Expanding the Pathogenic Potential Concept To Incorporate Fulminancy, Time, and Virulence Factors

Cell wall melanin impedes growth of the Cryptococcus neoformans polysaccharide capsule by sequestering calcium

Cryptococcus neoformans has emerged as a frontrunner among deadly fungal pathogens and is particularly life-threatening for many HIV-infected individuals with compromised immunity. Multiple virulence factors contribute to the growth and survival of C. neoformans within the human host, the two most prominent of which are the polysaccharide capsule and melanin. As both of these features are associated with the cell wall, we were interested to explore possible cooperative or competitive interactions between these two virulence factors. Whereas capsule thickness had no effect on the rate at which cells became melanized, build-up of the melanin pigment layer resulted in a concomitant loss of polysaccharide material, leaving melanized cells with significantly thinner capsules than their nonmelanized counterparts. When melanin was provided exogenously to cells in a transwell culture system we observed a similar inhibition of capsule growth and maintenance. Our results show that melanin sequesters calcium thereby limiting its availability to form divalent bridges between polysaccharide subunits required for outer capsule assembly. The decreased ability of melanized cells to incorporate exported polysaccharide into the growing capsule correlated with the amount of shed polysaccharide, which could have profound negative impacts on the host immune response.

Comparative Cross-Kingdom DDA- and DIA-PASEF Proteomic Profiling Reveals Novel Determinants of Fungal Virulence and a Putative Druggable Target

Accurate and reliable detection of fungal pathogens presents an important hurdle to manage infections, especially considering that fungal pathogens, including the globally important human pathogen, Cryptococcus neoformans, have adapted diverse mechanisms to survive the hostile host environment and moderate virulence determinant production during coinfections. These pathogen adaptations present an opportunity for improvements (e.g., technological and computational) to better understand the interplay between a host and a pathogen during disease to uncover new strategies to overcome infection. In this study, we performed comparative proteomic profiling of an in vitro coinfection model across a range of fungal and bacterial burden loads in macrophages. Comparing data-dependent acquisition and data-independent acquisition enabled with parallel accumulation serial fragmentation technology, we quantified changes in dual-perspective proteome remodeling. We report enhanced and novel detection of pathogen proteins with data-independent acquisition-parallel accumulation serial fragmentation (DIA-PASEF), especially for fungal proteins during single and dual infection of macrophages. Further characterization of a fungal protein detected only with DIA-PASEF uncovered a novel determinant of fungal virulence, including altered capsule and melanin production, thermotolerance, and macrophage infectivity, supporting proteomics advances for the discovery of a novel putative druggable target to suppress C. neoformans pathogenicity.

mSphere of Influence: How host genetics impact microbial pathogenesis and treatment of infectious disease

Emily Rosowski works in the field of host-pathogen interactions, studying how host innate immune mechanisms control pathogens. In this mSphere of Influence article, she reflects on how "Host genotype-specific therapies can optimize the inflammatory response to mycobacterial infections" by D. M. Tobin, F. J. Roca, S. F. Oh, R. McFarland, et al. (Cell 148:434-446, 2012, https://doi.org/10.1016/j.cell.2011.12.023) made an impact on her by investigating how differences in host genetics can affect modes of microbial pathogenesis and inform treatments for infectious disease.

High-throughput functional trait testing for bacterial pathogens

Functional traits are characteristics that affect the fitness and metabolic function of a microorganism. There is growing interest in using high-throughput methods to characterize bacterial pathogens based on functional virulence traits. Traditional methods that phenotype a single organism for a single virulence trait can be time consuming and labor intensive. Alternatively, machine learning of whole-genome sequences (WGS) has shown some success in predicting virulence. However, relying solely on WGS can miss functional traits, particularly for organisms lacking classical virulence factors. We propose that high-throughput assays for functional virulence trait identification should become a prominent method of characterizing bacterial pathogens on a population scale. This work is critical as we move from compiling lists of bacterial species associated with disease to pathogen-agnostic approaches capable of detecting novel microbes. We discuss six key areas of functional trait testing and how advancing high-throughput methods could provide a greater understanding of pathogens.

Co-evolution within the plant holobiont drives host performance

Plants interact with a diversity of microorganisms that influence their growth and resilience, and they can therefore be considered as ecological entities, namely "plant holobionts," rather than as singular organisms. In a plant holobiont, the assembly of above- and belowground microbiota is ruled by host, microbial, and environmental factors. Upon microorganism perception, plants activate immune signaling resulting in the secretion of factors that modulate microbiota composition. Additionally, metabolic interdependencies and antagonism between microbes are driving forces for community assemblies. We argue that complex plant-microbe and intermicrobial interactions have been selected for during evolution and may promote the survival and fitness of plants and their associated microorganisms as holobionts. As part of this process, plants evolved metabolite-mediated strategies to selectively recruit beneficial microorganisms in their microbiota. Some of these microbiota members show host-adaptation, from which mutualism may rapidly arise. In the holobiont, microbiota members also co-evolved antagonistic activities that restrict proliferation of microbes with high pathogenic potential and can therefore prevent disease development. Co-evolution within holobionts thus ultimately drives plant performance.