Constructing microbial biocatalysts that produce biorenewables at economically viable yields and titers is often hampered by product toxicity. For production of short chain fatty acids, membrane damage is considered the primary mechanism of toxicity, particularly in regards to membrane integrity. Previous engineering efforts in Escherichia coli to increase membrane integrity, with the goal of increasing fatty acid tolerance and production, have had mixed results.

Herein, a novel approach was used to reconstruct the E. colimembrane by enabling production of a novel membrane component. Specifically, trans unsaturated fatty acids (TUFA) were produced and incorporated into the membrane of E. coli MG1655 by expression of cis-trans isomerase (Cti) from Pseudomonas aeruginosa. While the engineered strain was found to have no increase in membrane integrity, a significant decrease in membrane fluidity was observed, meaning that membrane polarization and rigidity were increased by TUFA incorporation. As a result, tolerance to exogenously added octanoic acid and production of octanoic acid were both increased relative to the wild-type strain.


  • Substrate or product tolerance in bacteria
  • Fermentation of biomass-derived sugars
  • Reverse engineering evolved bacteria
  • Soil attachment by bacteria

Terms & topics represented in Dr. Jarboe's 2018 publications

The Research Group


Front row (left to right): Yingxi Chen, Kirsten Davis, Aric Warner
Back row (left to right): Laura Jarboe, Eleanor Wettstein, Efrain Rodriguez Ocasio, Miguel Chavez-Santoscoy


Front row (left to right): Yingxi Chen, Kirsten Davis, Aric Warner, Jong Moon Yoon
Back row (left to right): Laura Jarboe, Zaigao Tan


Left to right: Jieni Lian, Chunyu Lio, Laura Jarboe, Liam Royce, Yingxi Chen, Tao Jin, Ping Liu



Research Projects

  • Engineering Microbial Robustness

    The field of metabolic engineering has reached the point where microbes can be engineered for the production of an amazing array of chemicals. However, a hurdle to the economic viability of such fermentative processes is the need to produce the target molecule at a high concentration. Processes in which the target is dilute in the fermentation broth incur high downstream processing costs. This need to produce the target molecule at a high concentration is often harmful to the organism. Consider the fact that alcohol is used to sterilize surfaces, and yet we want our microbes to produce alcohols at a high concentration. This problem of product toxicity is not limited to alcohols, it is widespread in the microbial production of biorenewable fuels and chemicals.

    Microbial inhibition is also problematic in the utilization of biomass-derived sugars. We aim to release sugars from biomass in a manner that is fast and cheap, but the resulting sugar solutions are “dirty” and contain compounds that are inhibitory to the microbial biocatalyst. This limits the amount of sugars that can be fed to the organism and, in turn, limits the product concentration. Adding processing steps to detoxify the sugars can incur an unacceptable increase in process cost. An alternative approach is to develop organisms that can tolerate these inhibitors.

    Historically, evolution has been used to increase microbial robustness. Our long-term goal as engineers is to employ a rational, predictive approach for improving microbial tolerance. Thus, the main theme of the Jarboe research program has been understanding the mechanisms by which toxic molecules are harmful to microbes and then using a rational approach to modify the microbe for increased robustness. This approach started with my PhD work, with characterization of the inhibition of Escherichia coli by two compounds produced by the mammalian immune system, with the goal of understanding pathogenesis. My postdoctoral research included characterization of the inhibition of ethanol-producing E. coli by furfural, an inhibitory compound that is abundant in biomass-derived sugars.

    At Iowa State University, the Jarboe group has characterized inhibition of E. coli and Saccharomyces cerevisiae by carboxylic acids, as funded by the NSF Center for Biorenewable Chemicals Engineering Research Center (CBiRC). A variety of our publications have rigorously and quantitatively demonstrated that (a) membrane damage is the main mode of carboxylic acid toxicity to E. coli and S. cerevisiae; and (b) that changes in the membrane composition are associated with increased carboxylic acid tolerance and production. My group has also demonstrated that membrane damage is a challenge during production of other important biorenewable fuels and chemicals, such as styrene.


    Rational engineering of membrane composition

    Given the findings that membrane damage is a common theme during the production of biorenewable fuels and chemicals, the microbial membrane is an obvious target for rational modification efforts. The analogy that we like to use here is of a standard reaction vessel. If, during the course of the chemical reaction, you observe that the reaction vessel is vulnerable to corrosion by the vessel contents, you would modify your process so that a vessel with a different composition is used. Here we propose to take the same approach with the microbial cell membrane – if the membrane is vulnerable to damage by the metabolic product, then the composition of the membrane should be altered in order to increase resistance to this damage.

    Previous efforts by my own group and others have focused on changing the distribution of naturally-occurring membrane components. Specifically, efforts have focuses on changing the distribution of saturated lipids and unsaturated lipids. These efforts were effective in increasing tolerance of the inhibitory compound, but there have been no successful reports of this sort of membrane engineering approach actually improving production.

    The Jarboe group has shown that modification of E. coli to produce a non-native membrane component that improves the structural integrity of the membrane. Specifically, we introduced an enzyme that isomerizes some of the existing cis lipids to the trans form, a strategy that is used by Pseudomonas bacteria to improve tolerance to membrane-damaging compounds. This modification decreased the membrane fluidity and improved fatty acid production and styrene production. Tolerance to other important biorenewable products, such as the second-generation biofuel n-butanol, and factors important to the cost-effective production of biorenewable fuels and chemicals, such as tolerance to high temperature and low pH, were also significantly increased.

    Membrane engineering for increased microbial robustness is an ongoing area of research in the Jarboe lab.

  • Enabling Microbial Utilization of Depolymerized Biomass

    Utilization of monomers produced by biomass depolymerization, including both the sugar monomers and the lignin monomers, requires changes in both tolerance and metabolic pathways.
    Anhydrosugars, such as levoglucosan and cellobiosan, are best known in the context of the thermal depolymerization of biomass. If we aim to use thermal depolymerization to release fermentable substrates from biomass, our fermentation organisms need to be able to utilize these sugars. While traditional fermentation organisms lack the ability to metabolize anhydrosugars, my group has demonstrated that “plug and play” type expression of a single gene can enable levoglucosan utilization. The Jarboe group has also isolated, identified and characterized six microbes that are capable of utilizing cellobiosan. This is a crucial first step in identifying the enzymes responsible for cellobiosan utilization.
    Ongoing efforts to identify and characterize the enzymes and transporters responsible for anhydrosugar utilization is relevant not just to the utilization of biomass-derived sugars, but also in our understanding of the global carbon cycle. According to our calculations, 100 million metric tons of anhydrosugars are produced in naturally-occurring biomass burning events every year. Thus, characterization of anhydrosugar metabolism is important to understanding of the global carbon cycle.
    The Jarboe group is also involved the efforts to improve microbial utilization of aromatic monomers produced from the lignin fraction of biomass.

  • Microbial Attachment

    The Jarboe lab is also involved in characterization of the attachment of microbes to environmental particles. Microbial attachment to environmental particles is relevant to the safety of drinking water and recreational waters. One of the first publications from the Jarboe group demonstrated that among E. coli strains isolated from livestock facilities, there is a significant association between the propensity to attach to a model environmental particle (quartz) and antibiotic resistance. Amidst increasing concerns about antibiotic-resistant bacteria, this finding that resistant bacteria are “stickier” in regards to abiotic surfaces is relevant to public health.

    As part of an ongoing collaboration with Michelle Soupir (ISU Ag & Biosystems Engineering), we obtained funding from the NSF Environmental Engineering program in 2012 to characterize the genetic and environmental factor influencing E. coli’s attachment to environmental particles. This project led to the characterization of allelic variation in E. coli’s outer membrane protein A (OmpA) that was much more extensive than previously reported. We have shown that this allelic variation can actually serve as a means of tuning the cell surface hydrophobicity and cell surface charge, as well as resulting in significantly altered resistance to inhibitory compounds, such as phenol (in preparation). Thus, this ties into the membrane engineering strategies described above.