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Death by a thousand cuts: the challenges and diverse landscape of lignocellulosic hydrolysate inhibitors

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Frontiers in Microbiology
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March, 2014
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PERSPECTIVE ARTICLE
published: 14 March 2014
doi: 10.3389/fmicb.2014.00090

Death by a thousand cuts: the challenges and diverse
landscape of lignocellulosic hydrolysate inhibitors
Jeff S. Piotrowski*, Yaoping Zhang , Donna M. Bates , David H. Keating , Trey K. Sato , Irene M. Ong
and Robert Landick
DOE Great Lakes Bioenergy Research Center, University of Wisconsin-Madison, Madison, WI, USA

Edited by:
Shihui (Shane) Yang, National
Renewable Energy Laboratory, USA
Reviewed by:
Shiyong Peng, National Institute of
Health, USA
Qiang J. Fei, National Renewable
Energy Lab, USA
*Correspondence:
Jeff S. Piotrowski, DOE Great Lakes
Bioenergy Research Center,
University of Wisconsin-Madison,
1552 University Ave., WEI4152,
Madison, WI 53726, USA
e-mail: jpiotrowski@wisc.edu

Lignocellulosic hydrolysate (LCH) inhibitors are a large class of bioactive molecules that
arise from pretreatment, hydrolysis, and fermentation of plant biomass. These diverse
compounds reduce lignocellulosic biofuel yields by inhibiting cellular processes and
diverting energy into cellular responses. LCH inhibitors present one of the most significant
challenges to efficient biofuel production by microbes. Development of new strains that
lessen the effects of LCH inhibitors is an economically favorable strategy relative to
expensive detoxification methods that also can reduce sugar content in deconstructed
biomass. Systems biology analyses and metabolic modeling combined with directed
evolution and synthetic biology are successful strategies for biocatalyst development,
and methods that leverage state-of-the-art tools are needed to overcome inhibitors
more completely. This perspective considers the energetic costs of LCH inhibitors and
technologies that can be used to overcome their drain on conversion efficiency. We
suggest academic and commercial research groups could benefit by sharing data on LCH
inhibitors and implementing “translational biofuel research.”
Keywords: cellulosic biofuels, lignocellulosic hydrolysate inhibitors, systems biolog; y, chemical genomics,
metabolic modeling, ethanologens

INTRODUCTION
Lignocellulosic biofuels offer the promise of sustainable,
domestically produced fuels with favorable carbon balances. Fastgrowing grasses like Miscanthus and agricultural residues provide fermentable sugars at lower energy and fertilizer costs than
grains (Schmer et al., 2008), making them preferable feedstocks
for advanced biofuels. Cellulosic ethanol is an obvious nextgeneration biofuel to implement given its production and delivery
infrastructures are compatible with existing fuels.
Central to the success of cellulosic ethanol is efficient conversion of biomass-derived sugars to ethanol by microbes such as
Saccharomyces cerevisiae, Escherichia coli, and Zymomonas mobilis
(Alper and Stephanopoulos, 2009; Lau et al., 2010; Yang et al.,
2010a). Under optimal conditions, these microbes are powerful
ethanologens; however, lignocellulosic hydrolysates (LCH) and
industrial scale fermentation tanks are not optimal conditions.
Thermal, osmotic, and ethanol stresses are just some of the environmental factors that inhibit fermentation and reduce yield
(Attfield, 1997; Gibson et al., 2007; Jin et al., 2013). Industrial
microbes are pushed to the limits of stress tolerance to make
biofuel production energetically favorable.
Although environmental stressors limit yields in present day
ethanol facilities, cellulosic biomass conversion comes with new
challenges. Specifically, LCH inhibitors, a group of small, bioactive molecules can significantly reduce conversion efficiency. LCH
inhibitors such as aliphatic acids, furans, and phenolics are
released or condensed from cellulose and hemicellulose during
pretreatment and hydrolysis (Larsson et al., 1999, 2000; Yang
et al., 2010a); however, chemical residues from newer hydrolysis strategies and synergies with biofuel end products (ethanol,

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isobutanol) are less well studied. Removal of these inhibitors
can be expensive and may reduce titers of fermentable sugars;
some estimates suggest that detoxification can remove up to 26%
of total fermentable sugars (Larsson et al., 1999). Thus, a preferred strategy is to develop microbial strains with properties that
minimize the effects of LCH inhibitors on biofuel yields.
With commercially available industrial stains that are robust
to thermal and ethanol stress (e.g., Ethanol Red, Fermentis,
Milwaukee, WI, USA), recent attention has been directed to
overcoming the challenge of LCH inhibitors. These compounds
are ubiquitous in hydrolysates, and their abundance and composition depends on pretreatment (Chundawat et al., 2010),
feedstock (Klinke et al., 2004; Almeida et al., 2007), and seasonality (Bunnell et al., 2013; Greenhalf et al., 2013). Given their
chemical diversity, these compounds can target many cellular
processes. LCH inhibitors can also generate a substantial cellular energy drain. Cells have evolved to detoxify, excrete inhibitors,
or repair the resultant cellular damage fast enough to reproduce.
However, evolved coping mechanisms may also negatively affect
the efficiency of conversion by competing for cellular resources
(Bellissimi et al., 2009; Miller et al., 2009). Although it is in the
microbe’s best interest to use its resources to limit the effects
of LCH inhibitors and maintain cellular viability, this may be
reducing biofuel production. In this perspective, we consider the
diversity and cellular costs of LCH inhibitors from traditional
and novel pretreatment and hydrolysis strategies, describe new
technologies and their application to strain development, and
finally identify key needs of the cellulosic biofuel community that
will empower “translational biofuel research” to take discoveries
quickly to industrial scale.

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Challenges of lignocellulosic fermentation inhibitors

DIVERSITY OF FERMENTATION INHIBITORS
Prior to microbial conversion of lignocellulosic sugars into biofuel, biomass must be deconstructed into monomeric sugars by
enzymatic or chemical hydrolysis. This hydrolysis step is often
preceded by a pretreatment step that expands the plant fibers and
allows cellulolytic enzymes access to the polysaccharide matrices.
The resulting hydrolysates are complex, ill-defined mixtures that
include sugars and a diversity of bioactive molecules (Table 1).

Small acids and phenolic compounds are released from cellulose and hemicelluloses during hydrolysis and furans arise from
the dehydration of pentose and hexose monomers (Klinke et al.,
2004). Pretreatments such as acid hydrolysis, steam explosion
or NH3 expansion each impart their own “profile” of LCH
inhibitors. For example, AFEX (Ammonia Fiber EXpansion)
uses high-pressure/temperature ammonia to alter the cellulose
matrix to allow hydrolysis by cellulases (Lau and Dale, 2009), and

Table 1 | Classes of lignocellulosic hydrolysate (LCH) inhibitors and their described modes of toxicity.
Inhibitors

Mode of action

References

Decreases cellular pH,
Decreases cellular ATP,
Inhibits macromolecule
biosynthesis,
Inhibits DNA synthesis/repair,
Inhibits glycolytic enzymes

Sinha, 1986; Cherrington et al.,
1990; Holyoak et al., 1996;
Stratford and Anslow, 1998;
Bellissimi et al., 2009; Ullah et al.,
2012; Ding et al., 2013

Damages membranes,
Oxidative damage,
Damages nucleic acids,
Damages proteins,
Limits sulfur assimilation,
Reduces NADH/NADPH pools,
Inhibits enzymes

Ingram, 1976; Hadi et al., 1989;
Khan et al., 1995; Zaldivar and
Ingram, 1999; Modig et al., 2002;
Miller et al., 2009; Allen et al.,
2010; Wang et al., 2013

Damages membranes,
Decreases cellular pH,
Decrease cellular ATP,
Inhibits translation,
Oxidative damage, Denatures
proteins, Damages cytoskeleton,
DNA mutagenesis, Induces
apoptosis

Krebs et al., 1983; Mikulášová
et al., 1990; Verduyn et al., 1992;
Chambel et al., 1999; Fitzgerald
et al., 2004; Iwaki et al., 2013;
Nguyen et al., 2014

Unknown

Docherty and Kulpa, 2005

Damages membranes

King et al., 1991; Laouar et al.,
1996

Copper, Sodium, Zirconium

Damages membranes,
nucleic acids, and enzymes

Shapiro and Ling, 1998; Schmitt
and Tampé, 2002

γ-valerolactone

Unknown

Luterbacher et al., 2014

Damages membranes
Damages DNA
Inhibits enzymes

Nagodawithana and Steinkraus,
1976; Dombek and Ingram, 1984;
Alexandre et al., 1994; Ibeas and
Jimenez, 1997; Huffer et al., 2011

LIGNOCELLULOSE DERIVED
Small acids
Acetic acid, formic acid, levulic acid

Furans
Furfural, HMF, 2-furoic acid

Phenolics
Ferulic acid, coumaric acid
Vanillin, Syringealdehyde
Coniferyl alcohol, Eugenol
Acetovanillin, Feruloyl amide, Coumaryl
amide

PROCESS DERIVED INHIBITORS
Ionic liquids
1-Ethyl-3-methylimidazolium-Ac
Surfactants
Triton-X, Tween

Metal ions

End product inhibitors
Ethanol
Isobutanol

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Piotrowski et al.

this produces amide versions of inhibitors (e.g., feruloyl amide
from ferulic acid) with potentially new biological properties
(Chundawat et al., 2010).
Besides these common inhibitors, residual pretreatment chemicals may complicate fermentation. Ionic liquids are pretreatment
and hydrolysis solvents, but are toxic to many microorganisms
(Docherty and Kulpa, 2005; Ouellet et al., 2011). Alkaline hydrogen peroxide (AHP) pretreatment limits the production of furans;
however this method introduces significant amounts of Na+
from NaOH, which can cause osmotic stress (Sato et al., 2014).
Copper(II) 2,2 -bipyridine is a catalyst that enhances AHP pretreatment by reducing the H2 O2 requirement (Li et al., 2013), but
copper is toxic to most microbes. Next-generation pre-treatments
and hydrolysis methods like γ-valerolactone (Luterbacher et al.,
2014), surfactants (Sindhu et al., 2013), zirconium phosphate
catalysts (Gliozzi et al., 2014), and other incipient hydrolysis
technologies may imbue hydrolysates with novel toxicities and
synergisms with common inhibitors.
Biofuel end-products themselves are inhibitory. Ethanol can
directly damage cellular membranes, DNA, as well as inhibit
enzymes (Nagodawithana and Steinkraus, 1976; Dombek and
Ingram, 1984; Alexandre et al., 1994; Ibeas and Jimenez, 1997;
Huffer et al., 2011). The ethanologens S. cerevisiae and Z. mobilis
are not immune to ethanol toxicity at high concentrations
(Carmona-Gutierrez et al., 2012; Yang et al., 2013). Advanced
biofuels like isobutanol are toxic at significantly lower concentrations than ethanol (Brynildsen and Liao, 2009; Atsumi et al., 2010;
Huffer et al., 2011; Minty et al., 2011). Inhibition by end products
has been an area of research interest (Baez et al., 2011; McEwen
and Atsumi, 2012; Zingaro et al., 2013), and ethanol tolerance is
a pre-requisite for all industrial yeast.
Effects of inhibitors at a minimum can be additive, but an
even greater concern is the potential for synergy between LCH
inhibitors and fermentation condition, including high osmolarity and absence of O2 . Some studies have described synergies
between acetic acid, furfural, and phenolics in yeast (Oliva et al.,
2006; Ding et al., 2011), but a comprehensive evaluation of synergisms between compounds and conditions on both growth
rate and fermentation will be essential. Such assessment will be
a massive undertaking that will also require defined synthetic
hydrolysate media to permit meaningful definition of minimum
inhibitory concentrations (MICs) of the individual inhibitors,
and how this value will change in combination with other LCH
inhibitors (synergy/antagonism) and fermentation conditions.
Nevertheless, documenting interactions between inhibitors on
sugar conversion is crucial to prioritizing future research for
improved biofuel microbes.

SMALL MOLECULE INHIBITORS DEPLETE CELLULAR
RESOURCES
LCH inhibitors directly affect biofuel yield as well as the production rate, which can extend fermentation time and result
in higher operating costs. In simplest terms, these inhibitors
affect conversion efficiency by depleting cellular energy resources
(e.g., ATP, NADH, NADPH; Figure 1). Some inhibitors can act
broadly and damage key enzymes of fermentation pathways

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Challenges of lignocellulosic fermentation inhibitors

(Modig et al., 2002). The coping mechanisms available to the
biofuel microbes fall into 4 main categories: (i) detoxification, (ii) efflux, (iii) repair, or (iv) tolerance. The first three
are of most concern given their effects on cellular energy and
resources.
Detoxification and efflux are the most well characterized
mechanisms of inhibitor tolerance in microbes, with deep literature not only from biofuel research but also in the medical literature from a wealth of antibiotic and pharmaceutical research.
Detoxification is the major route of tolerance for aldehydes in
both bacteria and yeast. Reduction of compounds like furfural
and vanillin to less toxic alcohols by NADH/NADPH dependent reductases occurs in ethanol fermentation using S. cerevisiae
and E. coli (Jarboe, 2011). Oxidoreductase expression significantly increases in yeast and E. coli in the presence of aldehydes (Liu et al., 2008; Wang et al., 2011), depleting cellular
NADH/NADPH. This results in inhibition of NADPH-dependent
processes (e.g., assimilation of sulfur) leading ultimately to
slower conversion of sugars (Miller et al., 2009). Interestingly,
yields can be increased by disabling the detoxification pathway, suggesting that tolerance may be more energetically efficient
than detoxification (Wang et al., 2013). Alternatively, changing the source of reducing equivalent for aldehyde detox from
NADPH to NADH also can improve biofuel yield (Wang et al.,
2013).
Efflux is mediated by ATP-dependent trans-membrane pumps
that selectively or non-selectively pump out toxic compounds
usually at the cost of 1 ATP per molecule (Shapiro and Ling, 1998;
Schmitt and Tampé, 2002). The yeast S. cerevisiae has 29 different
ATP-binding cassette (ABC) efflux transporters (Decottignies and
Goffeau, 1997) and 5% of the E. coli genome is composed of genes
with ABC-transporter domains, many involved in efflux (Linton
and Higgins, 1998). In both yeast and E. coli, expression of transporters increases in response to LCH inhibitors (Schüller et al.,
2004; Lee et al., 2012; Schwalbach et al., 2012). The yeast weak acid
response is mediated by increased expression of ABC-transporters
(e.g., Pdr1p and Pdr5p) (Schüller et al., 2004; Pereira Rangel et al.,
2010). As long as cells are exposed to LCH inhibitors, a significant
fraction of cellular ATP will be diverted to efflux pumps. Of particular importance for overall efficiency of LCH conversion, ATPdepletion may have a disproportionate effect on xylose conversion
compared to glucose conversion because xylose produces less cellular energy per molecule transported (Matsushika et al., 2013).
Bellissimi et al. (2009) found that acetic acid could specifically
inhibit xylose fermentation, but that this effect could be reversed
with glucose addition. The authors posit that ATP generated from
xylose fermentation cannot match ATP depletion from the weak
acid response and efflux/proton pumps used to maintain cellular
pH. Inhibitors that require ATP-dependent coping mechanisms
can directly reduce xylose conversion. Some inhibitors can be particularly draining, affecting both NADPH and ATP pools. Ask
et al. (2013) found that furfural and HMF not only reduced cellular NAPDH in S. cerevisiae, but also elicited increased expression
of ATP-dependent efflux pumps Pdr5p and Yor1p. This suggests
that coping with furans requires both NADPH dependent detoxification and ATP-dependent efflux.

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Piotrowski et al.

FIGURE 1 | Inhibitor classes and the cellular energy consequences of
LCH inhibitors. Presented are examples from three main classes of
inhibitors and the ways cells can cope with these: efflux via pumps,
detoxification via enzymes, and repair of the damage caused by the

An unanswered question is whether the ATP-dependent action
of efflux pumps has a net positive (by inhibitor removal) or
net negative (by energy consumption) effect on biofuel production. Although the answer may vary by inhibitor, because some
inhibitors like aldehydes are more damaging to cells than others,
a general test of the positive or negative consequences of efflux
pumps for biofuel yield will help advance strategies for biofuel
microbe design.
The cellular energy costs of maintenance and repair are more
difficult to quantify but could account for significant energy
loss. If cells can repair the damage caused by fermentation
inhibitors quickly, then fermentation may proceed. Inhibitors
can acidify cells (Verduyn et al., 1992), damage cellular membranes (Russell, 1992), DNA (Allen et al., 2010), and individual
proteins (Modig et al., 2002). Repairing structures requires biogenesis; this comes at the expense of ATP, NADPH, carbon, and
nitrogen. Maintaining pH is mediated by ATP-dependent proton
pump Pma1p; and ATP cost under acidic conditions is the primary cause of reduced cellular growth (Verduyn et al., 1992; Ullah
et al., 2012). Biogenesis requires NADPH-dependent assimilation
of nutrients like sulfur, which is drained by repair enzymes.
Phenolics and furans can damage membranes, requiring more

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Challenges of lignocellulosic fermentation inhibitors

compounds. Each coping strategy comes at the expense of cellular energy
that is diverted from the stores used to maintain cell integrity by
chemiosmotic exchange and assimilation as well as energy required for
fermentation of sugars to fuel.

energy to maintain the proton gradient required for basic
metabolism (Ding et al., 2012; Schwalbach et al., 2012; Stratford
et al., 2013). Growth and sugar conversion will be slowed as
resources are diverted to maintenance and repair. Given that
hydrolysates contain a mixture of inhibitors with diverse modes of
action requiring all of these coping mechanisms simultaneously,
the energy drain from fermentation inhibitors is truly death by a
thousand cuts.

ADAPTING TO THE CHANGING LANDSCAPE OF LCH
INHIBITORS
Strain development is an economical route to deal with
LCH inhibitors. Resistance to the suite of inhibitors requires
complex response of many cellular systems, and as such
is not easily conferred by engineering of individual genes.
Moreover, inhibitor pools can vary between hydrolysate preparations, thus even the most robust strains in ammoniapretreated hydrolysate may wither in dilute acid pretreated
hydrolysates. It is unlikely that one strain will be optimal
for all conditions. The reality is that microbial strains will
need to be tailored to specific hydrolysates through engineering and directed evolution. Accelerating this process is

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Piotrowski et al.

crucial to making new cellulosic technologies industrially
viable.
The tools of systems biology can give a detailed view of
the microbial stress response (Jozefczuk et al., 2010; Lee et al.,
2011). Transcriptomic, proteomic, and metabolomic responses
to inhibitors can be tracked in detail, and this “multiomic”
approach can give high-resolution insight to the global cellular
consequences (Miller et al., 2009; Yang et al., 2010b; Schwalbach
et al., 2012; Skerker et al., 2013; Yang et al., 2013). Advanced
techniques such as ribosome profiling can give a view into relationships between transcription and protein abundance in the
presence of LCH inhibitors (Ingolia et al., 2009; Brar et al., 2012).
Metabolic and flux-balance models are valuable in determining
energy balances within cells (Varma and Palsson, 1994; Fiaux
et al., 2003; Jin and Jeffries, 2004; Herrgård et al., 2008; TaymazNikerel et al., 2010). These models combined with a systems
biology view of protein and gene expression can be used to identify key energetic bottlenecks as targets for engineering. Recently,
Wei et al. (2013) demonstrated an elegant way to overcome the
redox cofactor imbalance in yeast designed to ferment xylose by
engineering acetate metabolism from E. coli into S. cerevisiae.
The authors combined an acetate utilization pathway that consumes NADH with a xylose utilization pathway that produces
NADH to overcome the redox imbalance of engineered xylose
fermentation. The resultant strain has both better xylose conversion and the ability to detoxify acetate (Wei et al., 2013).
Detailed accounting of ATP and NAD(P)H in the presence of
LCH inhibitors and industrial conditions will be necessary to
disentangle and understand points for rational engineering of
microbial catalysts.
Model biofuel microbes like S. cerevisiae and E. coli benefit from well-developed suites of functional genomics resources,
such as deletion mutant or overexpression collections (Giaever
et al., 2002; Baba et al., 2006; Kitagawa et al., 2006). These tools
have revealed effects of some inhibitors such as furfural (Gorsich
et al., 2006), vanillin (Endo et al., 2008; Iwaki et al., 2013),
and acetic acid (Mira et al., 2010). Genome-wide mutant collections also allow powerful studies of inhibitors via “chemical
genomics” (Giaever et al., 2004; Parsons et al., 2006; Ho et al.,
2011). This new tool in the multiomic arsenal, when combined
with the information in genetic interaction networks (Butland
et al., 2008; Costanzo et al., 2010), can allow precise predictions of the cellular targets of fermentation inhibitors. Recently,
Skerker et al. (2013) used a chemical genomics approach to
discover a previously undescribed inhibitor in acid pretreated
hydrolysate, methyl glyoxal (MG), and identified mutations that
confer MG resistance. Chemical genomics can be used to identify the chemical biological signatures within hydrolysates and
mutations conferring resistance, but more broadly can serve as
a “biological fingerprinting” technique for hydrolysate to identify variation in production, and as a method to benchmark the
biological properties of novel hydrolysates. Resources such as
the MoBY-ORF collections (Ho et al., 2009; Magtanong et al.,
2011), which are barcoded plasmids collections carrying nearly all
S. cerevisiae used to assess the effects of increased gene dose and
gene complementation, could be used with industrial, wild, and
engineered yeast to identify genetic interactions within diverse

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Challenges of lignocellulosic fermentation inhibitors

yeast strains. Combined with traditional selection for resistance
and directed evolution, systems biology tools offer great potential
for strain development.
Further, new tools are now available that can accelerate strain
development. Genome editing and optimization techniques such
as CRISPR/Cas9 and MAGE allows rapid, detailed genome editing in both bacteria and eukaryotes (Wang et al., 2012; Cong
et al., 2013; DiCarlo et al., 2013; Gilbert et al., 2013). Next steps in
strain development will be tuning genomes to inhibitor tolerance
by parallel disruption or activation of inhibitor responsive genes
using CRISPR/Cas9-based systems. Additionally, large-scale gene
synthesis can be used to identify genes that not only aid xylose
utilization, but also confer inhibitor tolerance. Systems biology
tools for biocatalyst development are coalescing to a pipeline
that can keep pace with the changing landscape of fermentation
inhibitors, allowing for the rapid tailoring of ethanologens with
robust and efficient sugar to biofuel conversion from any new
hydrolysate.

NECESSARY TOOLS TO MEET A COMMON GOAL
Much like the interface between commercial and academic drug
discovery communities, applying next-generation biofuel technologies will require cooperative translational research. Academic
biofuel communities are developing advanced system biology
techniques whereas the commercial community excels at scale-up
commercialization. However, similar to drug discovery, these two
groups are often isolated in their research. In both cases, publicprivate partnerships such as the NIH translational medicine
initiatives (Zerhouni, 2003) and NCERC industrial partnerships
(http://www.siue.edu/ethanolresearch/), as well as shared computational resources like Kbase (http://www.kbase.us/) can help
bridge the divide.
How can researchers and funding agencies best enhance
collaboration? A key advance would be greater data sharing
about LCH inhibitors. Diverse hydrolysates and their respective
inhibitors are major variables in the field of cellulosic biofuel production, but detailed information on specific hydrolysate compositions is not broadly available. Comprehensive efforts to identify
all major LCH components and inhibitors across feedstocks and
hydrolysis treatments are needed and will require a central repository of LCH data that includes data on feedstocks, pretreatment,
hydrolysis, nutrients, and inhibitor. Chemical genomic profiling
could be used to generate “biological fingerprints” of hydrolysates
to allow comparisons among hydrolysates and identify the biological effects of LCH components by standard analytical methods.
A central resource would allow researchers to compare the composition and biological fingerprints of new hydrolysates with
existing knowledge about tolerant microbes for further strain
development. The DOE’s Systems Biology Portal, KBase, offers
the best outlet for community resources, and could serve as the
authority of the response of biocatalysts to LCH inhibitors with
open-source, community-developed analytical tools for chemical
genomics datasets.

CONCLUSIONS
LCH inhibitors are major barriers for cellulosic biofuels. The
cellular energy costs of coping with these compounds are a

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Piotrowski et al.

significant drain on the already thin margins of biofuel production. However, the increasingly powerful tools of systems biology
can be used to gain a detailed understanding of the cellular consequences of individual and mixtures of fermentation inhibitors,
which will serve as a basis for rational engineering of customizable
microbes. The biofuel research community would benefit from
shared computational and database resources that can improve
communication between the academic and commercial sides of
biofuels.

ACKNOWLEDGMENTS
All authors are funded by the DOE Great Lakes Bioenergy
Research Center (DOE BER Office of Science DE-FC0207ER64494).

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Conflict of Interest Statement: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be
construed as a potential conflict of interest.
Received: 16 November 2013; accepted: 18 February 2014; published online: 14 March
2014.
Citation: Piotrowski JS, Zhang Y, Bates DM, Keating DH, Sato TK, Ong IM and
Landick R (2014) Death by a thousand cuts: the challenges and diverse landscape
of lignocellulosic hydrolysate inhibitors. Front. Microbiol. 5:90. doi: 10.3389/fmicb.
2014.00090
This article was submitted to Microbial Physiology and Metabolism, a section of the
journal Frontiers in Microbiology.
Copyright © 2014 Piotrowski, Zhang, Bates, Keating, Sato, Ong and Landick.
This is an open-access article distributed under the terms of the Creative Commons
Attribution License (CC BY). The use, distribution or reproduction in other forums is
permitted, provided the original author(s) or licensor are credited and that the original
publication in this journal is cited, in accordance with accepted academic practice. No
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