Microbiology

Group A streptococcus (GAS) is a Gram-positive human pathogen that causes invasive infections with mild to life-threatening severity, like toxic shock syndrome, rheumatic heart disease, and necrotizing fasciitis (NF). NF is characterized by a clinical presentation of widespread tissue destruction due to the rapid spread of GAS infection into fascial planes. Despite quick medical interventions, mortality from NF is high. The early onset of the disease is difficult to diagnose because of non-specific clinical symptoms. Moreover, the unavailability of an effective vaccine against GAS warrants a genuine need for alternative treatments against GAS NF. One endoplasmic reticulum stress signaling pathway (PERK pathway) gets triggered in the host upon GAS infection. Bacteria utilize asparagine release as an output of this pathway for its pathogenesis. We reported that the combination of sub-cutaneous (SC) and intraperitoneal (IP) administration of PERK pathway inhibitors (GSK2656157 and ISRIB) cures local as well as systemic GAS infection in a NF murine model, by reducing asparagine release at the infection site. This protocol's methodology is detailed below.

Basic and translational research needs rapid methods to test antimicrobial formulations. Bioluminescent bacteria and advanced imaging systems capable of acquiring bioluminescence enable us to quickly and longitudinally evaluate the efficacy of antimicrobials. Conventional approaches, such as radial diffusion and viable count assays, are time-consuming and do not allow for longitudinal analysis. Bioluminescence imaging is sensitive and gives vital spatial and temporal information on the infection status in the body. Here, using bioluminescent Pseudomonas aeruginosa, we describe an in vitro and an in vivo approach to rapidly evaluate the antimicrobial efficacy of the host-defense peptide TCP-25.

Graphic abstract:

Evaluation of antimicrobials using bioluminescent bacteria.

Experimental pneumonia models are important tools to study the pathophysiology of lung inflammation caused by microbial infections and the efficacy of (novel) drugs. We have applied a murine model of pneumonia induced by Pseudomonas (P.) aeruginosa infection to study acute host antibacterial defense in lungs, and assess epithelial cell specific responses as well as leukocyte recruitment to the alveolar space. To study host responses during disseminating pneumonia, we also applied a model of infecting mice with hypermucoviscous Klebsiella (K.) pneumoniae. In the latter model, K. pneumoniae is restricted to lung during the early phase of infection and at the later time points disseminates to the circulation and distal organs resulting in sepsis. Detailed procedures for induction of pneumonia in mice by Pseudomonas and Klebsiella and for isolation and analysis of infected organs, bronchoalveolar fluid, and bronchial brushes are provided in this article.

Pneumococcal (PN) meningitis is a life-threatening disease with high mortality rates that leads to permanent neurological sequelae. Studies of the process of bacterial crossing of the blood brain barrier (BBB) are hampered by the lack of relevant in vitro and in vivo models of meningitis that recapitulate the human disease. PN meningitis involves bacterial access to the bloodstream preceding translocation across the BBB. A large number of PN meningitis models have been developed in mice, with intravenous administration via the lateral tail vein representing the main way to study BBB crossing by PN. While in humans, meningitis is not always associated with bacteremia, PN meningitis after intravenous injection in mice usually develops following sustained and very high bacteremic titers. High grade bacteremia, however, is known to favor inflammation and BBB permeabilization, thereby increasing PN translocation across the BBB and associated damages. Therefore, specific processes associated with early events of PN translocation may be blurred by overall changes in the inflammatory environment and potentially systemic dysfunction in the case of severe sepsis. Here, we report a mouse meningitis model induced by PN injection in the retro-orbital (RO) sinus. We show that, in this model, mice appear to control bacteremic levels during the first 13 h post-infection, while PN crossing of the BBB can be clearly detected by fluorescence confocal microscopy analysis of brain slices as early as 6 h post-infection. Because of the low frequency of events, however, PN translocation across brain parenchymal vessels at early time points requires a rigorous and systematic examination of the brain volume.

Most vaccines require co-delivery of an adjuvant in order to generate the desired immune responses. However, many currently available adjuvants are non-biodegradable, have limited efficacy, and/or poor safety profile. Thus, new adjuvants, or self-adjuvanting vaccine delivery systems, are required. Here, we proposed a self-adjuvanting delivery system that is fully defined, biodegradable, and non-toxic. The system is produced by conjugation of polyleucine to peptide antigen, followed by self-assembly of the conjugate into nanoparticles. The protocol includes solid-phase peptide synthesis of the vaccine conjugate, purification, self-assembly and physicochemical characterization of the product. Overall, this protocol describes, in detail, the production of a well-defined and effective self-adjuvanting delivery system for peptide antigens, along with tips for troubleshooting.

Several in-cell spectroscopic techniques have been developed recently to investigate the structure and mechanism of proteins in their native environment. Conditions in vivo differ dramatically from those selected for in vitro experiments. Accordingly, the cellular environment can affect the protein mechanism for example by molecular crowding or binding of small molecules. Fourier transform infrared (FTIR) difference spectroscopy is a well-suited method to study the light-induced structural responses of photoreceptors including changes in cofactor, side chains and secondary structure. Here, we describe a protocol to study the response of cofactor and protein in living E. coli cells via in-cell infrared difference (ICIRD) spectroscopy using the attenuated total reflection (ATR) configuration. Proteins are overexpressed in E. coli, the cells are transferred into saline solution and the copy number per cell is determined using fluorescence spectroscopy. The suspension is centrifuged and the concentrated cells transferred onto the ATR cell inside the FTIR spectrometer. The thermostatted cell is sealed and illuminated from the top with an LED. Intensity spectra are recorded before and after illumination to generate the difference spectrum of the receptor inside the living cell. With ICIRD spectroscopy, structural changes of soluble photoreceptors are resolved in a near-native environment. The approach works in H₂O at ambient conditions, is label free, without any limitations in protein size and does not require any purification step.

Graphic abstract:

In-cell infrared difference spectroscopy on photoreceptors in living E. coli using attenuated total reflection.

Animal infection models play significant roles in studying bacterial pathogenic mechanisms, host pathogen interaction as well as evaluating drug and vaccine efficacies. We have been utilizing an acute pneumonia model to study bacterial colonization in lungs and assess virulence to the host by determination of bacterial loads and survival assays, as well as examine the bacterial gene expression in vivo. Additionally, the host's immune response to the pathogen can be explored through this infection model.

During swarming, high density flagella-driven bacteria migrate collectively in a swirling pattern on wet agar surfaces, immersed in a thin viscous fluid layer called “swarm fluid”. Though the fluid environment has essential role in the emergence of swarming behavior, the microscopic mechanisms of it in mediating the cooperation of bacteria populations are not fully understood. Here, instead of micro-sized tracers used in previous research, we use gold nanorods as single particle tracers to probe the dynamics of the swarm fluid. This protocol includes five major parts: (1) the culture of swarming bacterial colony; (2) the preparations of gold nanorod tracers and the micro-spraying technique which are used to put the nanotracers into the upper fluid of bacterial swarms; (3) imaging and tracking; (4) other necessary control experiments; (5) data analysis and fitting of physical models. With this method, the nano-sized tracers could move long distances above motile cells without direct collisions with the bacteria bodies. In this way, the microscopic dynamics of the swarm fluid could be tracked with high spatiotemporal resolution. Moreover, the comprehensive analysis of multi-particle trajectories provides systematic visualization of the fluid dynamics. The method is promising to probe the fluid dynamics of other natural or artificial active matter systems.

This protocol was developed to qualitatively and quantitatively detect protein-protein interactions in all compartments of Escherichia coli by Förster Resonance Energy Transfer (FRET) using the Superfolder mTurquoise2 ox-mNeonGreen FRET pair (sfTq2^ox-mNG). This FRET pair has more than twice the detection range for FRET interaction studies in the cytoplasm or periplasm of E. coli compared to other pairs to date. These protein-interaction studies can be performed in vivo because fluorescent proteins can be genetically encoded as fusions to proteins of interest and expressed in the cell. sfTq2^ox and mNG fluorescent protein fusions are co-expressed in bacterial cells and the fluorescence emission spectra are measured. By also measuring reference spectra for the background, sfTq2^ox-only and mNG-only samples, expected emission spectra can be calculated. Sensitized emission for mNG above the expected spectrum can be attributed to FRET and quantified by spectral unmixing. This bio-protocol discusses the sfTq2^ox-mNG FRET pair and provides a practical guide in preparing the protein fusions, setting up and running the FRET experiments, measuring fluorescence spectra and gives the tools to analyze the collected data.

Splenectomy in an animal model requires a standardized technique utilizing best practice to avoid variability which can result in adverse impact to the animal resulting in flawed physiologic responses simply due to technique rather than to the studied variables. In the case of the spleen, often investigators are analyzing the animal immune or inflammatory responses. Surgical splenectomy involves many variables from the training and expertise of the surgeon, which directly correlates to surgical technique to the length of operation and ease of the procedure. This operation, in turn, impacts blood loss and insensible fluid losses, sterile technique, unintended trauma to the spleen and surrounding organs, the length of the incision and the duration of the operation with more prolonged exposure to anesthetic agents. All these variables ultimately play a significant role in the experiment since they directly affect the response of the model in terms of inflammation, immune activation, or even suppression. Undesired variables such as these go unnoticed and lead to inaccurate and misleading data.