime, alone
and in combination with clavulanic
acid.)
Results of initial screen test
For K. pneumoniae, K. oxytoca, and E.
Result of phenotypic
confirmatory Test
A ≥ 5-mm increase in a zone diameter for either
antimicrobial agent tested in
coli:
Cefpodoxime zone
≤
17
mm
combination with clavulanic acid versus its
Ceftazidime zone
≤
22
mm
Aztreonam zone
≤
27
mm zone when tested alone = ESBL (e.g., ceftazidime
Cefotaxime zone
≤
27
mm
Ceftriaxone zone
≤ 25 mm
zone
=
16;
ceftazidimeFor P. mirabilisa:
Cefpodoxime zone
≤ 22 mm Ceftazidime clavulanic acid zone = 21).
zone
≤ 22 mm Cefotaxime zone
≤
27 mm
Zones above may indicate ESBL production
Screening of Proteus mirabilis for
ESBL
• Screening of Proteus mirabilis for ESBL
production is recommended only when it is
deemed clinically relevant (e.g., a bacteremic
isolate).
• For all confirmed ESBL-producing strains,
the test interpretation should be reported
as
resistant for all penicillins, cephalosporins, and
aztreonam.
staphylococcus
Penicillin-susceptible staphylococci are also
susceptible to other penicillins, cephems,
and carbapenems approved for use by the
FDA for staphylococcal infections. (slide4)
MRSA
• Penicillin-resistant, oxacillin- susceptible
strains are resistant to penicillinase-labile
penicillins, but susceptible to other
penicillinase-stable penicillins, β-lactam/βlactamase inhibitor combinations, relevant
cephems, and carbapenems. slides 5,6
MRSA
• Oxacillin-resistant staphylococci are resistant
to all currently available β-lactam antibiotics.
Thus, susceptibility or resistance to a wide array
of β-lactam antibiotics may be deduced from
testing
only penicillin and oxacillin.
Routine testing of other penicillins, β-lactamase
inhibitor combinations, cephems, and
carbapenems is not advised.
• Historically, resistance to the penicillinasestable penicillins has been referred to as
“methicillin resistance,” thus the acronyms
MRSA (for “methicillin-resistant S. aureus”) or
MRS (for “methicillin-resistant staphylococci”)
are still commonly used, even though
methicillin is no longer the agent of choice for
testing or treatment.
Of the penicillinase-stable penicillins, oxacillin
is preferred for in vitro testing. Oxacillin
susceptibility test results can be applied to the
other penicillinase-stable penicillins, i.e.,
cloxacillin,dicloxacillin,flucloxacillin,methicillin,
and nafcillin.
Most resistance to oxacillin in staphylococci is mediated
by the mecA gene, which directs the production
of a supplemental penicillin-binding protein, PBP 2a,
and is expressed either homogeneously or
heterogeneously. Homogeneous resistance is easily
detected with standard testing methods, whereas
heterogeneous expression may be more difficult to
detect with some methods because only a fraction of
the population (e.g., 1 in 100 000 cells) expresses the
resistance phenotype
Detection of oxacillin resistance
• Detection of oxacillin resistance: Tests for mecA or for the protein
expressed by mecA, the penicillin-binding protein 2a (PBP 2a, also called
PBP2') are the most accurate methods for prediction of resistance to
oxacillin and can be used to confirm disk test results for isolates of
staphylococci from serious infections. Isolates of staphylococci that carry
the mecA gene, or that produce PBP 2a (the mecA gene product), should
be reported as oxacillin resistant. Isolates that do not carry mecA or do
not produce PBP 2a should be reported as oxacillin susceptible. Because
of the rare occurrence of resistance mechanisms other than mecA, if MIC
tests are performed in addition to disk diffusion, isolates for which
oxacillin MICs are ≥4 µg/mL and are mecA negative or PBP 2a negative
should be reported as oxacillin resistant.
•
Report isolates of staphylococci that carry mecA,
or that produce PBP 2a, the mecA gen
product, as
oxacillin resistant.
• Routine testing of urine isolates of S.
saprophyticus is not advised, because
infections respond to concentrations achieved
in urine of antimicrobial agents commonly
used to treat acute, uncomplicated urinary
tract infections (e.g., nitrofurantoin,
trimethoprim ± sulfamethoxazole, or a
fluoroquinolone).
•
• WARNING: For oxacillin-resistant S. aureus and
coagulase-negative staphylococci (MRS), other βlactam agents, i.e., penicillins, β-lactam/β-lactamase
inhibitor combinations, cephems, and carbapenems,
may appear active in vitro, but are not effective
clinically. Results for these drugs should be reported
as resistant or should not be reported. This is
because most cases of documented MRS infections
have responded poorly to β-lactam therapy, or
because convincing clinical data have yet to be
presented that document clinical efficacy for those
agents.
• For oxacillin-susceptible Staphylococcus
aureus and coagulase-negative staphylococci,
results for parenteral and oral cephems, βlactam/β-lactamase inhibitor combinations,
and carbapenems, if tested, should be
reported according to the results generated
using routine interpretive criteria
Penicillin-resistant,oxacillin-susceptible strains of
Staphylococcus aureus produce β-lactamase, and the testing
of the 10-unit penicillin disk instead of the ampicillin disk
is preferred.
Penicillin should be used to test the susceptibility of all βlactamase-labile penicillins, such as ampicillin, amoxicillin,
azlocillin, carbenicillin, mezlocillin, piperacillin, and
ticarcillin. Likewise, a positive β-lactamase test predicts
resistance to these agents. For oxacillin-resistant
staphylococci, report as resistant or do not report.
Vancomycin resistant Staphylococcus
Resistant strains are reliably detected by the
microdilution reference method, the disk
diffusion method and the vancomycin agar
screen test when the tests are incubated for a
full 24 hours at 35 ± 2 ºC.
All staphylococcal isolates for which vancomycin zone diameters are 14
mm or less should be tested by a reference MIC method. The disk
diffusion procedure will not differentiate strains with reduced
susceptibility to vancomycin (MICs 4-8 µg/mL) from susceptible strains
(MIC range 0.5 to 2 µg/mL) even when incubated for 24 hours.
Additionally, vancomycin- resistant S. aureus (VRSA) strains (MICs ≥16
µg/mL) may produce only subtle growth around a vancomycin disk. A BHI
vancomycin agar screen plate containing 6 µg/mL of vancomycin,
such as that used for detection of vancomycin-resistant enterococci, may
be inoculated to enhance the sensitivity of detecting vancomycin
intermediate and vancomycin-resistant strains of S. aureus. Send any
staphylococci determined to have an elevated MIC to vancomycin (≥4
µg/mL) to a reference laboratory.
Staphylococcus spp. may develop resistance
during prolonged therapy with quinolones.
Therefore, isolates that are initially susceptible
may become resistant within three to four
days after initiation of therapy. Testing of
repeat isolates may be warranted.
Cefoxitin Disk Diffusion Testa
The results of disk diffusion tests using a
30-μg cefoxitin disk and alternate
breakpoints can be used to predict mecAmediated oxacillin resistance in
staphylococci. The cefoxitin disk
test is equivalent to oxacillin MIC tests in
sensitivity and specificity for S. aureus. For
coagulasenegative
staphylococci, the cefoxitin disk test has
equivalent sensitivity to oxacillin MIC tests
but
greater specificity, i.e., the cefoxitin disk
test more accurately identifies oxacillinsusceptible strains
than the oxacillin MIC test.
Cefoxitin Disk Diffusion Testa for Prediction of
mecA-Mediated Resistance in Staphylococci
Antimicrobial
Agent
(Disk Content)
Cefoxitin (30 µg)
Organism Group
S. aureus and
S. lugdunensis
Zone Diameter,
Nearest Whole mm
Comme
nts
≤ 19
≥ 20
(30) S. aureus for
which cefoxitin disk
diffusion zones are ≤
19 mm should be
reported as oxacillin
resistant. Those for
which cefoxitin zones
are
≥ 20 mm should be
reported as oxacillin
susceptible.
Coagulase-negative ≤ 24
staphylococci except
S. lugdunensis
≥ 25
(31) Coagulasenegative
staphylococci for
which cefoxitin disk
diffusion zones are ≤
24 mm should be
reported as oxacillin
resistant. Those for
which cefoxitin zones
are
≥ 25 mm should be
reported as
oxacillinsusce.
WARNING: For Enterococcus spp.,
cephalosporins, aminoglycosides (except for
high-level resistance screening), clindamycin,
and trimethoprim- sulfamethoxazole may
appear active in vitro, but are not effective
clinically, and isolates should not be reported
as susceptible.
Synergy between ampicillin, penicillin, or vancomycin
and an aminoglycoside can be predicted for
enterococci by using a high-level aminoglycoside
(gentamicin and streptomycin) screening test. Other
aminoglycosides need not be tested, because their
activities against enterococci are not superior to
gentamicin and streptomycin.
high-level( AG) resistance screening(HLAR):
Gentamycin 120µ :6=R 7-9=Inconclusive ≥10=S
Because of limited alternatives,
chloramphenicol, erythromycin, tetracycline
(or doxycycline or minocycline), and rifampin
may be used for vancomycin- resistant
enterococci (VRE), and consultation with an
infectious disease practitioner is
recommended.
non-β-lactamase-producing
enterococci
Penicillin susceptibility may be used to predict the
susceptibility to ampicillin, amoxicillin, ampicillinsulbactam, amoxicillin-clavulanic acid,piperacillin,and
piperacillin-tazobactam for non-β-lactamaseproducing enterococci. Ampicillin susceptibility can
be used to predict imipenem susceptibility, provided
the species is confirmed to be E.faecalis.
Ampicillin is the class representative for
ampicillin and amoxicillin.Ampicillin results
may be used to predict susceptibility to
amoxicillin-clavulanic acid, ampicillinsulbactam, piperacillin, and piperacillintazobactam among non-β-lactamaseproducing enterococci.
The “susceptible” category for penicillin or
ampicillin implies the need for high-dose
therapy for serious enterococcal infections.
Enterococcal endocarditis requires combined
therapy with high-dose penicillin or high-dose
ampicillin, or vancomycin or teicoplanin plus
gentamicin or streptomycin for bactericidal
action.
Because ampicillin or penicillin resistance among enterococci
due to β-lactamase production is not reliably detected using
routine disk or dilution methods, a direct, nitrocefin-based
β-lactamase test is recommended for blood and
cerebrospinal fluid isolates. A positive β-lactamase test
predicts resistance to penicillin, as well as amino-,
carboxy-, and ureidopenicillins.
When testing vancomycin against enterococci,
plates should be held a full 24 hours and
examined using transmitted light; the
presence of a haze or any growth within
the zone of inhibition indicates resistance.
Organisms with intermediate zones should be
tested by an MIC method .
Oxacillin Screening Plates
Oxacillin Screening Plates
The oxacillin salt-agar screening-plate procedure can be used in addition to
the dilution methods described above for the detection of MRSA. Perform
the test by inoculating a S. aureus isolate onto Mueller-Hinton agar that
has been supplemented with NaCl (4% w/v; 0.68 mol/L) and that contains
6 μg oxacillin/mL. Inoculate the agar from a direct colony suspension
equivalent to a 0.5 McFarland standard using either a 1-μL loop or a swab.
Using a 1-μL loop, spread the inoculum in an area 10 to 15 mm in
diameter. Alternatively, using a swab, express excess fluid as for the disk
diffusion test and then spot an area at least 10 to 15 mm in diameter or
streak an entire quadrant. Incubate the plate at temperatures at 35
± 2 °C (testing at temperatures above 35 °C may not detect MRS) for 24 hours
and examine carefully with transmitted light for evidence of small
colonies (>1 colony) or a light film of growth, indicating oxacillin resistance
.Do not reuse plates after incubation.
•Based on the cefoxitin result, report oxacillin
as susceptible or resistant. ;
• report oxacillin as susceptible or resistant
based on the cefoxitin result.
• ESBL
Thank you
69
Health Reference Laboratory