Spotting the right location- imaging approaches to resolve the intracellular localization of invasive pathogens

Terahertz imaging for non-invasive classification of healthy and cimiciato-infected hazelnuts

The development of new non-invasive approaches able to recognize defective food is currently a lively field of research. In particular, a simple and non-destructive method able to recognize defective hazelnuts, such as cimiciato-infected ones, in real-time is still missing. This study has been designed to detect the presence of such damaged hazelnuts. To this aim, a measurement setup based on terahertz (THz) radiation has been developed. Images of a sample of 150 hazelnuts have been acquired in the low THz range by a compact and portable active imaging system equipped with a 0.14 THz source and identified as Healthy Hazelnuts (HH) or Cimiciato Hazelnut (CH) after visual inspection. All images have been analyzed to find the average transmission of the THz radiation within the sample area. The differences in the distribution of the two populations have been used to set up a classification scheme aimed at the discrimination between healthy and injured samples. The performance of the classification scheme has been assessed through the use of the confusion matrix on 50 samples. The False Positive Rate (FPR) and True Negative Rate (TNR) are 0% and 100%, respectively. On the other hand, the True Positive Rate (TPR) and False Negative Rate (FNR) are 75% and 25%, respectively. These results are relevant from the perspective of the development of a simple, automatic, real-time method for the discrimination of cimiciato-infected hazelnuts in the processing industry.

Generation of Sporothrix schenckii mutants expressing the green fluorescent protein suitable for the study of host-fungus interactions

Sporotrichosis is an infection caused by members of the Sporothrix genus, and among them, Sporothrix schenckii is one of the etiological agents. Both, the disease and the causative agent have gained interest in the recent years, because of the report of epidemic outbreaks, and the description of the disease transmission from animals to human beings. Despite the relevance of S. schenckii in the clinical field, there are basic aspects of its biology poorly explored. So far, Agrobacterium tumefaciens-mediated transformation has been reported as an alternative for genetic manipulation of this fungal pathogen. Here, we report the optimization of the transformation method and used this to generate insertional mutants that express the green fluorescent protein in S. schenckii. We obtained five mutant strains that showed mitotic stability and expression of the reporter gene. The strains displayed normal cell wall composition, and a similar ability to interact ex vivo with human monocytes and monocyte-derived macrophages. Moreover, the virulence in larvae of Galleria mellonella was similar to that obtained with the wild-type control strains. These data indicate that these fluorescent mutants with normal ability to interact with the host could be used in bioimaging to track the host-Sporothrix interaction in vivo.

Cytoplasmic access by intracellular bacterial pathogens

Entry into host cells is a strategy widely used by bacterial pathogens, after which they either remain within membrane-bound compartments or rupture the endocytic vacuole to reach the cytoplasm. During recent years, cytoplasmic access has been documented for an increasing number of pathogens. Here we review how classical cytoplasmic bacterial pathogens rupture their endocytic vacuoles as well as the mechanisms used to accomplish this task by bacterial species for which host cytoplasmic localization has only recently been identified. We also discuss the consequences for pathogenesis resulting from this change in intracellular localization, with a particular focus on the role of the host. What emerges is that cytoplasmic access plays an important role in the pathophysiology of an increasing number of intracellular bacterial pathogens.

Single cell measurements of vacuolar rupture caused by intracellular pathogens

Shigella flexneri are pathogenic bacteria that invade host cells entering into an endocytic vacuole. Subsequently, the rupture of this membrane-enclosed compartment allows bacteria to move within the cytosol, proliferate and further invade neighboring cells. Mycobacterium tuberculosis is phagocytosed by immune cells, and has recently been shown to rupture phagosomal membrane in macrophages. We developed a robust assay for tracking phagosomal membrane disruption after host cell entry of Shigella flexneri or Mycobacterium tuberculosis. The approach makes use of CCF4, a FRET reporter sensitive to β-lactamase that equilibrates in the cytosol of host cells. Upon invasion of host cells by bacterial pathogens, the probe remains intact as long as the bacteria reside in membrane-enclosed compartments. After disruption of the vacuole, β-lactamase activity on the surface of the intracellular pathogen cleaves CCF4 instantly leading to a loss of FRET signal and switching its emission spectrum. This robust ratiometric assay yields accurate information about the timing of vacuolar rupture induced by the invading bacteria, and it can be coupled to automated microscopy and image processing by specialized algorithms for the detection of the emission signals of the FRET donor and acceptor. Further, it allows investigating the dynamics of vacuolar disruption elicited by intracellular bacteria in real time in single cells. Finally, it is perfectly suited for high-throughput analysis with a spatio-temporal resolution exceeding previous methods. Here, we provide the experimental details of exemplary protocols for the CCF4 vacuolar rupture assay on HeLa cells and THP-1 macrophages for time-lapse experiments or end points experiments using Shigella flexneri as well as multiple mycobacterial strains such as Mycobacterium marinum, Mycobacterium bovis, and Mycobacterium tuberculosis.

Technical Advances in Veterinary Diagnostic Microbiology

Forming a significant part of biomass on earth, microorganisms are renowned for their abundance and diversity. From submicroscopic infectious particles (viruses), small unicellular cells (bacteria and yeasts) to multinucleate and multicellular organisms (filamentous fungi, protozoa, and helminths), microorganisms have found their way into virtually every environmental niche, and show little restrain in making their presence felt. While a majority of microorganisms are free-living and involved in the degradation of plant debris and other organic materials, others lead a symbiotic, mutually beneficial life within their hosts. In addition, some microorganisms have the capacity to take advantage of temporary weaknesses in animal and human hosts, causing notable morbidity and mortality. Because clinical manifestations in animals and humans resulting from infections with various microorganisms are often nonspecific (e.g., general malaise and fever), it is necessary to apply laboratory diagnostic means to identify the culprit organisms for treatment and prevention purposes.

Imaging inflammation: molecular strategies to visualize key components of the inflammatory cascade, from initiation to resolution

Dysregulation of inflammation is central to the pathogenesis of innumerable human diseases. Understanding and tracking the critical events in inflammation are crucial for disease monitoring and pharmacological drug discovery and development. Recent progress in molecular imaging has provided novel insights into spatial associations, molecular events and temporal sequelae in the inflammatory process. While remaining a burgeoning field in pre-clinical research, increasing application in man affords researchers the opportunity to study disease pathogenesis in humans in situ thereby revolutionizing conventional understanding of pathophysiology and potential therapeutic targets. This review provides a description of commonly used molecular imaging modalities, including optical, radionuclide and magnetic resonance imaging, and details key advances and translational opportunities in imaging inflammation from initiation to resolution.

Hierarchies of host factor dynamics at the entry site of Shigella flexneri during host cell invasion

Shigella flexneri, the causative agent of bacillary dysentery, induces massive cytoskeletal rearrangement, resulting in its entry into nonphagocytic epithelial cells. The bacterium-engulfing membrane ruffles are formed by polymerizing actin, a process activated through injected bacterial effectors that target host small GTPases and tyrosine kinases. Once inside the host cell, S. flexneri escapes from the endocytic vacuole within minutes to move intra- and intercellularly. We quantified the fluorescence signals from fluorescently tagged host factors that are recruited to the site of pathogen entry and vacuolar escape. Quantitative time lapse fluorescence imaging revealed simultaneous recruitment of polymerizing actin, small GTPases of the Rho family, and tyrosine kinases. In contrast, we found that actin surrounding the vacuole containing bacteria dispersed first from the disassembling membranes, whereas other host factors remained colocalized with the membrane remnants. Furthermore, we found that the disassembly of the membrane remnants took place rapidly, within minutes after bacterial release into the cytoplasm. Superresolution visualization of galectin 3 through photoactivated localization microscopy characterized these remnants as small, specular, patchy structures between 30 and 300 nm in diameter. Using our experimental setup to track the time course of infection, we identified the S. flexneri effector IpgB1 as an accelerator of the infection pace, specifically targeting the entry step, but not vacuolar progression or escape. Together, our studies show that bacterial entry into host cells follows precise kinetics and that this time course can be targeted by the pathogen.

Biochemistry of the phagosome: the challenge to study a transient organelle

Phagocytes are specialized cells of the immune system, designed to engulf and destroy harmful microorganisms inside the newly formed phagosome. The latter is an intracellular organelle that is transformed into a toxic environment within minutes and disappears once the pathogen is destroyed. Reactive oxygen species and reactive nitrogen species are produced inside the phagosome. Intracellular granules or lysosomes of the phagocyte fuse with the phagosome and liberate their destructive enzymes. This process of phagocytosis efficiently protects against most infections; however, some microorganisms avoid their destruction and cause severe damage. To understand such failure of phagosomal killing, we need to learn more about the actual destruction process in the phagosome. This paper summarizes methods to investigate the biochemistry of the phagosome and discusses some of their limitations. In accordance with the nature of the phagosome, the issue of localization and temporal dynamics is emphasized, and recent developments are highlighted.