S U P P L E M E N T A R T I C L E
In Vitro Antimicrobial Findings for Fusidic Acid
Tested Against Contemporary (2008–2009)
Gram-Positive Organisms Collected in the
Ronald N. Jones,1,2Rodrigo E. Mendes,1Helio S. Sader,1and Mariana Castanheira1
1JMI Laboratories, North Liberty, Iowa; and2Tufts University School of Medicine, Boston, Massachusetts
Fusidic acid has a long history of consistent activity against staphylococcal pathogens including methicillin-
resistant Staphylococcus aureus (MRSA). Fusidic acid (CEM-102) was susceptibility tested against a surveillance
study collection of 12,707 Gram-positive pathogens (2008–2009) from the United States. Reference broth
microdilution method results demonstrated the following MIC50/90 results: S. aureus (.12/.25 lg/mL),
coagulase-negative staphylococci (.12/.25 lg/mL), enterococci (4/4 lg/mL), Streptococcus pyogenes (4/8 lg/mL),
and viridans group Streptococcus spp.(.8/.8 lg/mL). At a proposed susceptible breakpoint (#1 lg/mL), fusidic
acid inhibited 99.7% of MRSA strains and 99.3% to 99.9% of multidrug-resistant phenotypes of S. aureus.
Furthermore, S. aureus strains nonsusceptible to fusidic acid (.35%) generally had detectable resistance
mechanisms (fusA, B, C, and E). Reviews of in vitro susceptibility test development confirm the accuracy and
intermethod reproducibility of various fusidic acid methods. Fusidic acid is a promising oral therapy for
staphylococcal skin and skin structure infections in the United States, where the contemporary S. aureus
population remains without significant resistance.
Fusidic acid, also known as CEM-102, is a steroid-class
antimicrobial agent initially identified from Fusidium
coccineum by Godtfredsen et al [1–3] in 1960. That or-
ganism was found in a primate (monkey) stool culture.
Such steroidal agents, however, have no corticosteroid
activity, yet exhibit a well-characterized potency against
staphylococci, including methicillin-resistant Staphylo-
coccus aureus (MRSA) and coagulase-negative staphy-
lococcalspecies (CoNS). Fusidic acid wasintroduced
into clinical trials in 1962 as a potential systemic and
topical therapy for staphylococcal skin and skin struc-
ture infections [4–6].
Thestructureoffusidicacid(Figure 1) as a sodium salt
is water soluble and active via the oral route (molecular
weight, 538.7; pK, 5.35) . Biedenbach and colleagues
recently redefined the fusidic acid spectrum and activity
against a wide range of pathogens as follows: S. aureus
(minimum inhibitory concentration [MIC], .25 lg/mL),
Micrococcus luteus (MIC, .25–.5 lg/mL), Corynebacterium
spp. (MIC, .06–.12 lg/mL), Moraxella catarrhalis (MIC,
.06–.12 lg/mL), and Neisseria meningitidis (MIC, .12–.25
lg/mL); Streptococcus spp., not S. pyogenes (MIC, 16–32
lg/mL), and enterococci (MIC, 2–8 lg/mL) were less
susceptible with Gram-negative bacilli being frankly re-
sistant at fusidic acid MIC values of $32 lg/mL . This
range of activity is the result of drug interactions with
elongation factor G (EF-G) that prevents its release from
the ribosome, thus compromising protein synthesis [8–
10], a mode of action that continues to be activelystudied
Fusidic acid resistance has long been thought to
be caused by mutations of the EF-G-encoding gene
Correspondence: Ronald N. Jones, MD, 345 Beaver Kreek Centre, Suite A, North
Liberty, Iowa 52317 (firstname.lastname@example.org).
Clinical Infectious Diseases
? The Author 2011. Published by Oxford University Press on behalf of the
Infectious Diseases Society of America. All rights reserved. For Permissions,
please e-mail: email@example.com.
Fusidic Acid Activity (United States 2008–2009)
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[12, 14]. More recently acquired mechanisms (fusB and C) were
detected as mobile elements that can either be chromosome- or
plasmid-mediated in staphylococci [15–21]. At least 5 mecha-
nisms exist (fusA–E), producing staphylococcal resistance cor-
relating with fusidic acid MIC values $2 lg/mL [22, 23]. As this
antimicrobial was used clinically worldwide, microbiologists in
some nations encountered Gram-positive pathogens with ele-
vated fusidic acid resistance rates (eg, United Kingdom, Ireland,
Greece); in contrast, in the United States (US), the US Food and
Drug Administration (FDA) has not approved this agent for
therapeutic use by any route of administration. Thus, this
unique steroidal antimicrobial, if used in the US, would be
prescribed for treatment of a naı ¨ve population of Staphylococcus
species [22, 23], as compared with a country like Australia [22,
In this report, we summarize the results of a fusidic acid
resistance surveillance study in the US for 2008 and 2009. We
also describe the recently published investigations defining fu-
sidic acidsusceptibilityratesglobally when testedagainst various
Gram-positive pathogens, as well as the status of in vitro di-
agnostic methods for clinical application.
MATERIALS AND METHODS
A total of 7339 S. aureus strains collected in 51 US medical
centers, located in the 9 census regions, were analyzed as part of
the SENTRY Antimicrobial Surveillance Program (2008–2009).
These isolates were obtained from bloodstream, respiratory tract
infections, and skin and skin structure infections, according to
defined protocols. An additional 5368 Gram-positive organisms
were sampled as follows: CoNS (1352), enterococci (2448),
b-hemolytic streptococci (1255), and viridans group Strepto-
coccus spp. (313) (Table 2). Only 1 isolate per patient from
documented infections was included. Species identification was
confirmed by standard biochemical tests, the Vitek 2 System
(bioMe ´rieux), or 16S ribosomal RNA sequencing, when neces-
Antimicrobial Susceptibility Testing
Isolates were susceptibility tested by a reference broth micro-
dilution procedure as described by the Clinical and Laboratory
Standards Institute (CLSI)  using validated microdilution
panels (TREK Diagnostics). Categorical interpretations for all
antimicrobials were those found in M100-S20-U , and
quality control was performed using Escherichia coli (ATCC
25922), S. aureus (ATCC 29213), and Enterococcus faecalis
(ATCC 29212). All quality control results were within specified
ranges as published in CLSI documents . For fusidic acid,
the interpretive susceptibility criteria of the European Com-
mittee on Antimicrobial Susceptibility Testing (EUCAST; 2010)
were applied at #1 lg/mL , and quality control ranges were
used based on the recent study reported by Jones and Ross .
Detection of Fusidic Acid Resistance Mechanisms
All strains displaying fusidic acid MIC at $2 lg/mL (EUCAST
resistant breakpoint) were tested for the presence of fusB, fusC,
and fusD in a multiplex polymerase chain reaction approach.
Detection of fusD (intrinsic in S. saprophyticus) was included in
this reaction to detect strains that were incorrectly identified as
other staphylococcal species.
Constitutive genes fusA and fusE were amplified and se-
quenced using Extensor Hi-Fidelity Master Mix (ABgene) as
well as custom and previously described oligonucleotides [22,
23]. Sequencing was performed in 5 and 2 reactions, re-
spectively. The nucleotide sequences and deduced amino acid
sequences were analyzed using Lasergene software version 8.1.5
(DNASTAR) and compared with sequences available through
the internet using BLAST, the Basic Local Alignment Search
RESULTS AND DISCUSSION
Fusidic Acid Activity Against S. aureus
A total of 7339 S. aureus strains among 12,707 Gram-positive
pathogens tested in the US SENTRY surveillance program in
2008–2009 were analyzed here. Of the S. aureus strains, 3876
(52.8%) were MRSA and the sample numbers were 3962 (2008)
and 3377 (2009) (Table 1). Most of the S. aureus strains were
from bacteremias (46.0%), skin and skin structure infections
(31.5%), and lower respiratory tract cultures (16.6%).
Overall, fusidic acid (MIC50/90, .12/.25 lg/mL) inhibited
99.65% of S. aureus strains at #1 lg/mL ; only .22% had
MIC results at $8 lg/mL (Table 1). No significant differences
were noted in MIC50, MIC90, or percentage of strains at #1 lg/
mL between years, but a slight trend toward more strains with
fusidic acid MIC values at $2 lg/mL was observed among
The chemical structure of fusidic acid.
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