Lack of correlation between reaction speed and analytical sensitivity in isothermal amplification reveals the value of digital methods for optimization: validation using digital real-time RT-LAMP.

In this paper, we asked if it is possible to identify the best primers and reaction conditions based on improvements in reaction speed when optimizing isothermal reactions. We used digital single-molecule, real-time analyses of both speed and efficiency of isothermal amplification reactions, which revealed that improvements in the speed of isothermal amplification reactions did not always correlate with improvements in digital efficiency (the fraction of molecules that amplify) or with analytical sensitivity. However, we observed that the speeds of amplification for single-molecule (in a digital device) and multi-molecule (e.g. in a PCR well plate) formats always correlated for the same conditions. Also, digital efficiency correlated with the analytical sensitivity of the same reaction performed in a multi-molecule format. Our finding was supported experimentally with examples of primer design, the use or exclusion of loop primers in different combinations, and the use of different enzyme mixtures in one-step reverse-transcription loop-mediated amplification (RT-LAMP). Our results show that measuring the digital efficiency of amplification of single-template molecules allows quick, reliable comparisons of the analytical sensitivity of reactions under any two tested conditions, independent of the speeds of the isothermal amplification reactions.

Digital Quantification of DNA Replication and Chromosome Segregation Enables Determination of Antimicrobial Susceptibility after only 15†Minutes of Antibiotic Exposure.

Rapid antimicrobial susceptibility testing (AST) would decrease misuse and overuse of antibiotics. The "holy grail" of AST is a phenotype-based test that can be performed within a doctor visit. Such a test requires the ability to determine a pathogen's susceptibility after only a short antibiotic exposure. Herein, digital PCR (dPCR) was employed to test whether measuring DNA replication of the target pathogen through digital single-molecule counting would shorten the required time of antibiotic exposure. Partitioning bacterial chromosomal DNA into many small volumes during dPCR enabled AST results after short exposure times by 1) precise quantification and 2) a measurement of how antibiotics affect the states of macromolecular assembly of bacterial chromosomes. This digital AST (dAST) determined susceptibility of clinical isolates from urinary tract infections (UTIs) after 15 min of exposure for all four antibiotic classes relevant to UTIs. This work lays the foundation to develop a rapid, point-of-care AST and strengthen global antibiotic stewardship.

Measuring fate and rate of single-molecule competition of amplification and restriction digestion, and its use for rapid genotyping tested with hepatitis C viral RNA.

We experimentally monitored, at the single-molecule level, the competition among reverse transcription, exponential amplification (RT-LAMP), and linear degradation (restriction enzymes) starting with hepatitis C viral RNA molecules. We found significant heterogeneity in the rate of single-molecule amplification; introduction of the restriction enzymes affected both the rate and the "fate" (the binary outcome) of single-molecule amplification. While end-point digital measurements were primarily sensitive to changes in fate, the bulk real-time kinetic measurements were dominated by the rate of amplification of the earliest molecules, and were not sensitive to fate of the rest of the molecules. We show how this competition of reactions can be used for rapid HCV genotyping with either digital or bulk readout. This work advances our understanding of single-molecule dynamics in reaction networks and may help bring genotyping capabilities out of clinical labs and into limited-resource settings.

Deletion of densin-180 results in abnormal behaviors associated with mental illness and reduces mGluR5 and DISC1 in the postsynaptic density fraction.

Densin is an abundant scaffold protein in the postsynaptic density (PSD) that forms a high-affinity complex with αCaMKII and α-actinin. To assess the function of densin, we created a mouse line with a null mutation in the gene encoding it (LRRC7). Homozygous knock-out mice display a wide variety of abnormal behaviors that are often considered endophenotypes of schizophrenia and autism spectrum disorders. At the cellular level, loss of densin results in reduced levels of α-actinin in the brain and selective reduction in the localization of mGluR5 and DISC1 in the PSD fraction, whereas the amounts of ionotropic glutamate receptors and other prominent PSD proteins are unchanged. In addition, deletion of densin results in impairment of mGluR- and NMDA receptor-dependent forms of long-term depression, alters the early dynamics of regulation of CaMKII by NMDA-type glutamate receptors, and produces a change in spine morphology. These results indicate that densin influences the function of mGluRs and CaMKII at synapses and contributes to localization of mGluR5 and DISC1 in the PSD fraction. They are consistent with the hypothesis that mutations that disrupt the organization and/or dynamics of postsynaptic signaling complexes in excitatory synapses can cause behavioral endophenotypes of mental illness.

Local monitoring of SARS-CoV-2 variants in two large California counties in 2021.

Coronavirus Disease 2019 (COVID-19), caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), continues to persist due to mutations resulting in newer, more infectious variants of concern. We aimed to leverage an ongoing private SARS-CoV-2 testing laboratory's infrastructure to monitor SARS-CoV-2 variants in two large California counties. Study enrollment was offered to adults aged 18 years or older in Los Angeles County and Riverside County who recently tested positive for SARS-CoV-2 with a polymerase chain reaction (PCR) assay. A cycle threshold value less than or equal to 30 cycles was considered a positive test for sequencing purposes. Within 5 days of study enrollment, clinician-monitored, self-collected oral fluid and anterior nares swab specimens were obtained from participants. Specimens were transported and stored at 8 °C or cooler. Samples underwent ribonucleic acid extraction, library preparation, and sequencing. SARS-CoV-2 lineages were identified using sequencing data. Participant and genomic data were analyzed using statistical tools and visualized with toolkits. The study was approved by Advarra Institutional Review Board (Pro00053729). From May 27, 2021 to September 9, 2021, 503 individuals were enrolled and underwent specimen collection. Of the 503 participants, 238 (47.3%) participants were women, 329 (63.6%) participants were vaccinated, and 221 (43.9%) participants were of Hispanic or Spanish origin. Of the cohort, 496 (98.6%) participants had symptoms at the time of collection. Among the 503 samples, 443 (88.1%) nasal specimens and 353 (70.2%) oral specimens yielded positive sequencing results. Over our study period, the prevalence of the Alpha variant of SARS-CoV-2 decreased (initially 23.1% [95% confidence interval (95% CI): 0-0.49%] to 0% [95% CI 0.0-0.0%]) as the prevalence of the Delta variant of SARS-CoV-2 increased (initially 33.3% [95% CI 0.0-100.0%] to 100.0% [95% CI 100.0-100.0%]). A strain that carried mutations of both Delta and Mu was identified. We found that outpatient SARS-CoV-2 variant surveillance could be conducted in a timely and accurate manner. The prevalence of different variants changed over time. A higher proportion of nasal specimens yielded results versus oral specimens. Timely and regional outpatient SARS-CoV-2 variant surveillance could be used for public health efforts to identify changes in SARS-CoV-2 strain epidemiology.

Microfluidic SlipChip device for multistep multiplexed biochemistry on a nanoliter scale.

We have developed a multistep microfluidic device that expands the current SlipChip capabilities by enabling multiple steps of droplet merging and multiplexing. Harnessing the interfacial energy between carrier and sample phases, this manually operated device accurately meters nanoliter volumes of reagents and transfers them into on-device reaction wells. Judiciously shaped microfeatures and surface-energy traps merge droplets in a parallel fashion. Wells can be tuned for different volumetric capacities and reagent types, including for pre-spotted reagents that allow for unique identification of original well contents even after their contents are pooled. We demonstrate the functionality of the multistep SlipChip by performing RNA transcript barcoding on-device for synthetic spiked-in standards and for biologically derived samples. This technology is a good candidate for a wide range of biological applications that require multiplexing of multistep reactions in nanoliter volumes, including single-cell analyses.