The biological importance and varied metabolic capabilities of specific microbial strains have long been established in the scientific community. Strains have, in the past, been largely defined and characterized based on microbial isolates. However, the emergence of new technologies and techniques has enabled assessments of their ecology and phenotypes within microbial communities and the human microbiome. While it is now more obvious how pathogenic strain variants are detrimental to human health, the consequences of subtle genetic variation in the microbiome have only recently been exposed. Here, we review the operational definitions of strains (e.g., genetic and structural variants) as they can now be identified from microbial communities using different high-throughput, often culture-independent techniques. We summarize the distribution and diversity of strains across the human body and their emerging links to health maintenance, disease risk and progression, and biochemical responses to perturbations, such as diet or drugs. We list methods for identifying, quantifying, and tracking strains, utilizing high-throughput sequencing along with other molecular and “culturomics” technologies. Finally, we discuss implications of population studies in bridging experimental gaps and leading to a better understanding of the health effects of strains in the human microbiome.

Rice (Oryza sativa. L) has been intensively studied to ensure a stable global supply of this commodity in the face of rapid global climate change. A critical factor that decreases crop yield is drought, which has been analyzed in various ways through many researches. Microbiome-based studies of rice investigate the symbiosis between rice and bacteria, which has been proposed as a way to overcome problems caused by drought. Several rice-associated metagenomic profiles obtained under drought conditions have been reported since the advent of next generation sequencing (NGS) technology. To elucidate the future diversity of plants and microorganisms and to promote sustainable agriculture, we reanalyzed 64 of the publicly available 16S amplicon sequencing data produced under drought condition. In the process of integrating data sets, however, we found an inconsistency that serves as a bottleneck for microbiome-based sustainability research. While this report provides clues about the composition of the microbiome under the drought conditions, the results are affected by differences in the location of the experiments, sampling conditions, and analysis protocols. Re-analysis of amplicon sequencing data of the soil microbiome in rice fields suggests that microbial composition shifts in response to drought condition and the presence of plants. Among the bacteria involved, the phylum Proteobacteria appears to play the most important role in the survival of rice under drought condition.

Worldwide 300 million children and adults are affected by asthma. The development of asthma is influenced by environmental and other exogenous factors synergizing with genetic predisposition, and shaping the lung microbiome especially during birth and in very early life. The healthy lung microbial composition is characterized by a prevalence of bacteria belonging to the phyla Bacteroidetes, Actinobacteria, and Firmicutes. However, viral respiratory infections are associated with an abundance of Proteobacteria with genera Haemophilus and Moraxella in young children and adult asthmatics. This dysbiosis supports the activation of inflammatory pathways and contributes to bronchoconstriction and bronchial hyperresponsiveness. Exogenous factors can affect the natural lung microbiota composition positively (farming environment) or negatively (allergens, air pollutants). It is evident that also gut microbiota dysbiosis has a high influence on asthma pathogenesis. Antibiotics, antiulcer medications, and other drugs severely impair gut as well as lung microbiota. Resulting dysbiosis and reduced microbial diversity dysregulate the bidirectional crosstalk across the gut-lung axis, resulting in hypersensitivity and hyperreactivity to respiratory and food allergens. Efforts are undertaken to reconstitute the microbiota and immune balance by probiotics and engineered bacteria, but results from human studies do not yet support their efficacy in asthma prevention or treatment. Overall, dysbiosis of gut and lung seem to be critical causes of the increased emergence of asthma.

The human microbiome harbors a diverse array of microbes which establishes a mutually beneficial relation with the host in healthy conditions, however, the dynamic homeostasis is influenced by both host and environmental factors. Smoking contributes to modifications of the oral, lung and gut microbiome, leading to various diseases, such as periodontitis, asthma, chronic obstructive pulmonary disease, Crohn’s disease, ulcerative colitis and cancers. However, the exact causal relationship between smoking and microbiome alteration remains to be further explored.

Allergies are characterized by a hypersensitive immune reaction to originally harmless antigens. In recent decades, the incidence of allergic diseases has markedly increased, especially in developed countries. The increase in the frequency of allergic diseases is thought to be primarily due to environmental changes related to a westernized lifestyle, which affects the commensal microbes in the human body. The human gut is the largest organ colonized by bacteria and contains more than 1000 bacterial species, called the “gut microbiota.” The recent development of sequencing technology has enabled researchers to genetically investigate and clarify the diversity of all species of commensal microbes. The collective genomes of commensal microbes are together called the “microbiome.” Although the detailed mechanisms remain unclear, it has been proposed that the microbiota/microbiome, especially that in the gut, impacts the systemic immunity and metabolism, thus affecting the development of various immunological diseases, including allergies. In this review, we summarize the recent findings regarding the importance of the microbiome/microbiota in the development of allergic diseases and also the results of interventional studies using probiotics or prebiotics to prevent allergies.

Background

Since the 1980s, numerous mutualistic Pseudomonas spp. strains have been used in studies on the biology of plant growth-promoting rhizobacteria (PGPR) and their interactions with host plants. In 1988, a strain from the Pseudomonas fluorescens group, WCS417, was isolated from lesions of wheat roots growing in a take-all disease-suppressive soil. In subsequent trials, WCS417 limited the build-up of take-all disease in field-grown wheat and significantly increased wheat yield. In 1991, WCS417 was featured in one of the first landmark studies on rhizobacteria-induced systemic resistance (ISR), in which it was shown to confer systemic immunity in carnation (Dianthus caryophyllus) against Fusarium wilt. The discovery that WCS417 conferred systemic immunity in the model plant species Arabidopsis thaliana in 1996 incited intensive research on the molecular mechanisms by which PGPR promote plant growth and induce broad-spectrum disease resistance in plants. Since then, the strain name appeared in over 750 studies on beneficial plant-microbe interactions.

Scope

In this review, we will highlight key discoveries in plant-microbe interactions research that have emerged from over 30 years of research featuring WCS417 as a model rhizobacterial strain. WCS417 was instrumental in improving our understanding of the microbial determinants that are involved in root colonization and the establishment of mutually beneficial interactions with the host plant. The model strain also provided novel insight into the molecular mechanisms of plant growth promotion and the onset and expression of rhizobacteria-ISR. More recently, WCS417 has been featured in studies on host immune evasion during root colonization, and chemical communication in the rhizosphere during root microbiome assembly.

Conclusions

Numerous studies on the modes of action of WCS417 have provided major conceptual advances in our understanding of how free-living mutualists colonize the rhizosphere, modulate plant immunity, and promote plant growth. The concepts may prove useful in our understanding of the molecular mechanisms involved in other binary plant-beneficial microbe interactions, and in more complex microbial community contexts, such as the root microbiome.

The two common species of house dust mites (HDMs), Dermatophagoides farinae and D. pteronyssinus, are major sources of allergens in human dwellings worldwide. Many allergens from HDMs have been described, but their extracts vary in immunogens. Mite strains may differ in their microbiomes, which affect mite allergen expression and contents of bacterial endotoxins. Some bacteria, such as the intracellular symbiont Cardinium, can affect both the sex ratio and biochemical pathways of mites, resulting in abundance variations of mite allergens/immunogens. Here, we investigated the bacterial microbiomes of D. farinae and D. pteronyssinus males and females using barcode 16S rDNA sequencing, qPCR, and genomic data analysis. We found a single species of Cardinium associated with D. farinae strains from the USA, China and Europe. Cardinium had high abundance relative to other bacterial taxa and represented 99% of all bacterial DNA reads from female mites from the USA. Cardinium was also abundant with respect to the number of host cells—we estimated 10.4–11.8 cells of Cardinium per single female mite cell. In a European D. farinae strain, Cardinium was more prevalent in females than in males (representing 92 and 67% of all bacterial taxa in females and males, respectively). In contrast, D. pteronyssinus lacked any Cardinium species, and the microbiomes of male and female mites were similar. We produced a Cardinium genome assembly (1.48 Mb; GenBank: PRJNA555788, GCA_007559345.1) associated with D. farinae. The ascertained ubiquity and abundance of Cardinium strongly suggest that this intracellular bacterium plays an important biological role in D. farinae.

The rumen microbiome plays a critical role in normal physiology and nutrition of ruminants. Alterations in the rumen microbiome have important physiological and pathological implications. The advent of next-generation sequencing technologies and rapid development of computational tools and reference databases provide powerful tools in rumen microbiome studies. Rumen metagenomics enables studies on the collective genetic structure and functional composition of the rumen microbial community in a culture-independent manner and can be simply divided into functional metagenomics and sequencing-based computational metagenomics. Recent progresses in mining the rumen microbial community for novel enzymes, such as fibrolytic enzymes, or other biomolecules for industry and biotechnology applications using functional screening are discussed. Rapid advances in computational metagenomic tools and methods are summarized. Metagenomics has provided novel insights into the structure and function of the rumen microbiome. Recent efforts suggest that the core rumen microbiome consists of 8 phyla and 15 families, which likely contribute to the basic function of the rumen. Systematic investigations of the rumen microbiome, including its viral (virome) and plasmid (plasmidome) fractions, have revealed previously unrecognized biodiversity in the rumen. Resistance and resilience of the rumen microbial community in response to perturbation is also discussed. Moreover, the need for mechanistic models and applications of general ecological theories and principles in rumen metagenomic studies is emphasized.

Nutrient availability is one of the major limiting factors affecting legume production in Africa. With the limited arable land resources, meeting the dietary requirement of the ever-increasing world population becomes a serious challenge. The most frequently deficient nutrient on crop fields is nitrogen (N). Inconvenient increase in prices of chemical nitrogen fertilizers together with the environmental problems associated with their excessive use calls for alternative low-cost and ecologically friendly soil-plant fertilization technologies. Soil microorganisms play significant roles in nutrient mineralization and supply to plant hence promoting plant growth. Soil microbes suppress soilborne plant diseases and destroy environmentally hazardous compounds in soil. Microbial inoculants are agricultural amendments that use microorganisms such as rhizobia and endophytes to promote legume growth. These microbes form symbiotic relationships with the target leguminous plant, and both parts benefit. The structure and function of the plant microbiome are major determinants of plant health and productivity. Microbial inoculants are the potential tools for sustainable agriculture.

Current agricultural practices demand for low-input technologies with an objective to scale down the synthetic fertilizers and pesticides usage in order to enhance the sustainability in food production and restore ecosystem functioning. Regardless of much understanding of the essential role played by the soil microbiome in agriculture, we still have a limited knowledge of the multifarious response of microbial heterogeneity. To explore this covert attribute of soil microbial diversity, there is a need to focus upon the infinite ways by virtue of which soil microbiome helps in sustainable agriculture. There is limited access to highly diverse and dynamic communities of microbiome in soil due to inability of culture techniques in laboratory. With the advent of next-generation sequencing (NGS) techniques and high-throughput analysis, researchers gained new opportunities to investigate undetermined composition of soil microorganisms. Among rapidly growing field of research, the role of metagenomics is crucial in studying uncultured microbes to comprehend the actual microbial diversity and pertinent cooperation, evolution, and functions in diverse environment. Soil microbiologists are putting efforts in analyzing the phylogenetic diversity of soil niches and subsequently attempting to describe the functions of these soil inhabitants at trophic levels for improvement of soil fertility and productivity for the future generation.