Redeploying β-Lactams Against Staphylococcus aureus: Repurposing With a Purpose
Redeploying β-Lactams Against Staphylococcus aureus: Repurposing With a Purpose
Arnold S. Bayer () 1
Yan Q. Xiong 0
0 David Geffen School of Medicine at UCLA , Los Angeles, California
1 LA Biomedical Research Institute at Harbor-UCLA , Torrance
β-Lactams; MRSA; MSSA; synergy; Staphylococcus aureus
β-Lactam antibiotics have been a
mainstay of clinical therapeutics for
approximately 70 years, especially for
aureus (MSSA) infections. Since
approximately one half of S. aureus bacteremias are
caused by MSSA , the
antistaphylococcal β-lactams remain key elements of
therapeutic strategies for such infections. Data
from a number of clinical trials have
documented the therapeutic superiority of
antistaphylococcal β-lactams over vancomycin
for MSSA bacteremic infections, including
endocarditis [2–4]. Further, the American
Heart Association has consistently
recommended β-lactams as the treatment of
choice for MSSA endocarditis .
Recently, the antistaphylococcal
βlactams have emerged as an additional
tool for treating recalcitrant
methicillinresistant S. aureus (MRSA) bacteremic
infections, often in combination with
daptomycin. The β-lactams that have
been deployed off label in combination
for this scenario include nafcillin,
oxacillin, and ceftaroline [6, 7]. This apparent
synergy extends to both persistent
MSSA infections and persistent MRSA
infections, suggesting that a novel
mechanism(s) is involved. Studies from several
laboratories [8, 9] have identified that the
key synergic event is likely the capacity
of the β-lactams of interest to block
penicillin-binding protein1 (PBP1). This
synergic event with daptomycin occurs
whether the PBP1 blockade is
promiscuous (ie, whether, like nafcillin, it binds
to PBP1–4) or occurs in a more
PBP1specific manner . Of interest, this
daptomycin–β-lactam synergy is also seen
with other cationic peptides, including
those of the innate host defense system
(eg, LL-37) . The 2 main theories
about the mechanism(s) of this synergy
between cationic peptides (eg, calcium
daptomycin) and PBP1-targeting β-lactams are
(1) enhanced binding of daptomycin to
the divisome, its principal site of action;
and/or (2) augmentation of the functional
activity of daptomycin without increasing
In the current issue of The Journal
of Infectious Diseases, Waters et al 
propose yet another role for β-lactams
in anti-staphylococcal therapeutics—an
antivirulence mechanism to enhance
clinical outcomes in MRSA infections,
using oxacillin as the proof-of-principle
β-lactam. It should be emphasized that
defining an agent’s specific antivirulence
properties is difficult. This difficulty
is because virulence per se potentially
encompasses the sum of a complex set
of pathophysiologic events and metrics:
(1) in vitro effects on growth rates and/or
growth yields, (2) organism
transmissibility (ie, the ability to colonize and persist on
biologic surfaces, such as on nasal
epithelium or on damaged cardiac valves), (3)
intrinsic pathogenicity at the site of
infection (including toxin production and
biofilm formation), and (4) the capacity to
evade the innate and adaptive immune
Repurposing existing compounds for
influencing bacterial virulence or the
outcomes of bacterial infections has become
de rigueur over the past 2 decades.
Examples include (1) using statins to improve
outcomes in sepsis and bacterial
pneumonias [12, 13]; (2) using statins to
reduce the capacity of S. aureus to
produce carotenoid pigments, thus improve
the ability of the host to eliminate this
organism via the oxidative limb of the
innate immune system ; (3) using
aspirin and its congeners to enhance
antistaphylococcal therapeutics ; and
(4) using azithromycin and other
macrolides as immunomodulating agents in
treating infections . The notion of
repurposing β-lactams to affect bacterial
virulence independently, over and above
their intrinsic bactericidal effects, is not
new. More than 30 years ago, a number
of experimental endocarditis
investigations confirmed that subbactericidal
exposures of viridans group streptococci
to β-lactam agents impeded the capacity
of these pathogens to adhere to and
colonize cardiac vegetations [17–19]. This
nonbactericidal impact of β-lactams
against endocarditis-causing pathogens,
confirmed experimentally, has been
leveraged into the current approach to
antimicrobial prophylaxis of
endocarditis, as recommended by the American
Heart Association . Thus, β-lactam
prophylaxis is recommended to be given
as late as 2 hours after invasive dental
procedures in patients with high-risk
underlying valve diseases, taking advantage
of the probable antiadherence plus
proopsonophagocytic properties of these
Waters et al  substantially advance
our understanding of the antivirulence
properties of the β-lactams both in vitro
and in vivo. In vitro, these investigators
show that β-lactams repress toxin
production in MRSA via an agr-inhibitory
mechanism. They provide several cogent
lines of evidence in this regard.
Using indirect evidence of cytolytic
toxin repression by β-lactams (in human
neutrophil lysis assays), they demonstrated
generally reduced lysis with oxacillin
exposures. Unfortunately, this effect was not
studied with murine neutrophils (which
is more pertinent to their experimental
in vivo model). Murine neutrophils are
relatively resistant to the effects of
important staphylococcal toxins, including
Panton-Valentine leukocidins . Moreover,
the outcomes of toxin suppression and
neutrophil lysis were medium dependent,
with oxacillin actually increasing lysis in
CCY medium. This increase in lysis brings
to mind the study by Kernodle et al ,
in which nafcillin increased the capacity of
37 S. aureus strains (both MSSA and
MRSA) to induce α toxin production
by both agr and non-agr mechanisms.
Moreover, in that latter investigation,
culture supernatants from nafcillin-induced
S. aureus strains actually increased murine
virulence in an intraperitoneal infection
model. We are left with the conclusion
that the antivirulence effects of β-lactams,
as documented by Waters et al , may
well be strain dependent.
By RNA sequencing (comparing
oxacillin-exposed vs control cells), the
authors show evidence of agr repression.
Moreover, agr knockouts of the study
strains demonstrated a reduced capacity
to lyse neutrophils.
Of note, oxacillin exposures led to
substantial effects on the cell wall
machinery of S. aureus, including activation
of wall teichoic acid production, which
translated into augmentation of C3b
complement deposition and enhanced
opsonophagocytosis. Again, the
readouts of these latter functional assays
were performed using human
phagocytes; the linkage with outcomes in
their murine infection models remains
Finally, in their murine in vivo studies,
Waters et al used 2 distinct models,
bacteremia and pneumonia. As with most in
vivo investigations, the devil is in the
details. For example, at 28 hours after
infection in the bacteremia model, there was
little bacterial load difference in kidneys
and only modest differences in spleens
of animals treated with 2 oxacillin doses
(7.5 and 75 mg/kg). At 7 days after
infection, there was significantly reduced
virulence in both kidneys and spleens,
although in kidneys, there was a
heterogeneous outcome from animal to animal,
featured by overlapping of untreated
control and oxacillin-treated animal bacterial
loads. Given that oxacillin exposures will
increase opsonophagocytic killing in
vitro, it makes sense that the spleen
would exhibit the largest readouts. In
contrast, in the pneumonia model, there
was a significant impact on both blood
culture clearances and in vivo survival
in the 2 oxacillin dose regimens.
Curiously, no lung bacterial load data were
The investigations by Waters et al 
are well done, clearly presented, and
hypothesis generating. Further investigations
with more animal models and additional
MRSA strains should be done. Moreover,
advanced randomized clinical trials
addressing the effects of β-lactams in
MRSA infections need to be performed.
It is encouraging that a recent, small (60
patient) open-label and randomized trial
in Australia of vancomycin, with or
without the β-lactam flucloxacillin,
demonstrated reduced duration of bacteremia
by approximately 1 day in the
combination therapy group . This bacteremia
duration difference, however, was not
statistically significant (P = .06), and there was
no discernible salutary effect of the
combination regimen on either 28-day or 90-day
mortalities or on apparent metastatic
There are substantial notes of caution in
metric outcomes and data interpretations
in the article by Waters et al. In addition,
there are limited clinical data published to
date on β-lactam enhancement of MRSA
treatment efficacy. Thus, the
recommendation by Waters et al that “β-lactam
antibiotics should be included in the
treatment regimen for patients with MRSA
infections”  seems somewhat premature.
This issue, albeit very important for
practitioners, awaits the outcomes of large,
randomized controlled clinical trials for
Potential conflict of interest. A. S. B. reports
grants and other supported activities from
ContraFect Pharmaceuticals, grants from
Debiopharma International, and grants from Trellis
Bioscience, all outside of the submitted work.
Y. Q. X. reports research grants from Trellis
Bioscience, ContraFect Pharmaceuticals, and
Debiopharma International, all outside of the
submitted work. Both authors have submitted
the ICMJE Form for Disclosure of Potential
Conflicts of Interest. Conflicts that the editors
consider relevant to the content of the manuscript have
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