Download Antimicrobial Resistance and Susceptibility Testing of Anaerobic

ANTIMICROBIAL RESISTANCE
INVITED ARTICLE
George M. Eliopoulos, Section Editor
Antimicrobial Resistance and Susceptibility
Testing of Anaerobic Bacteria
Audrey N. Schuetz1,2,3
1
Clinical Microbiology Laboratory, Departments of 2Pathology and Laboratory Medicine, and 3Internal Medicine, Weill Cornell Medical College/NewYork–
Presbyterian Hospital, New York, New York
Keywords. anaerobe; anaerobic bacteria; resistance; susceptibility; susceptibility test methods.
Anaerobic infections can be severe and life-threatening,
and their increasing incidence is due in part to the high
numbers of patients with complex underlying diseases
[1]. Studies demonstrate that failure to direct appropriate anaerobic therapy leads to poor clinical response [2].
Anaerobic collection, transport, and manipulation of
culture isolates can be time-consuming and difficult
to perform correctly. Empirical broad-spectrum antimicrobial therapy for anaerobic infections has often
been instituted long before results of susceptibility testing are available.
However, antimicrobial bacterial resistance among
anaerobic bacteria is increasing globally, as demonstrated by numerous surveys in Europe, the United States,
Canada, and New Zealand [3–6]. Differences in resistance patterns may be due to the variety of susceptibility
testing methodology used, the selective antibiotic pressures associated with antimicrobial usage, and the lack
of uniformity in adoption of interpretive breakpoints.
Received 10 November 2013; accepted 18 May 2014; electronically published 27
May 2014.
Correspondence: Audrey N. Schuetz, MD, MPH, Clinical Microbiology Laboratory,
Department of Pathology and Laboratory Medicine, Department of Internal
Medicine, Weill Cornell Medical College/NewYork–Presbyterian Hospital, 525 E
68th St, Starr 737C, New York, NY 10065 ([email protected]).
Clinical Infectious Diseases 2014;59(5):698–705
© The Author 2014. Published by Oxford University Press on behalf of the Infectious
Diseases Society of America. All rights reserved. For Permissions, please e-mail:
[email protected].
DOI: 10.1093/cid/ciu395
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This article covers typical resistance patterns and recent
changes in resistance of certain organisms over time.
Anaerobic susceptibility testing methods and their challenges are reviewed.
RESISTANCE PATTERNS OF ANAEROBIC
BACTERIA
Bacteroides fragilis Group
The members of the B. fragilis group are among some
of the least susceptible anaerobes to antibiotics. Resistance of Bacteroides species is linked to outcome, even
in the presence of mixed infections [4, 7]. The B. fragilis
group consists of 24 species including Bacteroides fragilis, Bacteroides vulgatus, Bacteroides ovatus, Bacteroides
thetaiotaomicron, Bacteroides uniformis, Bacteroides
caccae, and Parabacteroides distasonis.
Fewer than 3% of Bacteroides isolates are susceptible
to penicillin and ampicillin [8]. Penicillin and cephalosporin resistance is mediated by the cepA and cfxA
genes. The chromosomal cepA gene is a cephalosporinase encoding for resistance to cephalosporins and
aminopenicillins but not piperacillin or β-lactam–βlactamase inhibitor combinations (BLBLIs). The cfxA
gene encodes for high-level resistance to cefoxitin and
other β-lactams [9]. Although β-lactamases are the
principal mechanism of resistance to penicillins, the expression of altered penicillin-binding proteins can also
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Infections due to anaerobic bacteria can be severe and life-threatening. Susceptibility testing of anaerobes is not
frequently performed in laboratories, but such testing is important to direct appropriate therapy. Anaerobic
resistance is increasing globally, and resistance trends vary by geographic region. An overview of a variety of
susceptibility testing methods for anaerobes is provided, and the advantages and disadvantages of each method
are reviewed. Specific clinical situations warranting anaerobic susceptibility testing are discussed.
Prevotella, Porphyromonas, and Other Anaerobic
Gram-Negative Rods
Porphyromonas species are generally susceptible to β-lactams,
clindamycin, and metronidazole [21]. One-fourth to one-third
of Porphyromonas species produce β-lactamases, and clindamycin resistance has been observed in a minority of strains [22].
Compared with Porphyromonas, approximately 95% of Prevotella species is resistant to penicillin and ampicillin [22]. A
recent Belgian study showed that clindamycin susceptibility in
Prevotella species has been decreasing from 91% in 1993–1994
to 69% in 2012 [23]. Susceptibility to moxifloxacin, metronidazole, carbapenems, and amoxicillin-clavulanate is typically
≥90%. Penicillin resistance in Fusobacterium species ranges
from 4% to 15% and is generally due to the production of βlactamases [22]. Fusobacterium varium, Fusobacterium mortiferum, and Fusobacterium nucleatum are most often reported to
produce β-lactamases, and >90% of Fusobacterium necrophorum are susceptible to cephalosporins and cephamycins
[14, 24]. Fusobacterium species are typically susceptible to
metronidazole, BLBLIs, cephalosporins, carbapenems, and
clindamycin.
Gram-Positive, Non-Spore-Forming Rods
Actinomyces, Propionibacterium, Bifidobacterium, and the
Eubacterium group are usually susceptible to β-lactams and
BLBLIs [25, 26]. Propionibacterium, Actinomyces, Bifidobacterium, and Lactobacillus are usually resistant to metronidazole.
Clindamycin shows moderate activity against these bacteria
[27]. Decreasing clindamycin susceptibility of Propionibacterium
acnes is associated with prior topical acne therapy [28]. Vancomycin shows some activity against certain species of Lactobacillus; however, MICs >256 µg/mL are common for the most
frequently isolated lactobacilli from human specimens, Lactobacillus casei and Lactobacillus rhamnosus [29]. Telavancin has
shown good activity against some Lactobacillus species [30].
Newer antimicrobial agents such as linezolid, daptomycin, and
dalbavancin exhibit excellent in vitro activity against most anaerobic gram-positive species [31, 32].
Gram-Positive, Spore-Forming Rods
Of the clostridial species, Clostridium perfringens is one of the
most susceptible to penicillin, but clindamycin resistance is increasing, including up to 14% of blood isolates of C. perfringens
in one study [5, 26, 33]. Tetracycline resistance (defined as MIC
>2 µg/mL) has been documented in up to 75% of C. perfringens
isolates obtained from commercial poultry, but there are few data
in humans [34, 35]. The activities of tetracycline against other
clostridial species and of doxycycline against clostridia in general
have not been well documented [35]. Drugs that maintain activity
against non-perfringens Clostridium species include piperacillin,
BLBLIs, carbapenems, metronidazole, and vancomycin [36].
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lead to resistance [10]. Membrane modifications, such as porin
losses, may further increase the minimum inhibitory concentrations (MICs) to β-lactams and BLBLIs. Currently, ticarcillinclavulanate and piperacillin-tazobactam are active against
>90% of the B. fragilis group [3, 11, 12]. Carbapenems are generally active against the B. fragilis group, and most US-based
studies report susceptibility rates of 98.5%–99% [4, 13, 14].
Carbapenem resistance is usually mediated by a chromosomal
zinc metallo-β-lactamase enzyme encoded by the cfiA gene
[15]. Although cephamycins were used in the past for anaerobic
coverage, the Infectious Diseases Society of America (IDSA)
recently stated that cefotetan and clindamycin are no longer
recommended as therapy for community-acquired intraabdominal infections in adults due to increasing resistance
among the B. fragilis group [16]. Clindamycin resistance is mediated by erm genes located on transferable plasmids that can
also carry tetracycline resistance genes [17]. Worldwide, clindamycin resistance is increasing and approaches 60% [3, 4, 12, 18].
Although rare, metronidazole-resistant strains of B. fragilis
have been reported and are associated with the nim gene [3,
11, 15]. However, the presence of this gene does not invariably
lead to resistance, and MICs must be obtained. Of 206 B. fragilis
isolates in one study, the nim gene was detected in 7.3%, but
metronidazole MICs ranged from 1.5 mg/L (susceptible) to
>256 mg/L (resistant) [19]. Susceptibility rates to moxifloxacin
are highly variable across studies, and isolates are acquiring resistance rapidly. Moxifloxacin susceptibility rates range from
30% to 60% for the B. fragilis group, and such resistance is mediated by gyrA mutations, efflux pumps, or topoisomerase gene
modifications [12]. Tigecycline MICs are relatively low in surveillance studies, but there have been reports of resistance [6].
Susceptibility within the B. fragilis group varies by species,
with B. fragilis being the most susceptible. Parabacteroides distasonis demonstrates high MICs to β-lactams, and B. thetaiotaomicron also demonstrates higher resistance rates to various
antimicrobials in comparison to other members of the B. fragilis
group. A survey of 6574 B. fragilis group isolates collected from
10 US medical centers confirmed that susceptibility rates differed widely by species [4]. In this series, B. ovatus was more resistant to the carbapenems; B. vulgatus was more resistant to
piperacillin-tazobactam and showed the highest resistance
rates to moxifloxacin (54%); P. distasonis was more resistant
to ampicillin-sulbactam and cefoxitin; and B. ovatus and B.
uniformis showed higher resistance rates to moxifloxacin
(39% and 41%, respectively). However, a 2008 practice survey
of anaerobes across the United States reported that 33% of reference laboratories did not identify B. fragilis group isolates to
the species level [20]. Susceptibility rates among B. fragilis group
are decreasing worldwide. Snydman et al reported a decrease in
susceptibility of approximately 25% over 11 years to ampicillinsulbactam in P. distasonis and B. vulgatus [4].
Anaerobic Cocci
The anaerobic gram-positive cocci (GPC) have undergone
major taxonomic changes within the past 40 years; therefore,
susceptibility data have been difficult to interpret. More than
90% of anaerobic GPC are susceptible to penicillin, including
most Peptostreptococcus species [35]. Anaerobic GPC are also
generally susceptible to β-lactams, BLBLIs, cephalosporins,
and carbapenems [24]. Alterations in penicillin-binding
proteins account for the majority of β-lactam resistance [43].
Clindamycin resistance rates among anaerobic GPC range
from 7% to 20%, and this resistance is rising especially in Finegoldia magna and Peptoniphilus species [14, 35]. Among GPC,
90%–95% are susceptible to metronidazole, but rare nimBpositive, metronidazole-resistant strains of F. magna and
Parvimonas micra have been described [35, 44]. Strains of streptococci that grow well anaerobically, such as the Streptococcus
anginosus group, may initially be misidentified as anaerobic
GPC and will test resistant to metronidazole, so metronidazole
resistance in anaerobic GPC should be confirmed.
ANAEROBIC SUSCEPTIBILITY TESTING
Susceptibility testing is performed to obtain information on the
predicted response of the bacteria to antibiotics in the form of
an MIC, which is defined as the lowest concentration of the antibiotic that inhibits growth of organisms. Drugs are typically tested
at 2-fold doubling (log2) serial dilutions (ie, 2 µg/mL, 4 µg/mL,
8 µg/mL, etc). MICs may be obtained by utilizing the Clinical and
Laboratory Standards Institute (CLSI)–defined agar or broth microdilution methods or by using a variety of commercially available
methods. Commercial methods include Etest strips (bioMérieux,
Durham, North Carolina), M.I.C.Evaluator strips (M.I.C.E.; Thermo Fisher Scientific, Basingstoke, United Kingdom), and Sensititre panels (Trek Diagnostics Systems, Thermo Fisher Scientific,
Cleveland, Ohio). Etests and M.I.C.Evaluator strips are gradient
diffusion methods. Agar and broth microdilution methods are
usually employed by research or reference laboratories. Advantages and disadvantages of anaerobic antimicrobial susceptibility testing (AST) methods are listed in Table 1.
Table 1. Advantages and Disadvantages of Antimicrobial Susceptibility Testing Methods for Anaerobic Bacteria
Method
Advantages
Disadvantages
Agar dilution
•
Reference method against which other
methods are compared
•
•
•
•
Broth microdilution
•
• Currently recommended by CLSI only for Bacteroides fragilis
group organisms
• Shelf life of frozen panels may be limited
• Poor growth by some strains has been shown
•
Multiple antimicrobials (≥10) can be tested
per isolate
Commercial panels are available
Laboratories can customize their own
panel of antibiotics to test
Ease of use
Precise MIC value is obtained
Convenient for testing of a few individual
isolates
Multiple drugs can be tested at one time
•
Rapid (at most 30 min to results)
• Only tests for β-lactamases
• Can be used on a limited number of bacterial species
• Negative result must be followed up with an MIC test for accurate
prediction of β-lactam susceptibility
•
•
MIC gradient diffusion
method
Rapid β-lactamase disk
testing
•
•
•
Labor-intensive
Expertise required for performance
Considerable time is involved in setting up testing
Not amenable/appropriate for testing small numbers of organisms
• Expensive for surveillance purposes
• Metronidazole resistance can be overestimated if anaerobiosis
is inadequate
Abbreviations: CLSI, Clinical and Laboratory Standards Institute; MIC, minimum inhibitory concentration.
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Antimicrobial agents lacking significant activity include ampicillin, aminoglycosides, trimethoprim-sulfamethoxazole, and clindamycin. Resistance to clindamycin is typical for many species,
including Clostridium tertium, Clostridium difficile, and the
“RIC” group of clostridia—namely, Clostridium ramosum, Clostridium innocuum, and Clostridium clostridioforme. In a study of
540 clinical isolates of Clostridium species, tigecycline was highly
active, with an MIC90 of 0.25 µg/mL [37].
Clostridium difficile is usually susceptible to metronidazole
and vancomycin but resistant to β-lactams, fluoroquinolones,
and clindamycin [38]. Outbreaks of fluoroquinolone-resistant
C. difficile strains have been associated with increased mortality
[39]. Strains resistant to rifampin or rifaximin associated with
rpoB mutations have also been linked to clinical failure [38].
Metronidazole-resistant strains are rare but have been documented [40]. A recent surveillance study of C. difficile isolates
collected from 14 European countries reported that more
than one-half of isolates were multidrug resistant, with the majority resistant to moxifloxacin, clindamycin, erythromycin, and
rifampicin [41]. Fidaxomicin shows excellent activity against
C. difficile, but reduced susceptibility due to drug target modifications has been documented in vitro [42].
48 hours of incubation. Multiple antibiotics (10 or more) can be
tested at one time, and the laboratory can design its own drug
panels. However, this technique is limited by poor growth of
certain oxygen-sensitive anaerobes, and is currently validated
by CLSI for use only with B. fragilis group [46].
MIC Gradient Diffusion Method
The Etest and M.I.C.E. strips are thin plastic strips that are impregnated on one face with an increasing gradient concentration of an antimicrobial agent (Figure 1). The other face of
the strip is marked with an MIC scale with increasing antibiotic
concentrations, including increments between doubling dilutions (ie, 1 µg/mL, 1.5 µg/mL, 1.75 µg/mL, 2 µg/mL, etc). A
Brucella blood agar plate supplemented with hemin and vitamin K1 is prereduced in an anaerobic environment to rid the
media of toxic oxygen metabolites. A standard inoculum of bacterial cells is swabbed onto the plate. The Etest or M.I.C.E. strip
is placed antibiotic face down. The MIC is read at the intersection of the lower part of the ellipse with the corresponding
number on the test strip. Results are generally available at
48 hours; however, clostridial species and B. fragilis group isolates usually grow rapidly enough to be read at 24 hours. MIC
gradient diffusion methods are easier to use in routine clinical
laboratory practice than agar dilution. A significant advantage
of the MIC gradient diffusion method is the ability to generate
a quantitative and precise MIC. Additionally, several drugs can
be tested at one time, and laboratories can tailor testing to their
needs. However, Etest strips cost approximately US$2–$3
apiece. Etest results generally correlate well with agar dilution,
but the MIC gradient diffusion method can overestimate
Agar Dilution
The CLSI agar dilution procedure is the gold standard reference method for anaerobic AST [45]. Different concentrations of antimicrobial agents are prepared in 2-fold dilutions,
added to molten agar, poured into Petri dishes, and allowed
to solidify. A standardized inoculum of bacterial cells is spotted
onto the surface of each plate with an inoculator, replicator, or
pipette. After 48 hours of incubation, the plates are examined
visually for the lowest concentration of antibiotic that inhibits
growth. Agar dilution testing requires considerable time,
labor, and expertise and is not intended for testing of single isolates. Thus, it is not a technique that many clinical laboratories
employ.
Broth Microdilution
The broth microdilution method is less labor-intensive than
agar dilution. In this assay, drugs in serial 2-fold dilutions are
added to small volumes of broth liquid media pipetted in
wells of plastic microdilution trays or plates. A standard inoculum of bacterial cells is added into each well. MICs are read after
Figure 1. Penicillin Etest (bioMérieux) of Bacteroides fragilis on Brucella
blood agar plate supplemented with hemin and vitamin K1 (Anaerobe
Systems). The minimum inhibitory concentration of this isolate equals
32 µg/mL.
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In a 2008 practice survey of anaerobic culture and AST in US
hospitals, Goldstein and colleagues reported that only 21% (21/
98) of hospital laboratories performed anaerobic AST in-house
(as opposed to outsourcing to a reference laboratory) [20].
Twenty hospital laboratories reported sending their isolates
out for susceptibility testing, and the remaining 60% (58/98)
of laboratories did not offer any anaerobic AST. In 1990, 70%
of hospital laboratories performed anaerobic AST, but only
33% of hospital laboratories did so in 1993. This decline may
be explained by the fact that the disk diffusion and broth disk
elution methods, which were the predominant methods of susceptibility testing in 1993, are no longer considered appropriate
for susceptibility testing. In the 2008 survey, 65% of hospital
laboratories that performed anaerobic AST used Etest methodology [20]. The remainder of hospital laboratories used microbroth dilution, and none used agar dilution.
Procedural guidelines for anaerobic AST have been published
by CLSI, but the US Food and Drug Administration and European Committee on Antimicrobial Susceptibility Testing also
develop breakpoints [45–47]. Occasionally, there are differences
in interpretive criteria between organizations, which may be explained by differences in dosages, administration intervals, inoculum size, or test media. It is important for clinicians to be
aware of which breakpoints are being used. Antimicrobial susceptibility testing in anaerobes relies primarily on phenotypic
characterization, although a few molecular assays exist (eg,
nim gene for metronidazole resistance and cfiA for carbapenem
resistance) [48]. As molecular data on anaerobic susceptibility
are gathered, it is possible that additional molecular assays
could become available.
Table 2. Indications for Susceptibility Testing of Anaerobic
Bacteriaa
Indication
Persistence of infection despite
adequate therapy with an
appropriate therapeutic
regimen
• Any anaerobe
Particular species of bacteria is
implicated in disease
• Bacteroides fragilis
• Prevotella species
• Clostridium species other than
C. perfringens
• Bilophila wadsworthia
• Fusobacterium species
• Sutterella wadsworthensis
• Anaerobic organisms involved
in osteomyelitis, endocarditis,
or brain abscess
• B. fragilis group implicated in
osteomyelitis or joint infection
Long-term therapy needed
metronidazole resistance if anaerobiosis is not adequate [49].
Because the activity of metronidazole is dependent upon an
active intermediate that requires the reduced atmosphere of
anaerobiosis, it is important to utilize anaerobic control organisms when setting up such testing.
Pivotal role of antimicrobial
agent in clinical outcome
Infections of specific body sites
• Brain abscess
• Endocarditis
• Prosthetic devices or graft
infections
• Bacteremia
a
The above suggestions are examples only and are not intended to be used as
all-inclusive lists.
proteins [52]. Therefore, a negative result by the rapid βlactamase disk test must be followed by an MIC test for accurate
prediction of β-lactam susceptibility. Also, the β-lactamase disk
testing should not be performed on members of the B. fragilis
group, as they are presumed to be resistant due to almost universal presence of β-lactamases.
Disk Diffusion
Commercially Available Broth Microdilution Panels
Two commercially available panels for anaerobic susceptibility
testing are available. The Anaerobe Sensititre panel (ANO2,
Thermo Fisher) is a panel of antimicrobials of varying 2-fold
dilutions (Figure 2) [50]. A second anaerobic susceptibility
panel is available through Oxoid (Thermo Fisher) [51].
Rapid β-Lactamase Disk Testing
This disk test evaluates for the presence of β-lactamase enzyme.
Bacterial colonies are smeared onto a disk embedded with a
chromogenic cephalosporin. If the organism expresses βlactamase, the disk will change colors after reagent is added.
Although most reactions occur within 5–10 minutes, some
β-lactamase–positive strains may react more slowly (up to 30
minutes). If the disk is positive, the organism is considered to
be resistant to penicillin, ampicillin, and amoxicillin. Because
β-lactamases are not the sole mediator of resistance in anaerobes, a negative test does not rule out penicillin or ampicillin
resistance resulting from alterations of penicillin-binding
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Disk diffusion (Kirby-Bauer) tests should not be performed for
anaerobic bacteria for the purpose of obtaining susceptibility results, as the results are inaccurate and do not correlate with the
agar dilution method [46]. Special-potency antibiotic disks of
vancomycin (5 µg), kanamycin (1000 µg), and colistin (10 µg)
are occasionally used in the laboratory as an aid in preliminary
identification of some anaerobes based on patterns of resistance
[53]. However, due to the high antibiotic concentrations in
these disks, these disks are not intended for use in determining
antibiotic susceptibility for therapeutic purposes.
STRATEGIES FOR TESTING AND REPORTING
OF SUSCEPTIBILITY DATA
Antimicrobial Testing Guidelines
For individual patient management, anaerobic AST should be
performed when (1) selection of an active agent is critical for
disease management, (2) long-term therapy is being considered,
(3) anaerobes are recovered from sterile body sites, or (4) the
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Figure 2. Anaerobic susceptibility testing panel ANO2 (Sensititre, Thermo Fisher Scientific) inoculated with Bacteroides fragilis ATCC 25285. Various antimicrobial agents in increasing 2-fold concentrations are present in
the plate. Growth of the bacteria in a well is indicated by a dot of organismal growth in the bottom of the well. In a series of increasing concentration of a single antibiotic, the first empty (no growth) well is considered
the minimum inhibitory concentration (MIC) of that antibiotic. For example,
clindamycin is present in the fourth row from the top, beginning at a concentration of 0.25 µg/mL (fourth row, first well) and extending to 8 µg/mL
(fourth row, sixth well from the left). Thus, the MIC for clindamycin is 2 µg/
mL (fourth row, fourth well from the left), which is susceptible. Photo was
provided courtesy of Thermo Fisher Scientific. Copying is prohibited.
Examples
Table 3. Suggested Antimicrobial Agents for Testing and
Reporting on Anaerobic Organismsa
Antimicrobials for Primary Testing and Reporting
Bacteroides fragilis Group and
Other Gram-Negative Anaerobes
Gram-Positive Anaerobes
Ampicillin
Penicillin
Amoxicillin-clavulanate
Ampicillin-sulbactam
Ampicillin-sulbactam
Piperacillin-tazobactam
Ticarcillin-clavulanate
Piperacillin-tazobactam
Ticarcillin-clavulanate
Clindamycin
Clindamycin
Doripenem
Ertapenem
Doripenem
Ertapenem
Imipenem
Imipenem
Meropenem
Metronidazole
Meropenem
Metronidazole
Antibiograms
Establishment of an antibiogram outlining patterns of resistance on a periodic basis is helpful in guiding antimicrobial
therapy. The CLSI suggests gathering susceptibility data from
at least 30 isolates of the same genus or species collected over
the period of a year to obtain a reasonable number of isolates
upon which to estimate susceptibilities [54]. Examples of cumulative antimicrobial susceptibility reports for B. fragilis group
and other anaerobes from across the United States are included
in the CLSI documents M100 and M11 [45, 46].
CONCLUSIONS
Ampicillin
Ampicillin
Susceptibility among anaerobes differs from species to species and
also from region to region. Surveillance studies have shown that
susceptibility trends of anaerobes to certain antimicrobial agents
are changing, with more resistance recognized in many parts of
the world. Despite the difficulties inherent in anaerobic AST, testing should be performed and observed for trends over time.
Penicillin
Ceftizoxime
Penicillin
Ceftizoxime
Note
Ceftriaxone
Ceftriaxone
Cefotetan
Cefoxitin
Cefotetan
Cefoxitin
Piperacillin
Piperacillin
Ticarcillin
Chloramphenicol
Ticarcillin
Tetracycline
Moxifloxacin
Moxifloxacin
Supplemental Antimicrobials for Selective Testing and Reporting
Bacteroides fragilis Group and
Other Gram-Negative Anaerobes
Gram-Positive Anaerobes
Table adapted with permission from the Clinical and Laboratory Standards
Institute [46].
a
Only 1 of the antimicrobial agents in each box needs to be tested under usual
circumstances, as antibiotics that are listed together demonstrate similar
clinical efficacy and interpretive results.
infection persists despite adequate therapy with an appropriate
therapeutic regimen (Table 2). In cases of a single recovered anaerobic pathogen from culture, AST should be performed. CLSI
suggests certain antimicrobial agents to be considered for testing and reporting on anaerobic organisms (Table 3) [46]. For
gram-negative anaerobes, penicillin or ampicillin may be tested
and/or reported selectively for all except the B. fragilis group,
which are almost uniformly resistant. For non-spore-forming
gram-positive anaerobes, resistance to metronidazole is common, and ampicillin and penicillin are recommended for primary testing. Although CLSI MIC breakpoints for amoxicillin
have not been established for anaerobes, amoxicillin breakpoints are considered equivalent to ampicillin [46]. The remaining supplemental antimicrobials are helpful to test when strains
Potential conflicts of interest. Author certifies no potential conflicts of
interest.
The author has submitted the ICMJE Form for Disclosure of Potential
Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed.
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