Menu
June 1, 2021  |  

Low-input long-read sequencing for complete microbial genomes and metagenomic community analysis

Microbial genome sequencing can be done quickly, easily, and efficiently with the PacBio sequencing instruments, resulting in complete de novo assemblies. Alternative protocols have been developed to reduce the amount of purified DNA required for SMRT Sequencing, to broaden applicability to lower-abundance samples. If 50-100 ng of microbial DNA is available, a 10-20 kb SMRTbell library can be made. The resulting library can be loaded onto multiple SMRT Cells, yielding more than enough data for complete assembly of microbial genomes using the SMRT Portal assembly program HGAP, plus base modification analysis. The entire process can be done in less than 3 days by standard laboratory personnel. This approach is particularly important for analysis of metagenomic communities, in which genomic DNA is often limited. From these samples, full-length 16S amplicons can be generated, prepped with the standard SMRTbell library prep protocol, and sequenced. Alternatively, a 2 kb sheared library, made from a few ng of input DNA, can also be used to elucidate the microbial composition of a community, and may provide information about biochemical pathways present in the sample. In both these cases, 1-2 kb reads with >99.9% accuracy can be obtained from Circular Consensus Sequencing.


June 1, 2021  |  

Minimization of chimera formation and substitution errors in full-length 16S PCR amplification

The constituents and intra-communal interactions of microbial populations have garnered increasing interest in areas such as water remediation, agriculture and human health. One popular, efficient method of profiling communities is to amplify and sequence the evolutionarily conserved 16S rRNA sequence. Currently, most targeted amplification focuses on short, hypervariable regions of the 16S sequence. Distinguishing information not spanned by the targeted region is lost and species-level classification is often not possible. SMRT Sequencing easily spans the entire 1.5 kb 16S gene, and in combination with highly-accurate single-molecule sequences, can improve the identification of individual species in a metapopulation. However, when amplifying a mixture of sequences with close similarities, the products may contain chimeras, or recombinant molecules, at rates as high as 20-30%. These PCR artifacts make it difficult to identify novel species, and reduce the amount of productive sequences. We investigated multiple factors that have been hypothesized to contribute to chimera formation, such as template damage, denaturing time before and during cycling, polymerase extension time, and reaction volume. Of the factors tested, we found two major related contributors to chimera formation: the amount of input template into the PCR reaction and the number of PCR cycles. Sequence errors generated during amplification and sequencing can also confound the analysis of complex populations. Circular Consensus Sequencing (CCS) can generate single-molecule reads with >99% accuracy, and the SMRT Analysis software provides filtering of these reads to >99.99% accuracies. Remaining substitution errors in these highly-filtered reads are likely dominated by mis-incorporations during amplification. Therefore, we compared the impact of several commercially-available high-fidelity PCR kits with full-length 16S amplification. We show results of our experiments and describe an optimized protocol for full-length 16S amplification for SMRT Sequencing. These optimizations have broader implications for other applications that use PCR amplification to phase variations across targeted regions and to generate highly accurate reference sequences.


June 1, 2021  |  

Application specific barcoding strategies for SMRT Sequencing

Over the last few years, several advances were implemented in the PacBio RS II System to maximize throughput and efficiency while reducing the cost per sample. The number of useable bases per SMRT Cell now exceeds 1 Gb with the latest P6-C4 chemistry and 6-hour movies. For applications such as microbial sequencing, targeted sequencing, Iso-Seq (full-length isoform sequencing) and Nimblegen’s target enrichment method, current SMRT Cell yields could be an excess relative to project requirements. To this end, barcoding is a viable option for multiplexing samples. For microbial sequencing, multiplexing can be accomplished by tagging sheared genomic DNA during library construction with modified SMRTbell adapters. We studied the performance of 2- to 8-plex microbial sequencing. For full-length amplicon sequencing such as HLA typing, amplicons as large as 5 kb may be barcoded during amplification using barcoded locus-specific primers. Alternatively, amplicons may be barcoded during SMRTbell library construction using barcoded SMRTbell adapters. The preferred barcoding strategy depends on the user’s existing workflow and flexibility to changing and/or updating existing workflows. Using barcoded adapters, five Class I and II genes (3.3 – 5.8 kb) x 96 patients can be multiplexed and typed. For Iso-Seq full-length cDNA sequencing, barcodes are incorporated during 1st-strand synthesis and are enabled by tailing the oligo-dT primer with any PacBio published 16-bp barcode sequences. RNA samples from 6 maize tissues were multiplexed to generate barcoded cDNA libraries. The NimbleGen SeqCap Target Enrichment method, combined with PacBio’s long-read sequencing, provides comprehensive view of multi-kilobase contiguous regions, both exonic and intronic regions. To make this cost effective, we recommend barcoding samples for pooling prior to target enrichment and capture. Here, we present specific examples of strategies and best practices for multiplexing samples for different applications for SMRT Sequencing. Additionally, we describe recommendations for analyzing barcoded samples.


June 1, 2021  |  

Minimization of chimera formation and substitution errors in full-length 16S PCR amplification

The constituents and intra-communal interactions of microbial populations have garnered increasing interest in areas such as water remediation, agriculture and human health. Amplification and sequencing of the evolutionarily conserved 16S rRNA gene is an efficient method of profiling communities. Currently, most targeted amplification focuses on short, hypervariable regions of the 16S sequence. Distinguishing information not spanned by the targeted region is lost, and species-level classification is often not possible. PacBio SMRT Sequencing easily spans the entire 1.5 kb 16S gene in a single read, producing highly accurate single-molecule sequences that can improve the identification of individual species in a metapopulation.However, this process still relies upon PCR amplification from a mixture of similar sequences, which may result in chimeras, or recombinant molecules, at rates upwards of 20%. These PCR artifacts make it difficult to identify novel species, and reduce the amount of informative sequences. We investigated multiple factors that may contribute to chimera formation, such as template damage, denaturation time before and during thermocycling, polymerase extension time, and reaction volume. We found two related factors that contribute to chimera formation: the amount of input template into the PCR reaction, and the number of PCR cycles.A second problem that can confound analysis is sequence errors generated during amplification and sequencing. With the updated algorithm for circular consensus sequencing (CCS2), single-molecule reads can be filtered to 99.99% predicted accuracy. Substitution errors in these highly filtered reads may be dominated by mis-incorporations during amplification. Sequence differences in full-length 16S amplicons from several commercial high-fidelity PCR kits were compared.We show results of our experiments and describe our optimized protocol for full-length 16S amplification for SMRT Sequencing. These optimizations have broader implications for other applications that use PCR amplification to phase variations across targeted regions and generate highly accurate reference sequences.


June 1, 2021  |  

Workflow for processing high-throughput, Single Molecule, Real-Time Sequencing data for analyzing the microbiome of patients undergoing fecal microbiota transplantation

There are many sequencing-based approaches to understanding complex metagenomic communities spanning targeted amplification to whole-sample shotgun sequencing. While targeted approaches provide valuable data at low sequencing depth, they are limited by primer design and PCR. Whole-sample shotgun experiments generally use short-read sequencing, which results in data processing difficulties. For example, reads less than 500 bp in length will rarely cover a complete gene or region of interest, and will require assembly. This not only introduces the possibility of incorrectly combining sequence from different community members, it requires a high depth of coverage. As such, rare community members may not be represented in the resulting assembly. Circular-consensus, Single Molecule, Real-Time (SMRT) Sequencing reads in the 1-3 kb range, with >99% accuracy can be generated using the previous generation PacBio RS II or, in much higher throughput, using the new Sequel System. While throughput is lower compared to short-read sequencing methods, the reads are a true random sampling of the underlying community since SMRT Sequencing has been shown to have very low sequence-context bias. With single-molecule reads >1 kb at >99% consensus accuracy, it is reasonable to expect a high percentage of reads to include genes or gene fragments useful for analysis without the need for de novo assembly. Here we present the results of circular consensus sequencing for an individual’s microbiome, before and after undergoing fecal microbiota transplantation (FMT) in order to treat a chronic Clostridium difficile infection. We show that even with relatively low sequencing depth, the long-read, assembly-free, random sampling allows us to profile low abundance community members at the species level. We also show that using shotgun sampling with long reads allows a level of functional insight not possible with classic targeted 16S, or short read sequencing, due to entire genes being covered in single reads.


June 1, 2021  |  

Multiplexing strategies for microbial whole genome SMRT Sequencing

As the throughput of the PacBio Systems continues to increase, so has the desire to fully utilize SMRT Cell sequencing capacity to multiplex microbes for whole genome sequencing. Multiplexing is readily achieved by incorporating a unique barcode for each microbe into the SMRTbell adapters and using a streamlined library preparation process. Incorporating barcodes without PCR amplification prevents the loss of epigenetic information and the generation of chimeric sequences, while eliminating the need to generate separate SMRTbell libraries. We multiplexed the genomes of up to 8 unique strains of H. pylori. Each genome was sheared and processed through adapter ligation in a single, addition-only reaction. The barcoded samples were pooled in equimolar quantities and a single SMRTbell library was prepared. We demonstrate successful de novo microbial assembly from all multiplexes tested (2- through 8-plex) using data generated from a single SMRTbell library, run on a single SMRT Cell with the PacBio RS II, and analyzed with standard SMRT Analysis assembly methods. This strategy was successful using both small (1.6 Mb, H. pylori) and medium (5 Mb, E. coli) genomes. This protocol facilitates the sequencing of multiple microbial genomes in a single run, greatly increasing throughput and reducing costs per genome.


June 1, 2021  |  

WGS SMRT Sequencing of patient samples from a fecal microbiota transplant trial

Fecal samples were obtained from human subjects in the first blinded, placebo-controlled trial to evaluate the efficacy and safety of fecal microbiota transplant (FMT) for treatment of recurrent C. difficile infection. Samples included pre-and post-FMT transplant, post-placebo transplant, and the donor control; samples were taken at 2 and 8 week post-FMT. Sequencing was done on the PacBio Sequel System, with the goal of obtaining high quality sequences covering whole genes or gene clusters, which will be used to better understand the relationship between the composition and functional capabilities of intestinal microbiomes and patient health. Methods: Samples were randomly sheared to 2-3 kb fragments, a sufficient length to cover most genes, and SMRTbell libraries were prepared using standard protocols. Libraries were run on the Sequel System, which has a throughput of hundreds of thousands of reads per SMRT Cell, adequate yield to sample the complex microbiomes of post-transplant and donor samples.Results: Here we characterize samples, describe library prep methods and detail Sequel System operation, including run conditions. Descriptive statistics of data output (primary analysis) are presented, along with SMRT Analysis reports on circular consensus sequence (CCS) reads generated using an updated algorithm (CCS2). Final sequencing yields are filtered at various levels of predicted accuracy from 90% to 99.9%. Previous studies done using the PacBio RS II System demonstrated the ability to profile at the species level, and in some cases the strain level, and provided functional insight. Conclusions: These results demonstrate that the Sequel System is well-suited for characterization of complex microbial communities, with the ability for high-throughput generation of extremely accurate single-molecule sequences, each several kilobases in length. The entire process from shearing and library prep through sequencing and CCS analysis can be completed in less than 48 hours.


June 1, 2021  |  

Phased diploid genome assembly with single-molecule real-time sequencing

While genome assembly projects have been successful in many haploid and inbred species, the assembly of non-inbred or rearranged heterozygous genomes remains a major challenge. To address this challenge, we introduce the open-source FALCON and FALCON-Unzip algorithms (https://github.com/PacificBiosciences/FALCON/) to assemble long-read sequencing data into highly accurate, contiguous, and correctly phased diploid genomes. We generate new reference sequences for heterozygous samples including an F1 hybrid of Arabidopsis thaliana, the widely cultivated Vitis vinifera cv. Cabernet Sauvignon, and the coral fungus Clavicorona pyxidata, samples that have challenged short-read assembly approaches. The FALCON-based assemblies are substantially more contiguous and complete than alternate short- or long-read approaches. The phased diploid assembly enabled the study of haplotype structure and heterozygosities between homologous chromosomes, including the identification of widespread heterozygous structural variation within coding sequences.


June 1, 2021  |  

Profiling complex population genomes with highly accurate single molecule reads: cow rumen microbiomes

Determining compositions and functional capabilities of complex populations is often challenging, especially for sequencing technologies with short reads that do not uniquely identify organisms or genes. Long-read sequencing improves the resolution of these mixed communities, but adoption for this application has been limited due to concerns about throughput, cost and accuracy. The recently introduced PacBio Sequel System generates hundreds of thousands of long and highly accurate single-molecule reads per SMRT Cell. We investigated how the Sequel System might increase understanding of metagenomic communities. In the past, focus was largely on taxonomic classification with 16S rRNA sequencing. Recent expansion to WGS sequencing enables functional profiling as well, with the ultimate goal of complete genome assemblies. Here we compare the complex microbiomes in 5 cow rumen samples, for which Illumina WGS sequence data was also available. To maximize the PacBio single-molecule sequence accuracy, libraries of 2 to 3 kb were generated, allowing many polymerase passes per molecule. The resulting reads were filtered at predicted single-molecule accuracy levels up to 99.99%. Community compositions of the 5 samples were compared with Illumina WGS assemblies from the same set of samples, indicating rare organisms were often missed with Illumina. Assembly from PacBio CCS reads yielded a contig >100 kb in length with 6-fold coverage. Mapping of Illumina reads to the 101 kb contig verified the PacBio assembly and contig sequence. These results illustrate ways in which long accurate reads benefit analysis of complex communities.


June 1, 2021  |  

Profiling complex communities with highly accurate single molecule reads: cow rumen microbiomes

Determining compositions and functional capabilities of complex populations is often challenging, especially for sequencing technologies with short reads that do not uniquely identify organisms or genes. Long-read sequencing improves the resolution of these mixed communities, but adoption for this application has been limited due to concerns about throughput, cost and accuracy. The recently introduced PacBio Sequel System generates hundreds of thousands of long and highly accurate single-molecule reads per SMRT Cell. We investigated how the Sequel System might increase understanding of metagenomic communities. In the past, focus was largely on taxonomic classification with 16S rRNA sequencing. Recent expansion to WGS sequencing enables functional profiling as well, with the ultimate goal of complete genome assemblies. Here we compare the complex microbiomes in 5 cow rumen samples, for which Illumina WGS sequence data was also available. To maximize the PacBio single-molecule sequence accuracy, libraries of 2 to 3 kb were generated, allowing many polymerase passes per molecule. The resulting reads were filtered at predicted single-molecule accuracy levels up to 99.99%. Community compositions of the 5 samples were compared with Illumina WGS assemblies from the same set of samples, indicating rare organisms were often missed with Illumina. Assembly from PacBio CCS reads yielded a contig >100 kb in length with 6-fold coverage. Mapping of Illumina reads to the 101 kb contig verified the PacBio assembly and contig sequence. Scaffolding with reads from a PacBio unsheared library produced a complete genome of 2.4 Mb. These results illustrate ways in which long accurate reads benefit analysis of complex communities.


June 1, 2021  |  

Using the PacBio Sequel System to taxonomically and functionally classify metagenomic samples in a trial of patients undergoing fecal microbiota transplantation

Whole-sample shotgun sequencing can provide a more detailed view of a metagenomic community than 16S sequencing, but its use in multi-sample experiments is limited by throughput, cost and analysis complexity. While short-read sequencing technologies offer higher throughput, read lengthss less fewer than 500 bp will rarely cover a gene of interest, and necessitate assembly before further analysis. Assembling large fragments requires sampling each community member at a high depth, significantly increasing the amount of sequencing needed, and limiting the analysis of rare community members. Assembly methods also risk It is also possible to incorrectly combine combining sequences from different community members.


June 1, 2021  |  

Multiplexing strategies for microbial whole genome sequencing using the Sequel System

For microbial sequencing on the PacBio Sequel System, the current yield per SMRT Cell is in excess relative to project requirements. Multiplexing offers a viable solution; greatly increasing throughput, efficiency, and reducing costs per genome. This approach is achieved by incorporating a unique barcode for each microbial sample into the SMRTbell adapters and using a streamlined library preparation process. To demonstrate performance,12 unique barcodes assigned to B. subtilis and sequenced on a single SMRT Cell. To further demonstrate the applicability of this method, we multiplexed the genomes of 16 strains of H. pylori. Each DNA was sheared to 10 kb, end-repaired and ligated with a barcoded adapter in a single-tube reaction. The barcoded samples were pooled in equimolar quantities and a single SMRTbell library was prepared. Successful de novo microbial assemblies were achieved from all multiplexes tested (12-, and 16-plex) using data generated from a single SMRTbell library, run on a single SMRT Cell 1M with the PacBio Sequel System, and analyzed with standard SMRT Analysis assembly methods. Here, we describe a protocol that facilitated the multiplexing up to 12-plex of microbial genomes in one SMRT Cell 1M on the Sequel System that produced near-complete microbial de novo assemblies of <10 contigs for genomes <5 Mb in size.


June 1, 2021  |  

Multiplexed complete microbial genomes on the Sequel System

Microbes play an important role in nearly every part of our world, as they affect human health, our environment, agriculture, and aid in waste management. Complete closed genome sequences, which have become the gold standard with PacBio long-read sequencing, can be key to understanding microbial functional characteristics. However, input requirements, consumables costs, and the labor required to prepare and sequence a microbial genome have in the past put PacBio sequencing out of reach for some larger projects. We have developed a multiplexed library prep approach that is simple, fast, and cost-effective, and can produce 4 to 16 closed bacterial genomes from one Sequel SMRT Cell. Additionally, we are introducing a streamlined analysis pipeline for processing multiplexed genome sequence data through de novo HGAP assembly, making the entire process easy for lab personnel to perform. Here we present the entire workflow from shearing through assembly, with times for each step. We show HGAP assembly results with single or very few contigs from bacteria from different size genomes, sequenced without or with size selection. These data illustrate the benefits and potential of the PacBio multiplexed library prep and the Sequel System for sequencing large numbers of microbial genomes.


June 1, 2021  |  

Full-length cDNA sequencing of prokaryotic transcriptome and metatranscriptome samples

Next-generation sequencing has become a useful tool for studying transcriptomes. However, these methods typically rely on sequencing short fragments of cDNA, then attempting to assemble the pieces into full-length transcripts. Here, we describe a method that uses PacBio long reads to sequence full-length cDNAs from individual transcriptomes and metatranscriptome samples. We have adapted the PacBio Iso-Seq protocol for use with prokaryotic samples by incorporating RNA polyadenylation and rRNA-depletion steps. In conjunction with SMRT Sequencing, which has average readlengths of 10-15 kb, we are able to sequence entire transcripts, including polycistronic RNAs, in a single read. Here, we show full-length bacterial transcriptomes with the ability to visualize transcription of operons. In the area of metatranscriptomics, long reads reveal unambiguous gene sequences without the need for post-sequencing transcript assembly. We also show full-length bacterial transcripts sequenced after being treated with NEB’s Cappable-Seq, which is an alternative method for depleting rRNA and enriching for full-length transcripts with intact 5’ ends. Combining Cappable-Seq with PacBio long reads allows for the detection of transcription start sites, with the additional benefit of sequencing entire transcripts.


June 1, 2021  |  

Single chromosomal genome assemblies on the Sequel System with Circulomics high molecular weight DNA extraction for microbes

Background: The Nanobind technology from Circulomics provides an elegant HMW DNA extraction solution for genome sequencing of Gram-positive and -negative microbes. Nanobind is a nanostructured magnetic disk that can be used for rapid extraction of high molecular weight (HMW) DNA from diverse sample types including cultured cells, blood, plant nuclei, and bacteria. Processing can be completed in <1 hour for most sample types and can be performed manually or automated with common instruments. Methods:We have validated several critical steps for generating high-quality microbial genome assemblies in a streamlined microbial multiplexing workflow. This new workflow enables high-volume, cost-effective sequencing of up to 16 microbes totaling 30 Mb in genome size on a single SMRT Cell 1M using a target shear size of 10 kb. We also evaluated this method on a pool of four “class 3” microbes that contain >7 kb repeats. Fragment size was increased to ~14 kb, with some fragments >30 kb. Results: Here we present a demonstration of these capabilities using isolates relevant to high-throughput sequencing applications, including common foodborne pathogens (Shigella, Listeria, Salmonella), and species often seen in hospital settings (Klebsiella, Staphylococcus). For nearly all microbes, including difficult-to-assemble class III microbes, we achieved complete de novo microbial assemblies of =5 chromosomal contigs with minimum quality scores of 40 (99.99% accuracy) using data from multiplexed SMRTbell libraries. Each library was sequenced on a single SMRT Cell 1M with the PacBio Sequel System and analyzed with streamlined SMRT Analysis assembly methods. Conclusions: We achieved high-quality, closed microbial genomes using a combination of Circulomics Nanobind extraction and PacBio SMRT Sequencing, along with a newly streamlined workflow that includes automated demultiplexing and push-button assembly.


Talk with an expert

If you have a question, need to check the status of an order, or are interested in purchasing an instrument, we're here to help.