Browsing by Author "Ajo-Franklin, Caroline M"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Expanding the Understanding of Extracellular Electron Transfer Mechanisms and the Physiological Impacts in Gram-Positive Bacteria
    (2022-11-30) Tolar, Joe G; Ajo-Franklin, Caroline M
    Lactic acid bacteria are named for their nearly exclusive fermentative metabolism. Thus, the recent observation of extracellular electron transfer (EET) activity - typically associated with anaerobic respiration in Gram-negative bacteria - in this class of organisms has forced researchers to rethink the rules governing microbial metabolic strategies (Tejedor-Sanz et al., 2022). Lactic acid bacteria also differ from model EET microorganisms in their cell envelope structure and the niches they inhabit. These differences open important questions: What mechanisms do lactic acid bacteria use to accomplish EET? What are the physiological effects of EET on lactic acid bacteria? Here I address these questions by exploring the role of quinones and flavins in EET by Lactiplantibacillus plantarum (Chapter 2), by developing methods to monitor gene expression in L. plantarum under EET conditions (Chapter 3), and by characterizing the physiological implications of EET in Enterococcus faecalis (Chapter 4). I discovered that either the quinone, 1,4-dihydroxy-2-naphthoic acid (DHNA), or riboflavin support EET via similar, but unique pathways. Using genetic knockouts of candidate genes and electrochemical techniques, I constructed a working model of the EET mechanism for DHNA and riboflavin. Next, I developed a methodology to screen and validate fluorescent reporters of promoter activity under EET conditions to provide insight into the mechanism of the EET-induced fermentative metabolism changes. Additionally, I found that growth conditions and carbon sources can dramatically influence EET activity in E. faecalis. Lastly, I developed a method to quantify interspecies competition between E. faecalis and Staphylococcus aureus and simultaneously measure EET activity. Understanding the role of EET in lactic acid bacteria is of great importance due to the significance of lactic acid bacteria in agriculture, bioremediation, food production, and gut health. This work expands our molecular-level understanding of EET in Gram-positive microbes and provides additional opportunities to manipulate EET for biotechnology.
  • Thumbnail Image
    Item
    Extracellular electron transfer increases fermentation in lactic acid bacteria via a hybrid metabolism
    (eLife Sciences Publications Ltd., 2022) Tejedor-Sanz, Sara; Stevens, Eric T; Li, Siliang; Finnegan, Peter; Nelson, James; Knoesen, Andre; Light, Samuel H; Ajo-Franklin, Caroline M; Marco, Maria L
    Energy conservation in microorganisms is classically categorized into respiration and fermentation; however, recent work shows some species can use mixed or alternative bioenergetic strategies. We explored the use of extracellular electron transfer for energy conservation in diverse lactic acid bacteria (LAB), microorganisms that mainly rely on fermentative metabolism and are important in food fermentations. The LAB Lactiplantibacillus plantarum uses extracellular electron transfer to increase its NAD+/NADH ratio, generate more ATP through substrate-level phosphorylation, and accumulate biomass more rapidly. This novel, hybrid metabolism is dependent on a type-II NADH dehydrogenase (Ndh2) and conditionally requires a flavin-binding extracellular lipoprotein (PplA) under laboratory conditions. It confers increased fermentation product yield, metabolic flux, and environmental acidification in laboratory media and during kale juice fermentation. The discovery of a single pathway that simultaneously blends features of fermentation and respiration in a primarily fermentative microorganism expands our knowledge of energy conservation and provides immediate biotechnology applications.