Bringing Microbiology to Light: Toward All-Optical Electrophysiology in Bacteria

Membrane potential dynamics unveil the promise of bioelectrical antimicrobial susceptibility Testing (BeAST) for anti-fungal screening

Membrane potential is a useful marker for antimicrobial susceptibility testing (AST) due to its fundamental roles in cell function. However, the difficulties associated with measuring the membrane potential in microbes restrict its broad application. In this study, we present bioelectrical AST (BeAST) using the model fungus Saccharomyces cerevisiae. Using fluorescent indicators [DiBAC4(3), ThT, and TMRM], we measured plasma and mitochondrial membrane-potential dynamics upon electric stimulation. We find that a 2.5 second electric stimulation induces hyperpolarization of plasma membrane lasting 20 minutes in vital S. cerevisiae, but depolarization in inhibited cells. The numerical simulation of FitzHugh-Nagumo model successfully recapitulates vitality-dependent dynamics. The model also suggests that the magnitude of plasma-membrane potential dynamics (PMD) correlates with the degree of inhibition. To test this prediction and to examine if BeAST can be used for assessing novel anti-fungal compounds, we treat cells with biogenic silver nanoparticles (bioAgNPs) synthesized using orange fruit flavonoids and Fusarium oxysporum. Comparing BeAST with optical density assay alongside various stressors, we show that PMD correlates with the magnitude of growth inhibitions. These results suggest that BeAST holds promise for screening anti-fungal compounds, offering a valuable approach to tackling antimicrobial resistance.

IMPORTANCE

Rapid assessment of the efficacy of antimicrobials is important for optimizing treatments, avoiding misuse and facilitating the screening of new antimicrobials. The need for rapid antimicrobial susceptibility testing (AST) is growing with the rise of antimicrobial resistance. Here, we present bioelectrical AST (BeAST). Combining time-lapse microscopy and mathematical modeling, we show that electrically induced membrane potential dynamics of yeast cells correspond to the strength of growth inhibition. Furthermore, we demonstrate the utility of BeAST for assessing antimicrobial activities of novel compounds using biogenic silver nanoparticles.

A membrane intercalating metal-free conjugated organic photosensitizer for bacterial photodynamic inactivation

Photodynamic inhibition (PDI) of bacteria represents a powerful strategy for dealing with multidrug-resistant pathogens and infections, as it exhibits minimal development of antibiotic resistance. The PDI action stems from the generation of a triplet state in the photosensitizer (PS), which subsequently transfers energy or electrons to molecular oxygen, resulting in the formation of reactive oxygen species (ROS). These ROS are then able to damage cells, eventually causing bacterial eradication. Enhancing the efficacy of PDI includes the introduction of heavy atoms to augment triplet generation in the PS, as well as membrane intercalation to circumvent the problem of the short lifetime of ROS. However, the former approach can pose safety and environmental concerns, while achieving stable membrane partitioning remains challenging due to the complex outer envelope of bacteria. Here, we introduce a novel PS, consisting of a metal-free donor-acceptor thiophene-based conjugate molecule (BV-1). It presents several advantageous features for achieving effective PDI, namely: (i) it exhibits strong light absorption due to the conjugated donor-acceptor moieties; (ii) it exhibits spontaneous and stable membrane partitioning thanks to its amphiphilicity, accompanied by a strong fluorescence turn-on; (iii) it undergoes metal-free intersystem crossing, which occurs preferentially when the molecule resides in the membrane. All these properties, which we rationalized via optical spectroscopies and calculations, enable the effective eradication of Escherichia coli, with an inhibition concentration that is below that of current state-of-the-art treatments. Our approach holds significant potential for the development of new PS for controlling bacterial infections, particularly those caused by Gram-negative bacteria.

Modeling control and transduction of electrochemical gradients in acid-stressed bacteria

Transmembrane electrochemical gradients drive solute uptake and constitute a substantial fraction of the cellular energy pool in bacteria. These gradients act not only as "homeostatic contributors," but also play a dynamic and keystone role in several bacterial functions, including sensing, stress response, and metabolism. At the system level, multiple gradients interact with ion transporters and bacterial behavior in a complex, rapid, and emergent manner; consequently, experiments alone cannot untangle their interdependencies. Electrochemical gradient modeling provides a general framework to understand these interactions and their underlying mechanisms. We quantify the generation, maintenance, and interactions of electrical, proton, and potassium potential gradients under lactic acid-stress and lactic acid fermentation. Further, we elucidate a gradient-mediated mechanism for intracellular pH sensing and stress response. We demonstrate that this gradient model can yield insights on the energetic limitations of membrane transport, and can predict bacterial behavior across changing environments.

Membrane Targeted Azobenzene Drives Optical Modulation of Bacterial Membrane Potential

Recent studies have shown that bacterial membrane potential is dynamic and plays signaling roles. Yet, little is still known about the mechanisms of membrane potential dynamics regulation-owing to a scarcity of appropriate research tools. Optical modulation of bacterial membrane potential could fill this gap and provide a new approach for studying and controlling bacterial physiology and electrical signaling. Here, the authors show that a membrane-targeted azobenzene (Ziapin2) can be used to photo-modulate the membrane potential in cells of the Gram-positive bacterium Bacillus subtilis. It is found that upon exposure to blue-green light (λ = 470 nm), isomerization of Ziapin2 in the bacteria membrane induces hyperpolarization of the potential. To investigate the origin of this phenomenon, ion-channel-deletion strains and ion channel blockers are examined. The authors found that in presence of the chloride channel blocker idanyloxyacetic acid-94 (IAA-94) or in absence of KtrAB potassium transporter, the hyperpolarization response is attenuated. These results reveal that the Ziapin2 isomerization can induce ion channel opening in the bacterial membrane and suggest that Ziapin2 can be used for studying and controlling bacterial electrical signaling. This new optical tool could contribute to better understand various microbial phenomena, such as biofilm electric signaling and antimicrobial resistance.

Sensitive bacterial Vm sensors revealed the excitability of bacterial Vm and its role in antibiotic tolerance

As an important free energy source, the membrane voltage (V_m) regulates many essential physiological processes in bacteria. However, in comparison with eukaryotic cells, knowledge of bacterial electrophysiology is very limited. Here, we developed a set of novel genetically encoded bacterial V_m sensors which allow single-cell recording of bacterial V_m dynamics in live cells with high temporal resolution. Using these new sensors, we reveal the electrically "excitable" and "resting" states of bacterial cells dependent on their metabolic status. In the electrically excitable state, frequent hyperpolarization spikes in bacterial V_m are observed, which are regulated by Na⁺/K⁺ ratio of the medium and facilitate increased antibiotic tolerance. In the electrically resting state, bacterial V_m displays significant cell-to-cell heterogeneity and is linked to the cell fate after antibiotic treatment. Our findings demonstrate the potential of our newly developed voltage sensors to reveal the underpinning connections between bacterial V_m and antibiotic tolerance.