Download Characteristics, Clinical Relevance, and the Role of Echinocandins

SUPPLEMENT ARTICLE
Characteristics, Clinical Relevance, and the
Role of Echinocandins in Fungal–Bacterial
Interactions
Marios Arvanitis1,2,3 and Eleftherios Mylonakis1,2
1
Infectious Diseases Division, Rhode Island Hospital, and 2Warren Alpert Medical School of Brown University, Providence, Rhode Island; and 3Internal
Medicine Department, Boston Medical Center, Massachusetts
Fungal–bacterial interactions are common in the environment. The interactions between invasive fungi (eg,
Candida species and Aspergillus species) and pathogenic bacteria can be particularly significant in the outcome
of human infections. Study of these interactions in vivo using murine or invertebrate models, such as Caenorhabditis elegans or Galleria mellonella, has been very helpful in increasing our understanding of the pathogenesis of mixed infections and in identifying ways to use this between-kingdom interplay to our advantage. Based
on their effect against fungal biofilms and their immunomodulatory properties, the newer class of antifungal
agents, known as echinocandins, has the potential to be useful in polymicrobial infections and in high-risk complex infections such as ventilator-associated pneumonia or sepsis where colonization by fungi can lead to worse
outcomes.
Keywords. candidiasis; aspergillosis; micafungin; caspofungin; Caenorhabditis elegans.
The interactions between different species have been an
essential feature of life and, in their struggle for survival,
the various living organisms interact with each other
in a variety of ways. Often, this interaction leads to
synergistic cooperation, from which both species will
eventually benefit, whereas in other circumstances the
interaction becomes antagonistic, in which case the
less powerful part must either adapt, by developing features that will help improve its chances of survival, or
die. This second type of interaction has had profound
implications in life as we know it today, as it has been
one of the driving forces of natural selection [1]. These
interspecies interactions become particularly important
in the microscopic level among monocellular organisms
such as bacteria and fungi.
Correspondence: Eleftherios Mylonakis, MD, PhD, FIDSA, Infectious Diseases
Department, Rhode Island Hospital, 593 Eddy St, 3rd Flr, Ste 328/330, Providence,
RI 02903 ([email protected]).
Clinical Infectious Diseases® 2015;61(S6):S630–4
© The Author 2015. 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/civ816
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FUNGAL–BACTERIAL INTERACTIONS
Interactions between prokaryotes and eukaryotes are
abundant in nature and have been described in detail
by several in vitro experiments. In some circumstances,
these interactions can be beneficial for both life forms.
For example, researchers recently found that Candida
albicans biofilms can support and promote growth of
certain anaerobic bacteria by protecting them from
ambient toxic conditions. To enhance this phenomenon, the anaerobes promote aggregation of yeast cells
into mini-biofilms [2]. A similar relationship between
C. albicans and Streptococcus mutans was identified in
another article. The presence of C. albicans increases
the production of exopolysaccharides and thus promotes S. mutans biofilm formation, while also inducing
the expression of several virulence factors of the bacteria
[3]. Similarly, ethanol production by C. albicans has
been shown to promote Pseudomonas aeruginosa biofilm development through stimulation of the diguanylatecyclase WspR [4].
However, not all interactions between bacteria and
fungi are synergistic. Indeed, physical interactions between Candida species and prokaryotes in which the
bacteria develop biofilms on the yeast hyphae, destroying them
in the process, are well known [5]. In addition, several secretory
molecules produced by fungi have been proven to be bactericidal, with, of course, the most widely known example being the
discovery of the antimicrobial activities of penicillin produced
by the fungus Penicillium rubens [6]. The research for identification of other fungal metabolites with antibacterial effects continues to date [7]. Similarly, bacteria are able to produce
secretory molecules that inhibit the growth of fungi. For example, volatile compounds produced by P. aeruginosa [8] or Burkholderia tropica [9] have been shown to inhibit cytopathogenic
fungi of the genus Fusarium. Finally, changes in environmental
conditions caused by certain bacterial species can impact the
growth of fungi. For example, lactobacilli are known to cause
a decrease in the pH of human surfaces, thus leading to elimination of pathogenic fungi such as Candida species [10].
While these between-kingdom effects have clearly been elucidated by in vitro experiments and environmental observations, the relevance and extent of these relationships to
human infections have only recently started to be realized.
Fungal species are abundant in nature and thus are frequent
colonizers of the gastrointestinal tract and other parts of
multicellular organisms. Therefore, bacterial pathogens that invade these organs are often faced with the presence of these
fungi. Furthermore, mixed bacterial and fungal infections can
occur in high-risk patients [11]. Under those circumstances,
the outcome of the infection frequently depends on the character of fungal–bacterial interactions. Experiments on invertebrate
model hosts have been important in studying the pathogenesis
of those mixed polymicrobial infections.
Invertebrate models are small multicellular organisms that
are easy to breed and use in a microbiology laboratory and
can provide useful information regarding human infections, including microbial virulence and host immune responses, thanks
to the similarities of their immune system to the innate immune
response used by humans against infectious pathogens. The
worm Caenorhabditis elegans [12, 13] and the insect Galleria
mellonella [14] are among the most widely used invertebrate
models in experimental models of infection pathogenesis and
microbial virulence. In a pivotal article, Peleg et al used C. elegans to study the in vivo interactions between C. albicans and
Acinetobacter baumannii [15]. The investigators discovered
that A. baumannii is able to inhibit yeast filaments, thus decreasing its virulence and partially protecting worms from a lethal C. albicans gut infection. On the other hand, when able to
develop a quorum, the fungus is able to respond to this offensive
move by inhibiting A. baumannii growth through secretion of
the quorum-sensing molecule farnesol. In a subsequent article,
the investigators advanced this experiment by assessing the interactions between C. albicans and Salmonella enterica. They
found that similar to the previous experiment, Salmonella
species are able to inhibit C. albicans filamentation in the gut
of C. elegans via a secretory molecule produced by the bacterium. Furthermore, the decrease in C. albicans virulence,
while most potent against filaments, also extends to yeast cells
as well as C. albicans biofilms [16]. Kim et al were able to identify this secreted molecule as SopB, which is an effector of a type
II secretion system produced by Salmonella species [17]. A
similar interaction has been found between gram-positive bacteria and C. albicans. Specifically, researchers using a C. elegans
gut infection model showed that coinfection with Enterococcus
faecalis and C. albicans was associated with reduced virulence
compared with infection with either species alone [18]. Conversely, there are some data suggesting that coinfection of
C. elegans with Saccharomyces species and Acinetobacter species
may result in increased bacterial virulence through the production of ethanol by the fungus [19].
An often underappreciated type of polymicrobial interaction
within a multicellular host relies on the effect of different pathogens on host immune defenses. These effects have been clearly
demonstrated in murine experiments. For example, using a murine model of lung infection, investigators showed that colonization of the respiratory tract by C. albicans protected mice
against P. aeruginosa–induced lung damage through recruitment of natural killer cells, dendritic cells, and innate lymphoid
cells and the secretion of the cytokine interleukin 22 [20]. A different relationship between C. albicans and pneumonia was
found in a study on rats, which showed that Candida species
colonization induced a Th1–Th17 immune response with
high circulating interferon-γ levels and favored the development
of bacterial pneumonia, whereas antifungal treatment was able
to reverse this phenomenon and decrease the incidence of bacterial infection [21]. In parallel, coinfecting mice with C. albicans and nonfermenting gram-negative commensal bacteria
results in exacerbation of the fungal infection through interferon-γ secretion [22]. Similarly, in a murine model, polymicrobial
peritonitis caused by C. albicans and Staphylococcus aureus results in increased virulence and mortality compared with singlepathogen infection. This increased virulence is mediated by
innate proinflammatory cytokines, leading to increased inflammation and end-organ damage [23].
EFFECT OF EUKARYOTE–PROKARYOTE
INTERACTIONS ON HUMAN INFECTIONS
Taken together, these observations imply that the interactions
between bacteria and fungi in a living host are diverse, and
their outcome may often depend on the specific environmental
conditions, the potency and characteristics of the host immune
response, and the species of the microorganisms involved
(Table 1). Similar to what has been described from in vitro studies and in vivo models of infection, fungal–bacterial interactions
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Table 1. Types of Fungal–Bacterial Interactions
Example of
Antagonistic
Interactions
Type of
Interaction
Example of Synergistic
Interactions
Physical
interaction
Formation of fungal
biofilms that encase
and protect bacterial
cells [2]
Formation of bacterial
biofilms on fungal
hyphae leading to
their destruction [5]
Secretory
molecules
Formation of ethanol by
Saccharomyces spp
leads to increased
virulence of
Acinetobacter
baumannii [4]
None
Pseudomonas spp
phenazines inhibit
Candida spp
hyphae [7]
Gram-negative
nonfermenting
bacteria exacerbate
infection of the gut of
mice by Candida
albicans through
interferon-γ [22]
Candida albicans recruits
lymphoid cells in the
respiratory tract of
mice, protecting them
from subsequent
Pseudomonas
aeruginosa
pneumonia [20]
Changes in the
environment
Alteration of
host immune
response
Lactobacillus spp can
lower the
environmental pH,
thus eliminating
Candida spp [10]
may play an important role in the outcome of certain human
infections. This becomes particularly significant in the case of
infections in organs where fungi can become colonizers and
thus interact with invading bacterial pathogens.
One such type of infection is ventilator-associated pneumonia (VAP). In populations of critically ill, intubated patients,
Candida species frequently become colonizers of the respiratory
tract via transmission through the endotracheal tube, with large
trials estimating the prevalence of Candida species colonization
up to 16% [24]. Moreover, the respirator is used by both bacteria
and fungi as an abiotic surface to promote biofilm formation
[25]. In this setting, it is anticipated that the interplay between
the yeast and bacterial pathogens could be an essential determinant of the outcome of pneumonia. Indeed, several observational studies have addressed this hypothesis. In the first study of its
kind, Azoulay et al found that in hospitalized critically ill patients, colonization of the respiratory tract with Candida species
was an independent risk factor for pneumonia, with a greatest
risk for Pseudomonas species pneumonia [26]. In a different
study, researchers investigated the importance of the presence
of Candida species in cultures of the respiratory tract of patients
with suspected VAP and found increased intensive care unit
(ICU) stay and 28-day mortality in patients who were Candida
colonized [27]. These findings were supported by 2 retrospective
studies which showed that Candida species colonization is associated with prolonged hospital stay and higher in-hospital mortality in VAP [28, 29]. On the other hand, a more recent trial
assessed 200 patients with VAP from a tertiary referral center
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within a 5-year period and found that colonized patients had
increased ICU stay but not higher 30-day mortality compared
with controls [30]. Taken in their totality, these studies suggest
that Candida species colonization may have a detrimental effect
on the outcome of patients with VAP.
Notably, studies on the implications of mixed bacterial–
fungal systemic infections are generally lacking. One older study
that evaluated the clinical course of mixed bacterial–fungal infections found that the outcome of bacteremia is worse when
Candida species are present in the blood concurrently with
the bacteria compared to single-bacteria infection [31]. This
study, however, as with all the other human studies of mixed
infections, is limited by its observational nature, thus being
unable to establish causation. Indeed, the worse outcomes
observed in patients who are coinfected with fungi or are
colonized by Candida species may be attributed to other
comorbidities.
THE ROLE OF ANTIFUNGAL AGENTS AND
FUTURE DIRECTIONS
Although randomized studies to directly address the comparative outcomes of mixed bacterial–fungal vs single-pathogen infections have profound ethical limitations, indirect trials that
could evaluate the effect of antifungal treatment in colonized individuals could be performed without similar implications. As
suggested by the studies described above, in most cases of
human bacterial infections, fungal presence is associated with
increased morbidity. Therefore, the intuitive thought is that
the use of antifungal agents would be beneficial by preventing
bacterial infections or even improving outcomes in actively infected individuals who are colonized or have a mixed infection.
Surprisingly, this has not been extensively studied. In a doubleblind randomized trial, Jacobs et al evaluated 71 patients with
septic shock who were randomized to receive daily fluconazole
therapy vs placebo [32]. Interestingly, the patients who received
fluconazole had significantly improved 30-day survival (78% vs
46%), with the benefit being more pronounced in patients with
intra-abdominal infections. Nevertheless, due to the study design, it was unclear whether this could be attributed to direct
antifungal effect of fluconazole or to immunomodulatory properties of the agent. Conversely, a more recent pilot randomized
trial failed to support the use of antifungal agents to treat Candida species colonization in patients with a clinical suspicion for
VAP [33]. However, due to the relatively small number of participants (30 patients per group), the study may have been underpowered to show statistical differences in clinical outcomes.
Therefore, further assessment of the role of antifungal agents in
mixed infections is warranted.
Of all the antifungal agents available to date, the relatively
new class of echinocandins holds a promising potential against
polymicrobial infections. Contrary to the azole and polyene
classes of antifungal agents, echinocandins seem to be active
against fungal biofilms. Indeed, several in vitro studies [34, 35],
as well as reports from mammalian infection models [36], show
that different echinocandins such as caspofungin, anidulafungin, and micafungin can eliminate mature Candida species biofilms [37]. Therefore, this class of antifungal agents may be a
particularly attractive option in mixed infections, especially
those that develop on abiotic surfaces (ie, VAP or catheter-related
infections). An additional advantage of the echinocandins that
could prove to be useful in polymicrobial infections is their
immunomodulatory ability. Several studies have shown that antifungal agents such as caspofungin and micafungin are able to
alter the host immune response. Specifically, using the invertebrate model host G. mellonella, Kelly et al showed that preexposure to caspofungin can prime the immune response of
the larvae and increase their survival after a subsequent lethal
C. albicans inoculation. Further, this immune potentiation was
nonspecific, as it also resulted in prolonged survival of larvae
that were subsequently infected with S. aureus [38]. Moreover,
caspofungin and micafungin have been shown to prime the
human neutrophil response to Aspergillus species hyphae by inducing exposure of β-glucan on the hyphal surface [39]. Finally,
different investigators showed that caspofungin can enhance the
activity of impaired neutrophils from renal transplant recipients
against Candida species [40]. These effects on the host immune
response have the potential to be significant in mixed infections,
both by a nonspecific potentiation of innate immune effectors
that can act against both bacteria and fungi and by increased exposure of mold and yeast hyphae to lymphoid cells, which could
be impaired in some polymicrobial infections [41].
Based on these findings, one could argue that echinocandins
have the potential to be of use in clinical practice to not only treat
active invasive fungal infections, but also to improve survival in
mixed infections or in critically ill septic individuals who are colonized by fungal species. On the basis of this assumption, a double-blind randomized trial recently recruited 222 ventilated ICU
patients who were receiving antibiotics and randomized them to
receive prophylactic or preemptive caspofungin vs placebo [42].
The results showed that caspofungin tended to reduce the incidence of candidemia in these patients, but the results did not
reach statistical significance. To give a clearer answer to the question of whether echinocandins can help in polymicrobial infections, a large multicenter randomized trial that compares
micafungin and placebo in Candida-colonized patients with sepsis in the ICU setting is currently under way [43].
CONCLUSIONS
Interactions between fungal and bacterial species are abundant
in nature and can be relevant in human infections. Study of the
pathogenesis of these interactions may provide an insight on
how to treat complicated mixed species infections in critically
ill individuals. With their immunomodulatory and antibiofilm
properties, the newer class of echinocandins has a theoretical
advantage over older antifungal agents in polymicrobial infections and in fungal-colonized patients with sepsis. Whether
this holds true in clinical practice remains to be seen in future
randomized trials.
Notes
Supplement sponsorship. This article appears as part of the supplement
“Advances and New Directions for Echinocandins,” sponsored by Astellas
Pharma Global Development, Inc.
Potential conflict of interest. Both authors: No potential conflicts of
interest.
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 been disclosed.
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