Candida albicans is a major fungal pathogen of humans. This yeast is carried by many individuals as a harmless commensal, but when immune defences are perturbed it causes mucosal infections (thrush). Additionally, when the immune system becomes severely compromised, C. albicans often causes life-threatening systemic infections. A battery of virulence factors and fitness attributes promote the pathogenicity of C. albicans. Fitness attributes include robust responses to local environmental stresses, the inactivation of which attenuates virulence. Stress signalling pathways in C. albicans include evolutionarily conserved modules. However, there has been rewiring of some stress regulatory circuitry such that the roles of a number of regulators in C. albicans have diverged relative to the benign model yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. This reflects the specific evolution of C. albicans as an opportunistic pathogen obligately associated with warm-blooded animals, compared with other yeasts that are found across diverse environmental niches. Our understanding of C. albicans stress signalling is based primarily on the in vitro responses of glucose-grown cells to individual stresses. However, in vivo this pathogen occupies complex and dynamic host niches characterised by alternative carbon sources and simultaneous exposure to combinations of stresses (rather than individual stresses). It has become apparent that changes in carbon source strongly influence stress resistance, and that some combinatorial stresses exert non-additive effects upon C. albicans. These effects, which are relevant to fungus–host interactions during disease progression, are mediated by multiple mechanisms that include signalling and chemical crosstalk, stress pathway interference and a biological transistor.

The β-glucan receptor Dectin-1 is a member of the C-type lectin family and functions as an innate pattern recognition receptor in antifungal immunity. In both mouse and man, Dectin-1 has been found to play an essential role in controlling infections with Candida albicans, a normally commensal fungus in man which can cause superficial mucocutaneous infections as well as life-threatening invasive diseases. Here, using in vivo models of infection, we show that the requirement for Dectin-1 in the control of systemic Candida albicans infections is fungal strain-specific; a phenotype that only becomes apparent during infection and cannot be recapitulated in vitro. Transcript analysis revealed that this differential requirement for Dectin-1 is due to variable adaptation of C. albicans strains in vivo, and that this results in substantial differences in the composition and nature of their cell walls. In particular, we established that differences in the levels of cell-wall chitin influence the role of Dectin-1, and that these effects can be modulated by antifungal drug treatment. Our results therefore provide substantial new insights into the interaction between C. albicans and the immune system and have significant implications for our understanding of susceptibility and treatment of human infections with this pathogen.

Roseburia inulinivorans is a recently identified motile representative of the Firmicutes that contributes to butyrate formation from a variety of dietary polysaccharide substrates in the human large intestine. Microarray analysis was used here to investigate substratedriven gene-expression changes in R. inulinivorans A2-194. A cluster of fructo-oligosaccharide/inulin utilization genes induced during growth on inulin included one encoding a β-fructofuranosidase protein that was prominent in the proteome of inulin-grown cells. This cluster also included a 6-phosphofructokinase and an ABC transport system, whereas a distinct inulin-induced 1-phosphofructokinase was linked to a fructose-specific phosphoenolpyruvatedependent sugar phosphotransferase system (PTS II transport enzyme). Real-time PCR analysis showed that the β-fructofuranosidase and adjacent ABC transport protein showed greatest induction during growth on inulin, whereas the 1-phosphofructokinase enzyme and linked sugar phosphotransferase transport system were most strongly up-regulated during growth on fructose, indicating that these two clusters play distinct roles in the use of inulin. The R. inulinivorans β-fructofuranosidase was overexpressed in Escherichia coli and shown to hydrolyze fructans ranging from inulin down to sucrose, with greatest activity on fructo-oligosaccharides. Genes induced on starch included the major extracellular α-amylase and two distinct α-glucanotransferases together with a gene encoding a flagellin protein. The latter response may be concerned with improving bacterial access to insoluble starch particles.anaerobic gut bacteria | differential gene expression | fructooligosaccharides | butyrate | prebiotic P lant cell wall polysaccharides, storage polysaccharides such as resistant starch and inulin, and many oligosaccharides of plant origin remain undigested in the upper gastrointestinal tract and become important substrates for the growth of colonic bacteria. Prebiotics are defined as dietary substrates that reach the colon where they selectively stimulate the growth of beneficial gut bacteria (1), with the most widely used prebiotics being the fructans inulin and fructo-oligosaccharides (FOS). Inulin is a linear polymer [degree of polymerization (DP) = 3-60] of β-2,1-linked fructose monomers with a terminal glucose residue, whereas FOS have the same backbone but a maximal chain length of 8 monomeric units. The increasing use of prebiotics to enhance gut health is driving research into their mode of action. Many studies have shown that both inulin and FOS selectively stimulate the growth of potentially beneficial gut bacteria such as bifidobacteria and lactobacilli (reviewed in ref.2). However, only a few studies have considered the potential for stimulation of other bacterial groups that may also be beneficial (3-6).The bifidogenic dose of inulin has been defined as 5-8 g/d (7), yet the effect of such supplementation on other beneficial groups of gut bacteria and the differences relating to different compositions of inulin...

Nutritional immunity – the withholding of nutrients by the host – has long been recognised as an important factor that shapes bacterial-host interactions. However, the dynamics of nutrient availability within local host niches during fungal infection are poorly defined. We have combined laser ablation-inductively coupled plasma mass spectrometry (LA-ICP MS), MALDI imaging and immunohistochemistry with microtranscriptomics to examine iron homeostasis in the host and pathogen in the murine model of systemic candidiasis. Dramatic changes in the renal iron landscape occur during disease progression. The infection perturbs global iron homeostasis in the host leading to iron accumulation in the renal medulla. Paradoxically, this is accompanied by nutritional immunity in the renal cortex as iron exclusion zones emerge locally around fungal lesions. These exclusion zones correlate with immune infiltrates and haem oxygenase 1-expressing host cells. This local nutritional immunity decreases iron availability, leading to a switch in iron acquisition mechanisms within mature fungal lesions, as revealed by laser capture microdissection and qRT-PCR analyses. Therefore, a complex interplay of systemic and local events influences iron homeostasis and pathogen-host dynamics during disease progression.

The butyrate-producing anaerobe Fusobacterium varium is an integral constituent of human gut microflora. Unlike many gut microorganisms, F. varium is capable of fermenting both amino acids and glucose. Although F. varium has been implicated in beneficial as well as pathological bacterium-host interactions, its genome has not been sequenced. To obtain a better understanding of the metabolic processes associated with amino acid fermentation by F. varium, we used a gel-based proteomic approach to examine the changes in the soluble proteome accompanying the utilization of eight different growth substrates: glucose, L-and D-glutamate, L-histidine, L-and D-lysine, and L-and D-serine. Using LC-MS/MS to analyze ,25% of the detected protein spots, we were able to identify 47 distinct proteins. While the intracellular concentrations of enzymes characteristic of a catabolic pathway for a specific amino acid were selectively increased in response to the presence of an excess of that amino acid in the growth medium, the concentrations of the core acetate-butyrate pathway enzymes remained relatively constant. Our analysis revealed (i) high intracellular concentrations of glutamate mutase and b-methylaspartate ammonia-lyase under all growth conditions, underscoring the importance of the methylaspartate pathway of glutamate catabolism in F. varium (ii) the presence of two enzymes of the hydroxyglutarate pathway of glutamate degradation in the proteome of F. varium ((R)-2-hydroxyglutaryl-CoA dehydratase and NAD-specific glutamate dehydrogenase) specifically when Lglutamate was the main energy source (iii) the presence of genes in the genome of F. varium encoding each of the enzymes of the hydroxyglutarate pathway (iv) the presence of both L-and Dserine ammonia-lyases (dehydratases) which permit F. varium to thrive on either L-or D-serine, respectively, and (v) the presence of aspartate-semialdehyde dehydrogenase and dihydrodipicolinate synthase, consistent with the ability of F. varium to synthesize meso-2,6-diaminopimelic acid as a component of its peptidoglycan. Proteins involved in other cellular processes, including oxidation-reduction reactions, protein synthesis and turnover, and transport were also identified.