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Consuming diets rich in plant versus animal products changes the microbes found in the human gut within days, with important implications for our health and evolution. Diet can rapidly alter gut microbiome Diet influences the structure and function of the gut microbiota in the long term, but it is not clear how rapidly the microbiota is affected by short-term dietary change. Peter Turnbaugh and colleagues studied the effect of transition to a diet consisting entirely of either animal products or plant products on the composition and function of the human gut microbiota. They find that the community changes rapidly, within a single day, overwhelming the pre-existing inter-individual differences in microbiota composition to recapitulate expected patterns of composition and metabolic function for carnivorous and herbivorous mammals. The animal-based diet was associated with higher levels of bile-tolerant microorganisms, including the bacterium Bilophila wadsworthia , which has previously been linked to inflammatory bowel disease. The authors also detected intact foodborne fungi, bacteria and viruses in the distal gut. Long-term dietary intake influences the structure and activity of the trillions of microorganisms residing in the human gut 1 , 2 , 3 , 4 , 5 , but it remains unclear how rapidly and reproducibly the human gut microbiome responds to short-term macronutrient change. Here we show that the short-term consumption of diets composed entirely of animal or plant products alters microbial community structure and overwhelms inter-individual differences in microbial gene expression. The animal-based diet increased the abundance of bile-tolerant microorganisms ( Alistipes , Bilophila and Bacteroides ) and decreased the levels of Firmicutes that metabolize dietary plant polysaccharides ( Roseburia , Eubacterium rectale and Ruminococcus bromii ). Microbial activity mirrored differences between herbivorous and carnivorous mammals 2 , reflecting trade-offs between carbohydrate and protein fermentation. Foodborne microbes from both diets transiently colonized the gut, including bacteria, fungi and even viruses. Finally, increases in the abundance and activity of Bilophila wadsworthia on the animal-based diet support a link between dietary fat, bile acids and the outgrowth of microorganisms capable of triggering inflammatory bowel disease 6 . In concert, these results demonstrate that the gut microbiome can rapidly respond to altered diet, potentially facilitating the diversity of human dietary lifestyles.

Functional metagenomic selections for resistance to 18 antibiotics in 18 different soils reveal that bacterial community composition is the primary determinant of soil antibiotic resistance gene content. The answer does not lie in the soil Antibiotic resistance genes readily move between unrelated bacteria in hospital settings, prompting speculation that the remarkable diversity of resistance genes in soil contributes to an increasing flow of antibiotic resistance from environmental to pathogenic organisms. This study refutes this notion. Kevin Forsberg et al . performed functional metagenomic selections for resistance to 18 antibiotics from a series of agricultural and grassland soils and find that soil bacteria rarely possess the sequence signatures of resistance gene exchange between species. It seems that particular organisms, rather than horizontally exchanged DNA elements, are the major disseminators of antibiotic resistance in the soil. Ancient and diverse antibiotic resistance genes (ARGs) have previously been identified from soil 1 , 2 , 3 , including genes identical to those in human pathogens 4 . Despite the apparent overlap between soil and clinical resistomes 4 , 5 , 6 , factors influencing ARG composition in soil and their movement between genomes and habitats remain largely unknown 3 . General metagenome functions often correlate with the underlying structure of bacterial communities 7 , 8 , 9 , 10 , 11 , 12 . However, ARGs are proposed to be highly mobile 4 , 5 , 13 , prompting speculation that resistomes may not correlate with phylogenetic signatures or ecological divisions 13 , 14 . To investigate these relationships, we performed functional metagenomic selections for resistance to 18 antibiotics from 18 agricultural and grassland soils. The 2,895 ARGs we discovered were mostly new, and represent all major resistance mechanisms 15 . We demonstrate that distinct soil types harbour distinct resistomes, and that the addition of nitrogen fertilizer strongly influenced soil ARG content. Resistome composition also correlated with microbial phylogenetic and taxonomic structure, both across and within soil types. Consistent with this strong correlation, mobility elements (genes responsible for horizontal gene transfer between bacteria such as transposases and integrases) syntenic with ARGs were rare in soil by comparison with sequenced pathogens, suggesting that ARGs may not transfer between soil bacteria as readily as is observed between human pathogens. Together, our results indicate that bacterial community composition is the primary determinant of soil ARG content, challenging previous hypotheses that horizontal gene transfer effectively decouples resistomes from phylogeny 13 , 14 .

Complex gene-environment interactions are considered important in the development of obesity(1). The composition of the gut microbiota can determine the efficacy of energy harvest from food(2-4) and changes in dietary composition have been associated with changes in the composition of gut microbial populations(5,6). The capacity to explore microbiota composition was markedly improved by the development of metagenomic approaches(7,8), which have already allowed production of the first human gut microbial gene catalogue(9) and stratifying individuals by their gut genomic profile into different enterotypes(10), but the analyses were carried out mainly in nonintervention settings. To investigate the temporal relationships between food intake, gut microbiota and metabolic and inflammatory phenotypes, we conducted diet-induced weight-loss and weight-stabilization interventions in a study sample of 38 obese and 11 overweight individuals. Here we report that individuals with reduced microbial gene richness (40%) present more pronounced dys-metabolism and low-grade inflammation, as observed concomitantly in the accompanying paper(11). Dietary intervention improves low gene richness and clinical phenotypes, but seems to be less efficient for inflammation variables in individuals with lower gene richness. Low gene richness may therefore have predictive potential for the efficacy of intervention.

By exploring the phageome in mice, antibiotic treatment is shown to lead to enrichment of phage-encoded genes that are related to antibiotic resistance. Gut bacteria top-up their antibiotic resistance from a phage-gene reservoir Phages naturally coexist in abundance with their bacterial hosts in the mammalian gut. Antibiotic treatment can negatively affect the gut environment and cause immune and metabolic deficiencies. Previously the disruption of intestinal homeostasis has been studied mainly at the level of bacterial species, but here James Collins and colleagues use comparative metagenomics to profile gut phage populations following antibiotic treatment in mice. They find that exposure to ciprofloxacin or ampicillin enriches phage-encoded genes related to antibiotic resistance. Furthermore, phages from antibiotic-treated mice are able to increase resistance in an aerobically cultured naive microbiota. These results suggest that antibiotic treatment increases the frequency of phage integration and stimulates broad host range, which promotes a functional reservoir that is both genetically diverse and highly accessible to gut bacteria. The mammalian gut ecosystem has considerable influence on host physiology 1 , 2 , 3 , 4 , but the mechanisms that sustain this complex environment in the face of different stresses remain obscure. Perturbations to the gut ecosystem, such as through antibiotic treatment or diet, are at present interpreted at the level of bacterial phylogeny 5 , 6 , 7 . Less is known about the contributions of the abundant population of phages to this ecological network. Here we explore the phageome as a potential genetic reservoir for bacterial adaptation by sequencing murine faecal phage populations following antibiotic perturbation. We show that antibiotic treatment leads to the enrichment of phage-encoded genes that confer resistance via disparate mechanisms to the administered drug, as well as genes that confer resistance to antibiotics unrelated to the administered drug, and we demonstrate experimentally that phages from treated mice provide aerobically cultured naive microbiota with increased resistance. Systems-wide analyses uncovered post-treatment phage-encoded processes related to host colonization and growth adaptation, indicating that the phageome becomes broadly enriched for functionally beneficial genes under stress-related conditions. We also show that antibiotic treatment expands the interactions between phage and bacterial species, leading to a more highly connected phage–bacterial network for gene exchange. Our work implicates the phageome in the emergence of multidrug resistance, and indicates that the adaptive capacity of the phageome may represent a community-based mechanism for protecting the gut microflora, preserving its functional robustness during antibiotic stress.

Soil microbiota represent one of the ancient evolutionary origins of antibiotic resistance and have been proposed as a reservoir of resistance genes available for exchange with clinical pathogens. Using a high-throughput functional metagenomic approach in conjunction with a pipeline for the de novo assembly of short-read sequence data from functional selections (termed PARFuMS), we provide evidence for recent exchange of antibiotic resistance genes between environmental bacteria and clinical pathogens. We describe multidrug-resistant soil bacteria containing resistance cassettes against five classes of antibiotics (β-lactams, aminoglycosides, amphenicols, sulfonamides, and tetracyclines) that have perfect nucleotide identity to genes from diverse human pathogens. This identity encompasses noncoding regions as well as multiple mobilization sequences, offering not only evidence of lateral exchange but also a mechanism by which antibiotic resistance disseminates.

Key Points The anthropogenic use of antibiotics has selected for an increase in the evolution and dissemination of antibiotic resistance in environmental and human-associated bacteria. The first generation of antibiotic resistance research coincided with the golden age of antibiotics and focused on single resistance genes in single (usually pathogenic) organisms. In recent decades, technical and computational advances in genomics and metagenomics have revealed widespread resistance across diverse microbial communities. Recent exceptional studies integrate a deep mechanistic understanding of resistance determinants with broad genomic analysis of microorganisms and microbial communities to improve both the surveillance of resistance threats and the proactive development of strategies to counter these threats. Antibiotic resistance is a global problem that threatens individual and societal well-being. In this Review, Crofts, Gasparrini and Dantas summarize how research has changed from the discovery of resistant bacteria to community-level resistome studies, and they propose future therapeutic and surveillance approaches. Antibiotic resistance is a natural feature of diverse microbial ecosystems. Although recent studies of the antibiotic resistome have highlighted barriers to the horizontal transfer of antibiotic resistance genes between habitats, the rapid global spread of genes that confer resistance to carbapenem, colistin and quinolone antibiotics illustrates the dire clinical and societal consequences of such events. Over time, the study of antibiotic resistance has grown from focusing on single pathogenic organisms in axenic culture to studying antibiotic resistance in pathogenic, commensal and environmental bacteria at the level of microbial communities. As the study of antibiotic resistance advances, it is important to incorporate this comprehensive approach to better inform global antibiotic resistance surveillance and antibiotic development. It is increasingly becoming apparent that although not all resistance genes are likely to geographically and phylogenetically disseminate, the threat presented by those that are is serious and warrants an interdisciplinary research focus. In this Review, we highlight seminal work in the resistome field, discuss recent advances in the studies of resistomes, and propose a resistome paradigm that can pave the way for the improved proactive identification and mitigation of emerging antibiotic resistance threats.

The human microbiome encodes a large repertoire of biochemical enzymes and pathways, most of which remain uncharacterized. Here, using a metagenomics-based search strategy, we discovered that bacterial members of the human gut and oral microbiome encode enzymes that selectively phosphorylate a clinically used antidiabetic drug, acarbose 1 , 2 , resulting in its inactivation. Acarbose is an inhibitor of both human and bacterial α-glucosidases 3 , limiting the ability of the target organism to metabolize complex carbohydrates. Using biochemical assays, X-ray crystallography and metagenomic analyses, we show that microbiome-derived acarbose kinases are specific for acarbose, provide their harbouring organism with a protective advantage against the activity of acarbose, and are widespread in the microbiomes of western and non-western human populations. These results provide an example of widespread microbiome resistance to a non-antibiotic drug, and suggest that acarbose resistance has disseminated in the human microbiome as a defensive strategy against a potential endogenous producer of a closely related molecule. Bacteria in the human gut and oral microbiome encode enzymes that selectively phosphorylate the antidiabetic drug acarbose—an inhibitor of both human and bacterial α-glucosidases—resulting in its inactivation and limiting the drug's effects on the ability of the host to metabolize complex carbohydrates.

Perturbations in the gut microbiota can lead to a state of dysbiosis, which may involve 'blooming' of potentially harmful bacteria. Here, Hardt and colleagues propose that such bacteria blooms promote horizontal gene transfer between members of the gut ecosystem, thereby facilitating pathogen evolution. Hundreds of bacterial species make up the mammalian intestinal microbiota. Following perturbations by antibiotics, diet, immune deficiency or infection, this ecosystem can shift to a state of dysbiosis. This can involve overgrowth (blooming) of otherwise under-represented or potentially harmful bacteria (for example, pathobionts). Here, we present evidence suggesting that dysbiosis fuels horizontal gene transfer between members of this ecosystem, facilitating the transfer of virulence and antibiotic resistance genes and thereby promoting pathogen evolution.

The distribution of potential clinically relevant antibiotic resistance (AR) genes across soil, water, animal, plant and human microbiomes is not well understood. We aimed to investigate if there were differences in the distribution and relative abundances of resistance genes across a variety of ecological niches. All sequence reads (human, animal, water, soil, plant and insect metagenomes) from the MG-RAST database were downloaded and assembled into a local sequence database. We show that there are many reservoirs of the basic form of resistance genes e.g. bla TEM, but the human and mammalian gut microbiomes contain the widest diversity of clinically relevant resistance genes using metagenomic analysis. The human microbiomes contained a high relative abundance of resistance genes, while the relative abundances varied greatly in the marine and soil metagenomes, when datasets with greater than one million genes were compared. While these results reflect a bias in the distribution of AR genes across the metagenomes, we note this interpretation with caution. Metagenomics analysis includes limits in terms of detection and identification of AR genes in complex and diverse microbiome population. Therefore, if we do not detect the AR gene is it in fact not there or just below the limits of our techniques? Distribution and relative abundances of antibiotic resistance genes across the world from the ocean to the human gut and everywhere in between. Graphical Abstract Figure. Distribution and relative abundances of antibiotic resistance genes across the world from the ocean to the human gut and everywhere in between.

Antibiotic treatment disturbs the commensal microbiota and is often followed by infection with enteric pathogens such as Salmonella typhimurium and Clostridium difficile; pathogen expansion is fuelled by antibiotic-driven accumulation of commensal-liberated host mucosal carbohydrates. Gut microbes support pathogen proliferation Intestinal microbiota can provide protection against invading pathogens through competition for resources and production of specific antimicrobial products. But disruption of the microbiota with antibiotics can contribute to the emergence of several enteric pathogens. Justin Sonnenburg and colleagues show here that two antibiotic-associated pathogens, Salmonella enterica serovar Typhimurium and Clostridium difficile , catabolize microbiota-liberated host sugars to fuel their growth in the mouse gut. In particular, the ability to use sialic acid cleaved from host polysaccharides by Bacteroides thetaiotaomicron is important for pathogen expansion. These findings identify a role for the gut microbiota in facilitating enteric pathogen infection and provide new options for developing therapeutics. The human intestine, colonized by a dense community of resident microbes, is a frequent target of bacterial pathogens. Undisturbed, this intestinal microbiota provides protection from bacterial infections. Conversely, disruption of the microbiota with oral antibiotics often precedes the emergence of several enteric pathogens 1 , 2 , 3 , 4 . How pathogens capitalize upon the failure of microbiota-afforded protection is largely unknown. Here we show that two antibiotic-associated pathogens, Salmonella enterica serovar Typhimurium ( S. typhimurium ) and Clostridium difficile , use a common strategy of catabolizing microbiota-liberated mucosal carbohydrates during their expansion within the gut. S. typhimurium accesses fucose and sialic acid within the lumen of the gut in a microbiota-dependent manner, and genetic ablation of the respective catabolic pathways reduces its competitiveness in vivo . Similarly, C. difficile expansion is aided by microbiota-induced elevation of sialic acid levels in vivo . Colonization of gnotobiotic mice with a sialidase-deficient mutant of Bacteroides thetaiotaomicron , a model gut symbiont, reduces free sialic acid levels resulting in C. difficile downregulating its sialic acid catabolic pathway and exhibiting impaired expansion. These effects are reversed by exogenous dietary administration of free sialic acid. Furthermore, antibiotic treatment of conventional mice induces a spike in free sialic acid and mutants of both Salmonella and C. difficile that are unable to catabolize sialic acid exhibit impaired expansion. These data show that antibiotic-induced disruption of the resident microbiota and subsequent alteration in mucosal carbohydrate availability are exploited by these two distantly related enteric pathogens in a similar manner. This insight suggests new therapeutic approaches for preventing diseases caused by antibiotic-associated pathogens.