Type IV pili trigger episymbiotic association of Saccharibacteria with its bacterial host.

Recent characterization of the obligate episymbiont Saccharibacteria (TM7) belonging to the candidate phyla radiation (CPR) has expanded the extent of microbial diversity. However, the episymbiotic lifestyle of TM7 is still underexploited due to the deficiency of cultivated representatives. Here, we describe gene-targeted TM7 cultivation guided by repurposing epicPCR (emulsion, paired isolation, and concatenation PCR) to capture in situ TM7‒host associations. Using this method, we obtained a novel Saccharibacteria isolate TM7i and its host Leucobacter aridicollis J1 from Cicadae Periostracum, the castoff shell of cicada. Genomic analyses and microscopic characterizations revealed that TM7i could bind to J1 through twitching-like motility mediated by type IV pili (T4P). We further showed that the inhibition of T4P extrusion suppressed the motility and host adherence of TM7i, resulting in its reduced growth. However, the inactivation of T4P had little effect on the growth of TM7i that had already adhered to J1, suggesting the essential role of T4P in host recognition by TM7i. By capturing CPR‒host association and elaborating the T4P-dependent episymbiotic association mechanism, our studies shed light on the distinct yet widespread lifestyle of CPR bacteria.

Tunable and Contamination-Free Injection with Microfluidics by Stepinjection.

Droplet microfluidics with picoinjection provides significant advantages to multistep reactions and screenings. The T-junction design for picoinjection is convenient in adding picoliter reagents into passing droplets to initiate reactions. However, conventional picoinjectors face difficulties in eliminating cross-contamination between droplets, preventing them from widespread use in sensitive biological and molecular assays. Here, we introduce stepinjection, which uses a T-junction with a stepped channel design to elevate the diffusional buffer zone into the main channel and consequently increases the pressure difference between droplets and the inlet of the injection channel. To demonstrate the stepinjector's ability to perform contamination-sensitive enzymatic assays, we inject casein fluorescein isothiocyanate (FITC-casein) into a mixture of savinase and savinase-free (labeled with a red fluorescent dye) droplets. We observe no cross-contamination using stepinjection but find a severe cross-talk using an optimal picoinjection design. We envision that the simple, tunable, and reliable stepinjector can be easily integrated in various droplet processing devices, and facilitate various biomedical and biochemical applications including multiplex digital PCR, single-cell sequencing, and enzymatic screening.

One cell at a time: droplet-based microbial cultivation, screening and sequencing.

Microbes thrive and, in turn, influence the earth's environment, but most are poorly understood because of our limited capacity to reveal their natural diversity and function. Developing novel tools and effective strategies are critical to ease this dilemma and will help to understand their roles in ecology and human health. Recently, droplet microfluidics is emerging as a promising technology for microbial studies with value in microbial cultivating, screening, and sequencing. This review aims to provide an overview of droplet microfluidics techniques for microbial research. First, some critical points or steps in the microfluidic system are introduced, such as droplet stabilization, manipulation, and detection. We then highlight the recent progress of droplet-based methods for microbiological applications, from high-throughput single-cell cultivation, screening to the targeted or whole-genome sequencing of single cells.

High-throughput single-cell cultivation reveals the underexplored rare biosphere in deep-sea sediments along the Southwest Indian Ridge.

Microorganisms in the deep sea play vital roles in marine ecosystems. However, despite great advances brought by high throughput sequencing and metagenomics, only a small portion of microorganisms living in the environment can be cultivated in the laboratory and systematically studied. In this study, an improved high-throughput microfluidic streak plate (MSP) platform was developed to speed up the isolation of microorganisms from deep-sea sediments and evaluated with deep-sea sediments collected from the Southwest Indian Ridge (SWIR). Based on our previously reported MSP method, we improved its isolation efficiency with a semi-automated droplet picker and improved humidity control to enable long-term cultivation with a low-nutrient medium for up to five months according to the slow-growing nature of most deep-sea species. The improved MSP method allows the isolation of microbes by selection and investigation of microbial diversity by high throughput sequencing of the pooled sample cultures. By picking individual droplets and scale-up cultivation, a total of 772 strains that were taxonomically assigned to 70 species were isolated from the deep-sea sediments in the SWIR, including 15 potential novel species. On the other hand, based on 16S rRNA gene amplicon sequencing analysis, the microbial diversity of the SWIR was studied and documented with culture-dependent and independent methods in this study. The superiority of the MSP platform in revealing the rare biosphere was also evaluated based on amplicon sequencing. The results show that droplet-based single-cell cultivation of the MSP has a much higher ability than traditional agar plate cultivation in obtaining microbial species and more than 90% of operational taxonomic units (OTUs) detected in the MSP pool belong to the rare biosphere. Our results indicate the high robustness and efficiency of the improved MSP platform in revealing the environmentally rare biosphere, especially for slow-growing species. Overall, the MSP platform has a superior ability to recover microbial diversity than conventional agar plates and it was found to hold great potential for recovering rare microbial resources from various environments.

Virgibacillus indicus sp. nov. and Virgibacillus profundi sp. nov, two moderately halophilic bacteria isolated from marine sediment by using microfluidic streak plates.

Three Gram-variable, moderately halophilic, motile, endospore-forming rods, designated P2-C2T, P3-H5T and P3-B8, were isolated from marine sediment of the Southwest Indian Ocean by using the microfluidic streak plate method. Phylogeny based on 16S rRNA gene sequences showed that strains P2-C2T and P3-H5T formed a robust cluster within the genus Virgibacillus and exhibited 16S rRNA gene similarity levels of 95.3-96.8 and 94.9-96.3 % to the type strains of Virgibacillus species, respectively. The 16S rRNA gene similarity between P2-C2T and P3-H5T was 97.6 %. Strain P3-B8 has an identical 16S rRNA gene sequence to strain P3-H5T. For all the novel strains, the predominant cellular fatty acids were anteiso-C15 : 0 and anteiso-C17 : 0, the main menaquinone was MK-7, and the polar lipid profiles contained diphosphatidylglycerol and phosphatidylglycerol. The genomic DNA G+C contents of strains P2-C2T, P3-H5T and P3-B8 were 38.3, 37.3 and 37.5 mol%, respectively. Combined data from phenotypic and genotypic studies demonstrated that strains P2-C2T and P3-H5T/P3-B8 are representatives of two different novel species of the genus Virgibacillus, for which the name Virgibacillus indicus sp. nov. and Virgibacillusprofundi are proposed. The type strains are P2-C2T (=CGMCC 1.16138T=NBRC 113014T) and P3-H5T (=CGMCC 1.16139T=NBRC 113015T).