Polymicrobial infections often include both fungi and bacteria and can complicate patient treatment and resolution of infection. Cross-kingdom interactions among bacteria, fungi, and/or the immune system during infection can enhance or block virulence mechanisms and influence disease progression. The fungus Candida albicans and the bacterium Pseudomonas aeruginosa are coisolated in the context of polymicrobial infection at a variety of sites throughout the body, including mucosal tissues such as the lung. In vitro, C. albicans and P. aeruginosa have a bidirectional and largely antagonistic relationship. Their interactions in vivo remain poorly understood, specifically regarding host responses in mediating infection. In this study, we examine trikingdom interactions using a transparent juvenile zebrafish to model mucosal lung infection and show that C. albicans and P. aeruginosa are synergistically virulent. We find that high C. albicans burden, fungal epithelial invasion, swimbladder edema, and epithelial extrusion events serve as predictive factors for mortality in our infection model. Longitudinal analyses of fungal, bacterial, and immune dynamics during coinfection suggest that enhanced morbidity is associated with exacerbated C. albicans pathogenesis and elevated inflammation. The P. aeruginosa quorum-sensingdeficient ΔlasR mutant also enhances C. albicans pathogenicity in coinfection and induces extrusion of the swimbladder. Together, these observations suggest that C. albicans-P. aeruginosa cross talk in vivo can benefit both organisms to the detriment of the host.

Bloodborne infections with Candida albicans are an increasingly recognized complication of modern medicine. Here, we present a mouse model of low-grade candidemia to determine the effect of disseminated infection on cerebral function and relevant immune determinants. We show that intravenous injection of 25,000 C. albicans cells causes a highly localized cerebritis marked by the accumulation of activated microglial and astroglial cells around yeast aggregates, forming fungal-induced glial granulomas. Amyloid precursor protein accumulates within the periphery of these granulomas, while cleaved amyloid beta (Aβ) peptides accumulate around the yeast cells. CNS-localized C. albicans further activate the transcription factor NF-κB and induce production of interleukin-1β (IL-1β), IL-6, and tumor necrosis factor (TNF), and Aβ peptides enhance both phagocytic and antifungal activity from BV-2 cells. Mice infected with C. albicans display mild memory impairment that resolves with fungal clearance. Our results warrant additional studies to understand the effect of chronic cerebritis on cognitive and immune function.

dimorphism is a crucial virulence factor during invasive candidiasis infections, which claim the lives of nearly one-half of those afflicted. It has long been believed that filaments drive tissue invasion and yeast mediate bloodstream dissemination, but observation of these activities during infection has been prevented by technical limitations. We used a transparent zebrafish infection model to more comprehensively analyze how utilizes shape to disseminate and invade. This model facilitated the use of diverse, complementary strategies to manipulate shape, allowing us to monitor dissemination, invasion, and pathogenesis via intravital imaging of individual fungal cells throughout the host. To control fungal cell shape, we employed three different strategies: gene deletion (Δ/ΔΔ/Δ, Δ/Δ), overexpression of master regulators ( or ), and modulation of infection temperature (21°C, 28°C, 33°C). The effects of these orthogonal manipulations were consistent, support the proposed specialized roles of yeast in dissemination and filaments in tissue invasion and pathogenesis, and indicate conserved mechanisms in zebrafish. To test if either morphotype changes the effectiveness of the other, we infected fish with a known mix of shape-locked strains. Surprisingly, mixed infections were associated with additive, but not synergistic, filament invasion and yeast dissemination. These findings provide the most complete view of morphotype-function relationships for to date, revealing independent roles of yeast and filaments during disseminated candidiasis.

Candida albicans is a ubiquitous mucosal commensal that is normally prevented from causing acute or chronic invasive disease. Neutrophils contribute to protection in oral infection but exacerbate vulvovaginal candidiasis. To dissect the role of neutrophils during mucosal candidiasis, we took advantage of a new, transparent zebrafish swim bladder infection model. Intravital microscopic tracking of individual animals revealed that the blocking of neutrophil recruitment leads to rapid mortality in this model through faster disease progression. Conversely, artificial recruitment of neutrophils during early infection reduces disease pressure. Noninvasive longitudinal tracking showed that mortality is a consequence of C. albicans breaching the epithelial barrier and invading surrounding tissues. Accordingly, we found that a hyperfilamentous C. albicans strain breaches the epithelial barrier more frequently and causes mortality in immunocompetent zebrafish. A lack of neutrophils at the infection site is associated with less fungusassociated extracellular DNA and less damage to fungal filaments, suggesting that neutrophil extracellular traps help to protect the epithelial barrier from C. albicans breach. We propose a homeostatic model where C. albicans disease pressure is balanced by neutrophil-mediated damage of fungi, maintaining this organism as a commensal while minimizing the risk of damage to host tissue. The unequaled ability to dissect infection dynamics at a high spatiotemporal resolution makes this zebrafish model a unique tool for understanding mucosal host-pathogen interactions.KEYWORDS Candida albicans, mucosal immunity, neutrophils, zebrafish C andida albicans is the most successful commensal fungus at causing opportunistic mucosal and invasive infections (1, 2). The opportunistic nature of C. albicans mucosal infection suggests that there is a constantly challenged impasse between our immune defenses and C. albicans virulence factors (3, 4). Chronic mucocutaneous candidiasis (CMC) is associated with inborn errors of adaptive immune pathways, especially Th17 immunity (5). However, acute oropharyngeal candidiasis (OPC) is more common in neutropenic patients and can result from a multitude of immunesuppressive treatments that affect both innate and adaptive immunity, such as radiation/chemotherapy and corticosteroid or antibiotic use (6, 7).Experimental work in murine models of OPC suggests that C. albicans is kept at bay in mucosal tissues through the action of epithelial cells, neutrophils, and macrophages (8). To protect from mucosal invasion in OPC, neutrophils must be present in the blood, be recruited to the site of infection, and possess a fully functional antimicrobial arsenal (9-12). Neutrophils attack both the yeast and filamentous forms of C. albicans and are associated with vulvovaginal candidiasis (13-15). Counterintuitively, because symptomatic infection is more closely associated with high neutrophil numbers than with the fungal burden, it is believed that neutrophils do more to cause...

Microbial infections early in life remain a major cause of infant mortality worldwide. This is consistent with immune deficiencies in this population. Interleukin (IL)‐27 is suppressive toward a variety of immune cell types, and we have shown that the production of IL‐27 is elevated in humans and mice early in life. We hypothesize that elevated levels of IL‐27 oppose protective responses to infection during the neonatal period. In this study, we extended previous findings in neonatal mice to identify a population of IL‐27 producers that express Gr‐1 and were further identified as myeloid‐derived suppressor cells (MDSCs) based on the expression of surface markers and functional studies. In neonates, MDSCs are more abundant and contribute to the elevated pool of IL‐27 in this population. Although the ability of MDSCs to regulate T lymphocyte activation has been well‐studied, sparingly few studies have investigated the influence of MDSCs on innate immune function during bacterial infection. We demonstrate that macrophages are impaired in their ability to control growth of Escherichia coli when cocultured with MDSCs. This bacterium is a significant concern for neonates as a common cause of bacterial sepsis and meningitis. The suppressive effect of MDSCs on macrophage function is mediated by IL‐27; inclusion of a reagent to neutralize IL‐27 promotes improved control of bacterial growth. Taken together, these results suggest that the increased abundance of MDSCs may contribute to early life susceptibility to infection and further highlight production of IL‐27 as a novel MDSC mechanism to suppress immunity.