Bifidobacteria are widely recognized important symbionts of the human gut microbiome, shaping host physiology from birth to adulthood to old age. Their evolutionary success is derived from metabolic versatility, ecological adaptation and intimate host interactions. Equipped with an expansive glycan-degrading repertoire, stress-response systems and specialized cell surface structures, bifidobacteria establish stable gut colonization and influence host immune, metabolic and neuroendocrine functions. Genomics and functional studies have revealed how human-associated members of this genus shape trophic networks, tolerance and resilience within the gut ecosystem. Integration of ecological, metabolic and immunological perspectives now positions the genus Bifidobacterium as an excellent model to support human gut health and its potential use as gut microbiota-based therapeutics.

Microsporidia are obligate intracellular eukaryotic parasites. These pathogens belong to an understudied lineage of fungi and can infect most types of animal. Although microsporidia were first discovered over 150 years ago, their impact on humans and agriculturally important animals has become clear only within the last several decades. Microsporidia have the smallest known eukaryotic genomes, and technological advances have been used to highlight the consequences of genomic reduction. Microsporidia have a unique structure known as the polar tube and the mechanisms of how this infectious apparatus is used for invasion of host cells are beginning to be understood. Microsporidia infection has wide-ranging consequences on hosts, and in turn, hosts have evolved immunity against these parasites. Progress has been made in both detecting and treating these infections. This Review provides an overview of the evolution, infection mechanisms and impact of these parasites on their hosts.

Bacteriophages have been applied therapeutically for more than a century for the management of bacterial infections in humans. Although their potential as a safe alternative or adjunct to antibiotics is well established, clinical success is difficult to predict from in vitro testing. There is currently no consensus on the optimal way to use them, nor are there any simple extrapolations from the use paradigms of conventional antibiotics. A rational approach to phage therapy requires informed phage selection and a comprehensive understanding of bacterial-phage-human host relationships and the management of bacterial and phage adaptations that occur in vivo. Phages may be used in ways that antibiotics are not and are a powerful approach for addressing the problem of antimicrobial resistance, not least because their optimal use requires a more thoughtful approach. In this Review, we explore key aspects of phage biology that underpin the effective use of phages for therapy. We outline clinical strategies for applying phages against a range of infections, highlighting their advantages and limitations. We then address challenges in production, scalability and regulation of phage preparations, identifying areas requiring further development. Finally, we discuss interactions between phages and the human immune system, and their importance for therapeutic success.

Malnutrition poses a substantial global health burden with long-lasting physical, social and economic effects. Now recognized as an spectrum of interrelated conditions, malnutrition manifests differently based on an individual's geography, health-care access and lifestyle. There is growing evidence that malnutrition is also driven by defects in the composition and function of the gut microbiota, a metabolically active 'virtual organ' known to play a central role in intestinal barrier function, energy harvest, immune regulation and metabolic signalling. Across the malnutrition spectrum, perturbed microbial composition and function have been consistently observed in both children and adults. This 'malnourished microbiota' is associated with impaired nutrient utilization, chronic inflammation and altered energy balance and is known to persist despite nutritional interventions. This Review synthesizes evidence from human cohorts and animal models showing that the gut microbiota functions as a central player in malnutrition. We discuss implications for diagnostic and therapeutic strategies, arguing that microbiome restoration could provide a partial solution for malnutrition. This reframing is particularly relevant given current challenges such as armed conflict, climate change and declining international support, all of which are associated with an increased global burden of malnutrition. Thus, integrating microbiota-based strategies could protect vulnerable populations and accelerate progress towards overcoming malnutrition.