Zouhaier Ben Belgacem

Bio: Zouhaier Ben Belgacem is an academic researcher from Katholieke Universiteit Leuven. The author has contributed to research in topics: Microbial ecology & Microbial population biology. The author has an hindex of 1, co-authored 1 publications receiving 120 citations.

Microbiology of sugar-rich environments: diversity, ecology and system constraints

Abstract: Microbial habitats that contain an excess of carbohydrate in the form of sugar are widespread in the microbial biosphere. Depending on the type of sugar, prevailing water activity and other substances present, sugar-rich environments can be highly dynamic or relatively stable, osmotically stressful, and/or destabilizing for macromolecular systems, and can thereby strongly impact the microbial ecology. Here, we review the microbiology of different high-sugar habitats, including their microbial diversity and physicochemical parameters, which act to impact microbial community assembly and constrain the ecosystem. Saturated sugar beet juice and floral nectar are used as case studies to explore the differences between the microbial ecologies of low and higher water-activity habitats respectively. Nectar is a paradigm of an open, dynamic and biodiverse habitat populated by many microbial taxa, often yeasts and bacteria such as, amongst many others, Metschnikowia spp. and Acinetobacter spp., respectively. By contrast, thick juice is a relatively stable, species-poor habitat and is typically dominated by a single, xerotolerant bacterium (Tetragenococcus halophilus). A number of high-sugar habitats contain chaotropic solutes (e.g. ethyl acetate, phenols, ethanol, fructose and glycerol) and hydrophobic stressors (e.g. ethyl octanoate, hexane, octanol and isoamyl acetate), all of which can induce chaotropicity-mediated stresses that inhibit or prevent multiplication of microbes. Additionally, temperature, pH, nutrition, microbial dispersion and habitat history can determine or constrain the microbiology of high-sugar milieux. Findings are discussed in relation to a number of unanswered scientific questions.

A new analysis of Mars "Special Regions": findings of the second MEPAG Special Regions Science Analysis Group (SR-SAG2)

Abstract: A committee of the Mars Exploration Program Analysis Group (MEPAG) has reviewed and updated the description of Special Regions on Mars as places where terrestrial organisms might replicate (per the COSPAR Planetary Protection Policy). This review and update was conducted by an international team (SR-SAG2) drawn from both the biological science and Mars exploration communities, focused on understanding when and where Special Regions could occur. The study applied recently available data about martian environments and about terrestrial organisms, building on a previous analysis of Mars Special Regions (2006) undertaken by a similar team. Since then, a new body of highly relevant information has been generated from the Mars Reconnaissance Orbiter (launched in 2005) and Phoenix (2007) and data from Mars Express and the twin Mars Exploration Rovers (all 2003). Results have also been gleaned from the Mars Science Laboratory (launched in 2011). In addition to Mars data, there is a considerable body of n...

The seed microbiome: Origins, interactions, and impacts

Abstract: The development and dispersal of seeds as well as their transition to seedlings represent perhaps the most critical stages of a plant’s life cycle The endophytic and epiphytic microbial interactions that take place in, on, and around seeds during these stages of the plant’s life cycle may have profound impacts on plant ecology, health, and productivity While our understanding of the seed microbiota has lagged far behind that of the rhizosphere and phyllosphere, many advances are now being made This review explores the microbial associations with seeds through various stages of the plant life cycle, beginning with the earliest stages of seed development on the parent plant and continuing through the development and establishment of seedlings in soil This review represents a broad synthesis of the ecological and agricultural literature focused on seed-microbe interactions as a means of better understanding how these interactions may ultimately influence plant ecology, health, and productivity in both natural and agricultural systems Our current understanding of seed-microbe associations will be discussed, with an emphasis on recent findings that specifically highlight the emerging contemporary understanding of how seed-microbe associations may ultimately impact plant health and productivity The diversity and dynamics of seed microbiomes represent the culmination of complex interactions with microbes throughout the plant life cycle The richness and dynamics of seed microbiomes is revealing exciting new opportunities for research into plant-microbe interactions Often neglected in plant microbiome studies, the renaissance of inquiry into seed microbiomes is offering exciting new insights into how the diversity and dynamics of the seed microbiome with plant and soil microbiomes as well as the microbiomes of dispersers and pollinators It is clear that the interactions taking place in and around seeds indeed have significant impacts on plant health and productivity in both agricultural and natural ecosystems

Is there a common water-activity limit for the three domains of life?

Abstract: Archaea and Bacteria constitute a majority of life systems on Earth but have long been considered inferior to Eukarya in terms of solute tolerance. Whereas the most halophilic prokaryotes are known for an ability to multiply at saturated NaCl (water activity (aw) 0.755) some xerophilic fungi can germinate, usually at high-sugar concentrations, at values as low as 0.650–0.605 aw. Here, we present evidence that halophilic prokayotes can grow down to water activities of <0.755 for Halanaerobium lacusrosei (0.748), Halobacterium strain 004.1 (0.728), Halobacterium sp. NRC-1 and Halococcus morrhuae (0.717), Haloquadratum walsbyi (0.709), Halococcus salifodinae (0.693), Halobacterium noricense (0.687), Natrinema pallidum (0.681) and haloarchaeal strains GN-2 and GN-5 (0.635 aw). Furthermore, extrapolation of growth curves (prone to giving conservative estimates) indicated theoretical minima down to 0.611 aw for extreme, obligately halophilic Archaea and Bacteria. These were compared with minima for the most solute-tolerant Bacteria in high-sugar (or other non-saline) media (Mycobacterium spp., Tetragenococcus halophilus, Saccharibacter floricola, Staphylococcus aureus and so on) and eukaryotic microbes in saline (Wallemia spp., Basipetospora halophila, Dunaliella spp. and so on) and high-sugar substrates (for example, Xeromyces bisporus, Zygosaccharomyces rouxii, Aspergillus and Eurotium spp.). We also manipulated the balance of chaotropic and kosmotropic stressors for the extreme, xerophilic fungi Aspergillus penicilloides and X. bisporus and, via this approach, their established water-activity limits for mycelial growth (∼0.65) were reduced to 0.640. Furthermore, extrapolations indicated theoretical limits of 0.632 and 0.636 aw for A. penicilloides and X. bisporus, respectively. Collectively, these findings suggest that there is a common water-activity limit that is determined by physicochemical constraints for the three domains of life.

Ecology of aspergillosis: insights into the pathogenic potency of Aspergillus fumigatus and some other Aspergillus species

Abstract: Fungi of the genus Aspergillus are widespread in the environment. Some Aspergillus species, most commonly Aspergillus fumigatus, may lead to a variety of allergic reactions and life-threatening systemic infections in humans. Invasive aspergillosis occurs primarily in patients with severe immunodeficiency, and has dramatically increased in recent years. There are several factors at play that contribute to aspergillosis, including both fungus and host-related factors such as strain virulence and host pulmonary structure/immune status, respectively. The environmental tenacity of Aspergilllus, its dominance in diverse microbial communities/habitats, and its ability to navigate the ecophysiological and biophysical challenges of host infection are attributable, in large part, to a robust stress-tolerance biology and exceptional capacity to generate cell-available energy. Aspects of its stress metabolism, ecology, interactions with diverse animal hosts, clinical presentations and treatment regimens have been well-studied over the past years. Here, we synthesize these findings in relation to the way in which some Aspergillus species have become successful opportunistic pathogens of human- and other animal hosts. We focus on the biophysical capabilities of Aspergillus pathogens, key aspects of their ecophysiology and the flexibility to undergo a sexual cycle or form cryptic species. Additionally, recent advances in diagnosis of the disease are discussed as well as implications in relation to questions that have yet to be resolved.

Water structure and chaotropicity: their uses, abuses and biological implications

Abstract: The concept of “water structure” has been invoked to explain all manner of aqueous phenomena. Here we look at the origins of this tendency to understand solute hydration in terms of structural changes in bulk water, and consider the validity of one particular example: the classification of small solutes as chaotropic or kosmotropic, and the putative relation of this terminology to notions of structure-making and structure-breaking in the solvent. We doubt whether complex phenomena such as Hofmeister and osmolyte effects on macromolecules can be understood simply on the basis of a change in solvent structure. Rather, we argue that chaotropicity, if understood in the original sense, arises from the activities that solutes exert on macromolecular systems, as well as from deviations of solvation water from bulk-like behaviour. If applied judiciously, chaotropicity remains a potent, biologically pertinent parameter useful for classifying and understanding solution phenomena in all types of living system.