Recombinations in staphylococcal cassette chromosome mec elements compromise the molecular detection of methicillin resistance in Staphylococcus aureus

One-tube RPA-CRISPR Cas12a/Cas13a rapid detection of methicillin-resistant Staphylococcus aureus

At present, methicillin-resistant Staphylococcus aureus (MRSA) has caused a serious impact on a global scale. The infection and carrier rate of MRSA in the community is increasing year by year, but there is still no convenient detection system for on-site rapid detection. It is very important to select a rapid detection system to accurately and quickly detect patients infected with MRSA. We have developed a high-efficient single-tube detection platform based on RPA and CRISPR reaction system to detect the genes of mecA and clfA of MRSA. Using this detection platform, visual MRSA detection could be achieved in 30 min. It was observed that this detection platform was capable to successfully detect the target genomic as low as 5 copies μL-1, and the reaction was completed in one step without opening the lid. This detection platform could only detect MRSA, but not other common clinical pathogenic bacteria, such as Salmonella, Pseudomonas aeruginosa, Staphylococcus xylosus, Aeromonas hydrophila, Escherichia coli and Staphylococcus warneri, indicated its satisfactory selectivity for MRSA without interference from other bacteria. The results of clinical samples show that the platform has outstanding advantages in sensitivity, specificity and identification of methicillin resistance. The entire reaction can be completed in one step in the handheld instrument without opening the cover, avoiding aerosol pollution during the reaction. The detection platform combined with handheld instruments will have great application potential in point-of-care testing.

Recent advances in PCR-free nucleic acid detection for SARS-COV-2

As the outbreak of Coronavirus disease 2019 (COVID-19) caused by severe acute respiratory disease coronavirus 2 (SARS-COV-2), fast, accurate, and economic detection of viral infection has become crucial for stopping the spread. Polymerase chain reaction (PCR) of viral nucleic acids has been the gold standard method for SARS-COV-2 detection, which, however, generally requires sophisticated facilities and laboratory space, and is time consuming. This review presents recent advances in PCR-free nucleic acid detection methods for SARS-CoV-2, including emerging methods of isothermal amplification, nucleic acid enzymes, electrochemistry and CRISPR.

Rapid and direct detection of hepatitis E virus in raw pork livers by recombinase polymerase amplification assays

Hepatitis E virus (HEV) is a zoonotic pathogen that causes global hepatitis E. Outbreaks of hepatitis E are directly linked to the consumption of pork liver products. Herein reverse transcription recombinase polymerase amplification assays targeting the ORF2 gene were developed for the rapid detection of HEV by integrating the fluorescence detection platform (qRT-RPA) and the visible lateral flow biosensor by naked eyes (LFB RT-RPA). The qRT-RPA assay effectively detected HEV RNA with a limit of detection (LOD) of 154 copies/μl (95%CI: 126–333 copies/µl) in Genie III at 41°C for 20 min. Besides this, the LFB RT-RPA detected the HEV RNA with a LOD of 24 copies/μl (95%CI: 20–57 copies/µl) in an incubator block at 41°C for 20 min. The developed RT-RPA assays also showed good specificity for HEV, with no cross-reactions with any of the other important swine pathogens examined in this work. The performance of the developed RT-RPA assays was validated on 14 HEV RNA-positive and 66 HEV RNA-negative raw pork liver samples identified by a previously described qRT-PCR. Consequently, 11 and 12 samples were HEV RNA-positive as detected by the qRT-RPA and the LFB RT-RPA, respectively. Compared to qRT-PCR, the qRT-RPA and LFB RT- RPA assays revealed a coincidence rate of 96.3 and 97.5% as well as a Kappa value of 0.858 and 0.908, respectively. These results ascertain that the developed RT-RPA assays are effective diagnostic tools for the point-of-care detection of HEV in resource-limited settings.

Characterization of SCCmec Instability in Methicillin-Resistant Staphylococcus aureus Affecting Adjacent Chromosomal Regions, Including the Gene for Staphylococcal Protein A (spa)

Staphylococcal cassette chromosome mec (SCCmec) represents a sequence of clear clinical and diagnostic importance in staphylococci. At a minimum the chromosomal cassette contains the mecA gene encoding PBP2a but frequently also includes additional antibiotic resistance genes (e.g., ermA and aadC; macrolide and aminoglycoside resistance, respectively). Certain regions within SCCmec elements are hot spots for sequence instability due to cassette-specific recombinases and a variety of internal mobile elements. SCCmec changes may affect not only cassette stability but the integrity of adjacent chromosomal sequences (e.g., the staphylococcal protein A gene; spa). We investigated SCCmec stability in methicillin-resistant Staphylococcus aureus (MRSA) strains carrying one of four SCCmec types cultured in the absence of antimicrobial selection over a 3-month period. SCCmec rearrangements were first detected in cefoxitin-susceptible variants after 2 months of passage, and most commonly showed precise excision of the SCCmec element. Sequence analysis after 3 months revealed both precise SCCmec excision and a variety of SCCmec internal deletions, some including extensive adjacent chromosomal loss, including spa. No empty cassettes (i.e., loss of just mecA from SCCmec) were observed among the variants. SCCmec stability was influenced both by internal mobile elements (IS431) as well as the host cell environment. Genotypically similar clinical isolates with deletions in the spa gene were also included for purposes of comparison. The results indicate a role for host-cell influence and the IS431 element on SCCmec stability.

Detection of Methicillin-Resistant Staphylococcus aureus Infections Using Molecular Methods

The application of molecular detection methods for bacterial pathogens has dramatically improved the outcomes of septic patients, including those with methicillin-resistant Staphylococcus aureus (MRSA) infections. Molecular methods can be applied to a variety of clinical specimens including nasal swabs, growth in blood culture bottles, and wounds. While data show that the overall accuracy of molecular tests for MRSA is high, results can be confounded by the presence of multiple staphylococcal species in a specimen, insertions and deletions of DNA in and around the Staphylococcal Cassette Chromosome mec (SCCmec) element, and point mutations in mecA. Herein, we explore the complexities of molecular approaches to MRSA detection and the instances where phenotypic methods should be pursued to resolve discrepancies between genotypic and phenotypic results.

Rapid Detection and Differentiation of Legionella pneumophila and Non-Legionella pneumophila Species by Using Recombinase Polymerase Amplification Combined With EuNPs-Based Lateral Flow Immunochromatography

Legionella, a waterborne pathogen, is the main cause of Legionnaires’ disease. Therefore, timely and accurate detection and differentiation of Legionella pneumophila and non-Legionella pneumophila species is crucial. In this study, we develop an easy and rapid recombinase polymerase amplification assay combined with EuNPs-based lateral flow immunochromatography (EuNPs-LFIC-RPA) to specifically distinguish Legionella pneumophila and non-Legionella pneumophila. We designed primers based on the mip gene of Legionella pneumophila and the 5S rRNA gene of non-Legionella pneumophila. The recombinase polymerase amplification reaction could go to completion in 10 min at 37°C, and the amplification products could be detected within 5 min with EuNPs-LFIC strips. Using a florescent test strip reader, the quantitative results were achieved by reading the colored signal intensities on the strips. The sensitivity was 1.6 × 10¹ CFU/ml, and a linear standard linear curve plotted from the test strip reader had a correlation coefficient for the determination of Legionella pneumophila (R² = 0.9516). Completed concordance for the presence or absence of Legionella pneumophila by EuNPs-LFIC-RPA and qPCR was 97.32% (κ = 0.79, 95% CI), according to an analysis of practical water samples (n = 112). In short, this work shows the feasibility of EuNPs-LFIC-RPA for efficient and rapid monitoring of Legionella pneumophila and non-Legionella pneumophila in water samples.

Sensitive and semiquantitative detection of soil-transmitted helminth infection in stool using a recombinase polymerase amplification-based assay

Background

Soil-transmitted helminths (STHs) are parasitic nematodes that inhabit the human intestine. They affect more than 1.5 billion people worldwide, causing physical and cognitive impairment in children. The global strategy to control STH infection includes periodic mass drug administration (MDA) based on the results of diagnostic testing among populations at risk, but the current microscopy method for detecting infection has diminished sensitivity as the intensity of infection decreases. Thus, improved diagnostic tools are needed to support decision-making for STH control programs.

Methodology

We developed a nucleic acid amplification test based on recombinase polymerase amplification (RPA) technology to detect STH in stool. We designed primers and probes for each of the four STH species, optimized the assay, and then verified its performance using clinical stool samples.

Principal findings

Each RPA assay was as sensitive as a real-time polymerase chain reaction (PCR) assay in detecting copies of cloned target DNA sequences. The RPA assay amplified the target in DNA extracted from human stool samples that were positive for STH based on the Kato-Katz method, with no cross-reactivity of the non-target genomic DNA. When tested with clinical stool samples from patients with infections of light, moderate, and heavy intensity, the RPA assays demonstrated performance comparable to that of real-time PCR, with better results than Kato-Katz. This new rapid, sensitive and field-deployable method for detecting STH infections can help STH control programs achieve their goals.

Conclusions

Semi-quantitation of target by RPA assay is possible and is comparable to real-time PCR. With proper instrumentation, RPA assays can provide robust, semi-quantification of STH DNA targets as an alternative field-deployable indicator to counts of helminth eggs for assessing infection intensity.

More than 1.5 billion people are infected with parasitic intestinal worms called soil-transmitted helminths. Infection is transmitted by helminth eggs in human feces, which contaminate soil in areas with poor sanitation. Adverse health effects include physical and cognitive impairment in children. A key strategy to control infection is periodic mass drug administration for populations with a high prevalence of disease based on the results of diagnostic testing. The current microscopy method for detecting infection, however, has limited ability to detect disease as the intensity of infection decreases with repeated mass drug administration. To address limitations of current diagnostic methods, we developed a novel technique to diagnose infections, including those at very low levels of intensity, by detecting helminth DNA in stool samples. Our initial studies suggest that the new diagnostic technique reliably detects the presence of intestinal worms, even at low intensities of infection, and may be more useful than currently available diagnostic tools for guiding the use of periodic mass drug administration to eliminate disease in low-resource settings.

Investigation of Plasmids Among Clinical Staphylococcus aureus and Staphylococcus haemolyticus Isolates From Egypt

Staphylococci can cause a wide array of infections that can be life threatening. These infections become more deadly when the isolates are antibiotic resistant and thus harder to treat. Many resistance determinants are plasmid-mediated; however, staphylococcal plasmids have not yet been fully characterized. In particular, plasmids and their contributions to antibiotic resistance have not been investigated within the Arab states, where antibiotic use is not universally regulated. Here, we characterized the putative plasmid content among 56 Staphylococcus aureus and 10 Staphylococcus haemolyticus clinical isolates from Alexandria, Egypt. Putative plasmid sequences were detected in over half of our collection. In total, we identified 72 putative plasmid sequences in 27 S. aureus and 1 S. haemolyticus isolates. While these isolates typically carried one or two plasmids, we identified one isolate-S. aureus AA53-with 11 putative plasmids. The plasmid sequences most frequently encoded a Rep_1, RepL, or PriCT_1 type replication protein. As expected, antibiotic resistance genes were widespread among the identified plasmid sequences. Related plasmids were identified amongst our clinical isolates; homologous plasmids present in multiple isolates clustered into 11 groups based upon sequence similarity. Plasmids from the same cluster often shared antibiotic resistance genes, including blaZ, which is associated with β-lactam resistance. Our analyses suggest that plasmids are a key factor in the pathology and epidemiology of S. aureus in Egypt. A better characterization of plasmids and the role they contribute to the success of Staphylococci as pathogens will guide the design of effective control strategies to limit their spread.

Characterisation of S. aureus/MRSA CC1153 and review of mobile genetic elements carrying the fusidic acid resistance gene fusC

While many data on molecular epidemiology of MRSA are available for North America, Western Europe and Australia, much less is known on the distribution of MRSA clones elsewhere. Here, we describe a poorly known lineage from the Middle East, CC1153, to which several strains from humans and livestock belong. Isolates were characterised using DNA microarrays and one isolate from the United Arab Emirates was sequenced using Nanopore technology. CC1153 carries agr II and capsule type 5 genes. Enterotoxin genes are rarely present, but PVL is common. Associated spa types include t504, t903 and t13507. PVL-positive CC1153-MSSA were found in Egyptian cattle suffering from mastitis. It was also identified among humans with skin and soft tissue infections in Saudi Arabia, France and Germany. CC1153-MRSA were mainly observed in Arabian Gulf countries. Some isolates presented with a previously unknown SCCmec/SCCfus chimeric element in which a mec B complex was found together with the fusidic acid resistance gene fusC and accompanying genes including ccrA/B-1 recombinase genes. Other isolates carried SCCmec V elements that usually also included fusC. Distribution and emergence of CC1153-MRSA show the necessity of molecular characterization of MRSA that are resistant to fusidic acid. These strains pose a public health threat as they combine resistance to beta-lactams used in hospitals as well as to fusidic acid used in the community. Because of the high prevalence of fusC-positive MRSA in the Middle East, sequences and descriptions of SCC elements harbouring fusC and/or mecA are reviewed. When comparing fusC and its surrounding regions from the CC1153 strain to available published sequences, it became obvious that there are four fusC alleles and five distinct types of fusC gene complexes reminiscent to the mec complexes in SCCmec elements. Likewise, they are associated with different sets of ccrA/B recombinase genes and additional payload that might include entire mec complexes or SCCmec elements.

Development and Evaluation of Real-Time Reverse Transcription Recombinase Polymerase Amplification Assay for Rapid and Sensitive Detection of West Nile Virus in Human Clinical Samples

West Nile virus (WNV) causes West Nile fever and encephalitis worldwide. Currently, there are no effective drugs or vaccines available in the market to treat WNV infection in humans. Hence, it is of paramount importance to detect WNV early for the success of the disease control programs and timely clinical management in endemic areas. In the present paper, we report the development of real-time reverse transcription recombinase polymerase amplification (RT-RPA) assay for rapid and real-time detection of WNV targeting the envelope (env) gene of the virus. The RPA reaction was performed successfully at 39°C for 15 min in a real-time thermal cycler. The sensitivity of this assay was found similar to that of the quantitative real-time RT PCR (RT-qPCR) assay, which could detect 10 copies of the gene. The efficacy of the assay was evaluated with a panel of 110 WN suspected human samples showing the signs of retinitis, febrile illness and acute posterior uveitis. In comparison with RT-qPCR, RT-RPA showed a specificity of 100% (CI, 95.07–100%) and sensitivity of 96.15% (CI, 80.36–99.90%) with a negative (NPV) and positive predictive value (PPV) of 98.65 and 100%, respectively. The level of agreement between RT-RPA and reference RT-qPCR assay was shown to be very high. The turnaround time of real-time RPA assay is about 10-20 times faster than the RT-qPCR, which confirms its utility in the rapid and sensitive diagnosis of WNV infection. To the best of our knowledge, this is the first report which deals with the development of real-time RT-RPA assay for simple, rapid, sensitive, and specific detection of WNV in human clinical samples. The present RT-RPA assay proves to be a powerful tool that can be used for the rapid diagnosis of a large number of patient samples in endemic settings.

An epidemic CC1-MRSA-IV clone yields false-negative test results in molecular MRSA identification assays: a note of caution, Austria, Germany, Ireland, 2020

We investigated why a clinical meticillin-resistant Staphylococcus aureus (MRSA) isolate yielded false-negative results with some commercial PCR tests for MRSA detection. We found that an epidemic European CC1-MRSA-IV clone generally exhibits this behaviour. The failure of the assays was attributable to a large insertion in the orfX/SCCmec integration site. To ensure the reliability of molecular MRSA tests, it is vital to monitor emergence of new SCCmec types and junction sites.

Mobile genetic elements responsible for discordant Staphylococcus aureus phenotypes and genotypes in the same blood culture bottle

Approximately 15-20% of the S. aureus genome contains mobile genetic elements that can cause discrepancies between phenotypic and genotypic identification methods. Three blood culture bottles (each from a different patient) that showed discordant results, were shown to contain 2 S. aureus isolates after additional subcultures. One bottle had MRSA and MSSA that by DNA sequence analysis differed only by 31 kb; however, the deletions encompassed parts of SCCmec including mecA and SCC_M1. The second bottle contained MRSA and MSSA that differed by 124 kb; the MSSA was missing the entire SCCmec and spa regions. The last bottle contained 2 MRSA, one with ACME II disrupting SCCmec and a 24 bp spa deletion. The deletions in SCCmec and the other elements gave rise to the discrepancies between molecular and the original culture results. Such discrepancies should prompt a search for additional strains in the blood culture bottle.

On point identification of species origin of food animals by recombinase polymerase amplification-lateral flow (RPA-LF) assay targeting mitochondrial gene sequences

The present study was aimed to develop and standardize Recombinase polymerase amplification-lateral flow (RPA-LF) assays for on point identification of species origin of food animals viz: cattle, buffalo and pig. Species specific RPA primers sets for cattle, buffalo and pig were designed by homology comparisons of the sequences of mitochondrial cytochrome b gene and d-loop region from common food species viz: cattle, buffalo, sheep, goat, pig and chicken. The RPA assays for designed primers sets were optimized using the reaction components from Twist Amp basic kit and instructions in its manual. Endpoint detection of species specific amplified RPA products were made by gel electrophoresis and designed species specific RPA-LFA strips. The developed assays were evaluated for their specificity, diagnostic sensitivity, and validated on coded samples and binary meat admixtures with relative percentage of 20, 10, 5 & 1% target species. The developed RPA assays resulted in amplification of DNA template exclusively of cattle, buffalo and pig origin to product sizes of 294, 405 and 283 bp respectively. The diagnostic sensitivities of developed assays were up to 10 pg of genomic DNA and highly correlated with species specific PCR assays taken as gold standard. Developed species specific RPA assays also identified the target species in coded samples and binary meat admixture up to 1%.

Rapid and visual detection of milk vetch dwarf virus using recombinase polymerase amplification combined with lateral flow strips

Background

Milk vetch dwarf virus (MDV) is an important ssDNA virus which causes yellowing, stunting and leaf rolling symptoms on legumes. In China, the virus causes great economic losses and has recently been found to infect tobacco. The expansion of its host range and its ability to spread rapidly has given rise to the urgent need for a sensitive, specific and rapid diagnostic assay that can assist in effective disease control.

Methods

Assays based on the polymerase chain reaction combined with lateral flow strip detection (PCR-LFS) and recombinase polymerase amplification combined with LFS (RPA-LFS) were developed targeting the coat protein (CP) gene of MDV.

Results

The PCR and RPA assays could detect respectively 10³ copies or 10¹ copies of MDV by agarose gel electrophoresis. The PCR-LFS and RPA-LFS assays developed could both detect as few as 10¹ copies per reaction at 37 °C. Both methods could detect MDV in crude leaf extracts.

Conclusions

The RPA-LFS assay developed is a rapid, sensitive and specific method for detecting MDV, which is convenient and has great potential for use in the field.

Updating Molecular Diagnostics for Detecting Methicillin-Susceptible and Methicillin-Resistant Staphylococcus aureus Isolates in Blood Culture Bottles

Molecular diagnostic tests can be used to provide rapid identification of staphylococcal species in blood culture bottles to help improve antimicrobial stewardship. However, alterations in the target nucleic acid sequences of the microorganisms or their antimicrobial resistance genes can lead to false-negative results.

Molecular diagnostic tests can be used to provide rapid identification of staphylococcal species in blood culture bottles to help improve antimicrobial stewardship. However, alterations in the target nucleic acid sequences of the microorganisms or their antimicrobial resistance genes can lead to false-negative results. We determined the whole-genome sequences of 4 blood culture isolates of Staphylococcus aureus and 2 control organisms to understand the genetic basis of genotype-phenotype discrepancies when using the Xpert MRSA/SA BC test (in vitro diagnostic medical device [IVD]). Three methicillin-resistant S. aureus (MRSA) isolates each had a different insertion of a genetic element in the staphylococcal cassette chromosome (SCCmec)-orfX junction region that led to a misclassification as methicillin-susceptible S. aureus (MSSA). One strain contained a deletion in spa, which produced a false S. aureus-negative result. A control strain of S. aureus that harbored an SCCmec element but no mecA (an empty cassette) was correctly called MSSA by the Xpert test. The second control contained an SCC_M1 insertion. The updated Xpert MRSA/SA BC test successfully detected both spa and SCCmec variants of MRSA and correctly identified empty-cassette strains of S. aureus as MSSA. Among a sample of 252 MSSA isolates from the United States and Europe, 3.9% contained empty SCCmec cassettes, 1.6% carried SCC_M1, <1% had spa deletions, and <1% contained SCCmec variants other than those with SCC_M1. These data suggest that genetic variations that may interfere with Xpert MRSA/SA BC test results remain rare. Results for all the isolates were correct when tested with the updated assay.

Rapid authentication of pharmaceuticals via DNA tagging and field detection

A small PCR-generated DNA fragment was introduced into a pharmaceutical grade ink as a molecular taggant, and the DNA tagged ink was delivered onto the surface of capsules by standard high-speed offset printing. The amount of DNA in the ink on each capsule is roughly 10⁻¹² fold lower than that allowed as safe by the United States Food and Drug Administration (FDA) and the WHO with regards to acceptable limits of residual DNA. The printed ink on the capsule surface was sampled by swabbing, followed by direct analysis of the DNA-swab complex, without subsequent DNA purification. It was shown that DNA recovered from the ink by swabbing was suitable for PCR-CE analysis—a widely used method in forensic science and was also suitable for qPCR and isothermal DNA amplification, when coupled with portable devices similar to those used for environmental sampling and food safety testing. The data set a precedent: A small DNA fragment could be introduced as an excipient into a pharmaceutical application, and thereafter tracked through the pharmaceutical supply chain via forensic DNA authentication.

Resistance diagnostics as a public health tool to combat antibiotic resistance: A model-based evaluation

Rapid point-of-care resistance diagnostics (POC-RD) are a key tool in the fight against antibiotic resistance. By tailoring drug choice to infection genotype, doctors can improve treatment efficacy while limiting costs of inappropriate antibiotic prescription. Here, we combine epidemiological theory and data to assess the potential of resistance diagnostics (RD) innovations in a public health context, as a means to limit or even reverse selection for antibiotic resistance. POC-RD can be used to impose a nonbiological fitness cost on resistant strains by enabling diagnostic-informed treatment and targeted interventions that reduce resistant strains' opportunities for transmission. We assess this diagnostic-imposed fitness cost in the context of a spectrum of bacterial population biologies and find that POC-RD have a greater potential against obligate pathogens than opportunistic pathogens already subject to selection under "bystander" antibiotic exposure during asymptomatic carriage (e.g., the pneumococcus). We close by generalizing the notion of RD-informed strategies to incorporate carriage surveillance information and illustrate that coupling transmission-control interventions to the discovery of resistant strains in carriage can potentially select against resistance in a broad range of opportunistic pathogens.

Community-acquired methicillin-resistant Staphylococcus aureus from ST1 lineage harboring a new SCCmec IV subtype (SCCmec IVm) containing the tetK gene

A pivotal event in the evolutionary path of methicillin-resistant Staphylococcus aureus (MRSA) is the acquisition of the staphylococcal cassette chromosome mec (SCCmec) element carrying the mecA gene, the determinant of methicillin resistance. Community-acquired (CA) MRSA is commonly associated with skin/soft tissue infections, and doxycycline is one of the drug choices for this purpose. Doxycycline resistance is associated with the acquisition of the tetK gene carried by the S. aureus plasmid pT181, which may also be integrated into SCCmec III and V. The aim of this study was to describe a novel SCCmec IV subtype (IVm) carrying tetK and reveal the genetic context of this element. The SCCmec sequence was obtained by whole-genome sequencing of the MRSA strain 2288 (ST1 CA-MRSA) and genomic analysis performed using different bioinformatics tools. A copy of pT181 was found to be integrated in the new SCCmec IVm of the strain 2288. The SCCmec IVm has high nucleotide identity (99%) with SCCmec IVa of the strain MW2, except for the J3 region, where the pT181 – carrying tetK gene – is inserted. Inverted repeats (IRs) flanking pT181 were found in this region, suggesting the occurrence of recombination events. The strain 2288 (spa type t125) shares most of the virulence attributes with MW2 (spa type t128), which is recognized in the past as a cause of severe infections in children in USA. The pattern of branching in the phylogenetic tree depicts a recent common ancestor shared by the 2228 strain and other MRSA from USA, including ERS410852, TCH70, CIG1835, CO-41, MW2, and USA400-0051, but none of them carried pT181. This study also showed that the tetK carried by SCCmec IVm is functional, determining resistance to doxycycline and tetracycline. The potential dissemination of the tetK and mecA genes in the same genetic event by the acquisition of this new SCCmec subtype is of concern for community infections.

The Enterococcus Cassette Chromosome, a Genomic Variation Enabler in Enterococci

Enterococcus faecium has a highly variable genome prone to recombination and horizontal gene transfer. Here, we have identified a novel genetic island with an insertion locus and mobilization genes similar to those of staphylococcus cassette chromosome elements SCCmec This novel element termed the enterococcus cassette chromosome (ECC) element was located in the 3' region of rlmH and encoded large serine recombinases ccrAB similar to SCCmec Horizontal transfer of an ECC element termed ECC::cat containing a knock-in cat chloramphenicol resistance determinant occurred in the presence of a conjugative rep _pLG1 plasmid. We determined the ECC::cat insertion site in the 3' region of rlmH in the E. faecium recipient by long-read sequencing. ECC::cat also mobilized by homologous recombination through sequence identity between flanking insertion sequence (IS) elements in ECC::cat and the conjugative plasmid. The ccrAB_Ent genes were found in 69 of 516 E. faecium genomes in GenBank. Full-length ECC elements were retrieved from 32 of these genomes. ECCs were flanked by attR and attL sites of approximately 50 bp. The attECC sequences were found by PCR and sequencing of circularized ECCs in three strains. The genes in ECCs contained an amalgam of common and rare E. faecium genes. Taken together, our data imply that ECC elements act as hot spots for genetic exchange and contribute to the large variation of accessory genes found in E. faecium IMPORTANCE Enterococcus faecium is a bacterium found in a great variety of environments, ranging from the clinic as a nosocomial pathogen to natural habitats such as mammalian intestines, water, and soil. They are known to exchange genetic material through horizontal gene transfer and recombination, leading to great variability of accessory genes and aiding environmental adaptation. Identifying mobile genetic elements causing sequence variation is important to understand how genetic content variation occurs. Here, a novel genetic island, the enterococcus cassette chromosome, is shown to contain a wealth of genes, which may aid E. faecium in adapting to new environments. The transmission mechanism involves the only two conserved genes within ECC, ccrAB _Ent, large serine recombinases that insert ECC into the host genome similarly to SCC elements found in staphylococci.

Laboratory-Based and Point-of-Care Testing for MSSA/MRSA Detection in the Age of Whole Genome Sequencing

Staphylococcus aureus is an opportunistic pathogen of animals and humans that is capable of both colonizing and infecting its eukaryotic host. It is frequently detected in the clinical microbiology routine laboratory. S. aureus is capable of acquiring antibiotic resistance traits with ease and, given its rapid global dissemination, resistance to meticillin in S. aureus has received extensive coverage in the popular and medical press. The detection of meticillin-resistant versus meticillin-susceptible S. aureus (MRSA and MSSA) is of significant clinical importance. Detection of meticillin resistance is relatively straightforward since it is defined by a single determinant, penicillin-binding protein 2a', which exists in a limited number of genetic variants carried on various Staphylococcal Cassette Chromosomes mec. Diagnosis of MRSA and MSSA has evolved significantly over the past decades and there has been a strong shift from culture-based, phenotypic methods toward molecular detection, especially given the close correlation between the presence of the mec genes and phenotypic resistance. This brief review summarizes the current state of affairs concerning the mostly polymerase chain reaction-mediated detection of MRSA and MSSA in either the classical laboratory setting or at the point of care. The potential diagnostic impact of the currently emerging whole genome sequencing (WGS) technology will be discussed against a background of diagnostic, surveillance, and infection control parameters. Adequate detection of MSSA and MRSA is at the basis of any subsequent, more generic antibiotic susceptibility testing, epidemiological characterization, and detection of virulence factors, whether performed with classical technology or WGS analyses.

Molecular diagnosis of protozoan parasites by Recombinase Polymerase Amplification

Infections caused by protozoan parasites affect millions of people around the world. Traditionally, diagnosis was made by microscopy, which is insensitive and in some cases not specific. Molecular methods are highly sensitive and specific, but equipment costs and personnel training limit its availability only to specialized centers, usually far from populations with the highest risk of infection. Inexpensive methods that can be applied at the point of care (POC), especially in places with limited health infrastructure, would be a major advantage. Isothermal amplification of nucleic acids does not require thermocyclers and is relatively inexpensive and easy to implement. Among isothermal methods, recombinase polymerase amplification (RPA) is sensitive and potentially applicable at POC. We and others have developed RPA diagnostic tests to detect protozoan parasites of medical importance. Overall, our results have shown high specificity with limits of detection similar to PCR. Currently, the optimization of RPA for use at the POC is under development, and in the near future the tests should become available to detect protozoan infections in the field. In this review we discuss the current status, challenges, and future of RPA in the field of molecular diagnosis of protozoan parasites.

Variability of SCCmec elements in livestock-associated CC398 MRSA

The most common livestock-associated lineage of methicillin-resistant Staphylococcus aureus (MRSA) in Western Europe is currently clonal complex (CC) 398. CC398-MRSA spread extensively across livestock populations in several Western European countries, and livestock-derived CC398-MRSA strains can also be detected in humans. Based on their SCCmec elements, different CC398 strains can be distinguished. SCCmec elements of 100 veterinary and human CC398-MRSA isolates from Germany and Austria were examined using DNA microarray-based assays. In addition, 589 published SCC and/or genome sequences of CC398-MRSA (including both, fully finished and partially assembled sequences) were analysed by mapping them to the probe sequences of the microarrays. Several isolates and sequences showed an insertion of a large fragment of CC9 genomic DNA into the CC398 chromosome. Fifteen subtypes of SCCmec elements were detected among the 100 CC398 isolates and 41 subtypes could be discerned among the published CC398 sequences. Eleven of these were also experimentally detected within our strain collection, while four subtypes identified in the isolates where not found among the sequences. A high prevalence of heavy metal resistance genes, especially of czrC, was observed among CC398-MRSA. A possible co-selection of resistances to antibiotics and zinc/copper supplements in animal feed as well as a spill-over of SCCmec elements that have evolved in CC398-MRSA to other, possibly more virulent and/or medically relevant S. aureus lineages might pose public health problems in future.

Introduction of a hydrolysis probe PCR assay for high-throughput screening of methicillin-resistant Staphylococcus aureus with the ability to include or exclude detection of Staphylococcus argenteus

Many countries using sensitive screening methods for detection of carriage of methicillin-resistant Staphylococcus aureus (MRSA) have a sustained low incidence of MRSA infections. For diagnostic laboratories with high sample volumes, MRSA screening requires stability, low maintenance and high performance at a low cost. Herein we designed oligonucleotides for a new nuc targeted hydrolysis probe PCR to replace the standard in-house nuc SybrGreen PCR assay. This new, more time-efficient, PCR assay resulted in a 40% increase in daily sample capacity, with maintained high specificity and sensitivity. The assay was also able to detect Staphylococcus aureus clonal cluster 75 (CC75) lineage strains, recently re-classified as Staphylococcus argenteus, with a sensitivity considerably increased compared to our previous assay. While awaiting consensus if the CC75 lineage of S. aureus should be considered as S. argenteus, and whether methicillin-resistant S. argenteus should be included in the MRSA definition, many diagnostic laboratories need to update their MRSA assay sensitivity/specificity towards this lineage/species. The MRSA screening assay presented in this manuscript is comprised of nuc oligonucleotides separately targeting S. aureus and CC75 lineage strains/S. argenteus, thus providing high user flexibility for the detection of CC75 lineage strains/S. argenteus.

Real-time quantitative isothermal detection of Ostreid herpesvirus-1 DNA in Scapharca subcrenata using recombinase polymerase amplification

Ostreid herpesvirus-1 (OsHV-1) is a well-known pathogen associated with high mortality rates in hatchery-reared larvae and juveniles of different bivalve species worldwide. Early, rapid and accurate diagnosis plays a fundamental role in disease prevention and control in aquaculture. Recombinase polymerase amplification (RPA) is a novel isothermal amplification method, which can amplify detectable amount of DNA at 37 °C-39 °C within 20 min. In the present study, two sets of specific primers and probes were designed for the real-time quantitative RPA (qRPA) detection of OsHV-1 DNA. The sensitivity and specificity of detection were evaluated by comparison with quantitative polymerase chain reaction (qPCR). The detection limit for qRPA assays was shown to be 5 copies DNA/reaction for the primer set ORF95, which was lower than the 100 copies required for the qPCR test. The optimal reaction temperature and time were 37 °C for 20 min, making this approach faster than qPCR. This is the first study to apply qPCR and qRPA methods to detect OsHV-1 in Scapharca subcrenata. The percentage of viral load sample detected by the two methods was 22% and the correlation of the two virus quantitative results was 0.8. Therefore, qRPA assays is sensitive, fast, and high-temperature independent relative to qPCR and is suitable for critical clinical diagnostics use and rapid field analysis in resource-limited settings.

Multiplexed isothermal nucleic acid amplification

Multiplexed isothermal amplification and detection of nucleic acid sequences and biomarkers is of increasing importance in diverse areas including advanced diagnostics, food quality control and environmental monitoring. Whilst there are several very elegant isothermal amplification approaches, multiplexed amplification remains a challenge, requiring careful experimental design and optimisation, from judicious primer design in order to avoid the formation of primer dimers and non-specific amplification, applied temperature as well as the ratio and concentration of primers. In this review, we describe the various approaches that have been reported to date for multiplexed isothermal amplification, for both "one-pot" multiplexing as well as parallelised multiplexing using loop-mediated isothermal amplification, strand-displacement amplification, helicase-dependent amplification, rolling circle amplification, nucleic acid sequence-based amplification, with a particular focus on recombinase polymerase amplification.

Recombinase polymerase amplification: Basics, applications and recent advances

Recombinase polymerase amplification (RPA) is a highly sensitive and selective isothermal amplification technique, operating at 37-42°C, with minimal sample preparation and capable of amplifying as low as 1-10 DNA target copies in less than 20 min. It has been used to amplify diverse targets, including RNA, miRNA, ssDNA and dsDNA from a wide variety of organisms and samples. An ever increasing number of publications detailing the use of RPA are appearing and amplification has been carried out in solution phase, solid phase as well as in a bridge amplification format. Furthermore, RPA has been successfully integrated with different detection strategies, from end-point lateral flow strips to real-time fluorescent detection amongst others. This review focuses on the different methodologies and advances related to RPA technology, as well as highlighting some of the advantages and drawbacks of the technique.

Real-time isothermal detection of Abalone herpes-like virus and red-spotted grouper nervous necrosis virus using recombinase polymerase amplification

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This is the first study to use RPA to detect AbHV and RGNNV.

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Reaction can be finished at 37 °C in 20 min; time can be further reduced to 5 min for high viral load sample.

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The detection limits are 100 viral DNA copies per reaction for both viruses.

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Detection methods for both viruses have good specificity without false positive results.

Abalone herpes-like virus (AbHV) and Red-spotted grouper nervous necrosis virus (RGNNV) are two serious viruses that infect animal populations in aquaculture. Both viruses cause diseases associated with high mortality rates, resulting in dramatic economic losses in the aquaculture industry. There are currently no effective treatments for either of these two viral diseases. Thus, early, rapid, and accurate diagnosis plays a fundamental role in disease prevention and control in aquaculture. Traditional methods of diagnosis, such as virus culture, enzyme-linked immunoassay, and polymerase chain reaction (PCR), are either time consuming or require sophisticated temperature control devices. In this study, one sets of specific primers and probes were designed for the real-time quantitative recombinase polymerase amplification (qRPA) detection of AbHV and RGNNV separately. The sensitivity and specificity of detection were evaluated by comparison with detection by conventional PCR and quantitative PCR. The optimal reaction temperature and time for virus detection is 37 °C for 20 min. The detection limit is 100 copies per reaction, making this approach faster and more sensitive than qPCR in this study. In a field application, the detection percentage of qRPA was higher than that of qPCR for both AbHV and NNV. Additionally, good correlation was found between qRPA and qPCR detection (R² > 0.8). The methods presented here can be used as alternatives to qPCR for quick and quantitative detection of pathogens infecting aquaculture species.

Resistance diagnosis and the changing epidemiology of antibiotic resistance

Widespread adoption of point-of-care resistance diagnostics (POCRD) reduces ineffective antibiotic use but could increase overall antibiotic use. Indeed, in the context of a standard susceptible-infected epidemiological model with a single antibiotic, POCRD accelerates the rise of resistance in the disease-causing bacterial population. When multiple antibiotics are available, however, POCRD may slow the rise of resistance even as more patients receive antibiotic treatment, belying the conventional wisdom that antibiotics are "exhaustible resources" whose increased use necessarily promotes the rise of resistance.

Establishment of a rapid and sensitive method based on recombinase polymerase amplification to detect mts90, a new molecular target of Mycobacterium tuberculosis

The paper is about rapid screening of tuberculosis for detecting mts90, a new molecular target ofMycobacterium tuberculosis.

Recombinase Polymerase Amplification for Diagnostic Applications

BACKGROUND

First introduced in 2006, recombinase polymerase amplification (RPA) has stirred great interest, as evidenced by 75 publications as of October 2015, with 56 of them just in the last 2 years. The widespread adoption of this isothermal molecular tool in many diagnostic fields represents an affordable (approximately 4.3 USD per test), simple (few and easy hands-on steps), fast (results within 5–20 min), and sensitive (single target copy number detected) method for the identification of pathogens and the detection of single nucleotide polymorphisms in human cancers and genetically modified organisms.

CONTENT

This review summarizes the current knowledge on RPA. The molecular diagnostics of various RNA/DNA pathogens is discussed while highlighting recent applications in clinical settings with focus on point-of-care (POC) bioassays and on automated fluidic platforms. The strengths and limitations of this isothermal method are also addressed.

SUMMARY

RPA is becoming a molecular tool of choice for the rapid, specific, and cost-effective identification of pathogens. Owing to minimal sample-preparation requirements, low operation temperature (25–42 °C), and commercial availability of freeze-dried reagents, this method has been applied outside laboratory settings, in remote areas, and interestingly, onboard automated sample-to-answer microfluidic devices. RPA is undoubtedly a promising isothermal molecular technique for clinical microbiology laboratories and emergence response in clinical settings.

Multiplexed Recombinase Polymerase Amplification Assay To Detect Intestinal Protozoa

This work describes a proof-of-concept multiplex recombinase polymerase amplification (RPA) assay with lateral flow readout that is capable of simultaneously detecting and differentiating DNA from any of the diarrhea-causing protozoa Giardia, Cryptosporidium, and Entamoeba. Together, these parasites contribute significantly to the global burden of diarrheal illness. Differential diagnosis of these parasites is traditionally accomplished via stool microscopy. However, microscopy is insensitive and can miss up to half of all cases. DNA-based diagnostics such as polymerase chain reaction (PCR) are far more sensitive; however, they rely on expensive thermal cycling equipment, limiting their availability to centralized reference laboratories. Isothermal DNA amplification platforms, such as the RPA platform used in this study, alleviate the need for thermal cycling equipment and have the potential to broaden access to more sensitive diagnostics. Until now, multiplex RPA assays have not been developed that are capable of simultaneously detecting and differentiating infections caused by different pathogens. We developed a multiplex RPA assay to detect the presence of DNA from Giardia, Cryptosporidium, and Entamoeba. The multiplex assay was characterized using synthetic DNA, where the limits-of-detection were calculated to be 403, 425, and 368 gene copies per reaction of the synthetic Giardia, Cryptosporidium, and Entamoeba targets, respectively (roughly 1.5 orders of magnitude higher than for the same targets in a singleplex RPA assay). The multiplex assay was also characterized using DNA extracted from live parasites spiked into stool samples where the limits-of-detection were calculated to be 444, 6, and 9 parasites per reaction for Giardia, Cryptosporidium, and Entamoeba parasites, respectively. This proof-of-concept assay may be reconfigured to detect a wide variety of targets by re-designing the primer and probe sequences.

Characterization of a Novel Composite Staphylococcal Cassette Chromosome mec in Methicillin-Resistant Staphylococcus pseudintermedius from Thailand

A novel staphylococcal cassette chromosome mec (SCCmec) composite island (SCCmecAI16-SCCczrAI16-CI) was identified in Staphylococcus pseudintermedius. Four integration site sequences for SCC subdivided the 60,734-bp island into 41,232-bp SCCmecAI16, 19,400-bp SCCczrAI16, and 102-bp SCC-likeAI16 elements. SCCmecAI16 represents a new combination of ccrA1B3 genes with a class A mec complex. SCCczrAI16 contains ccrA1B6 and genes related to restriction modification and heavy metal resistance. SCCmecAI16-SCCczrAI16-CI was found in methicillin-resistant S. pseudintermedius sequence type 112 (ST112) and ST111 isolated from dogs and veterinarians in Thailand.

Detection of mecA- and mecC-Positive Methicillin-Resistant Staphylococcus aureus (MRSA) Isolates by the New Xpert MRSA Gen 3 PCR Assay

An advanced methicillin-resistant Staphylococcus aureus (MRSA) detection PCR approach targeting SCCmec-orfX along with mecA and mecC was evaluated for S. aureus and coagulase-negative staphylococci. The possession of mecA and/or mecC was correctly confirmed in all cases. All methicillin-susceptible S. aureus strains (n = 98; including staphylococcal cassette chromosome mec element [SCCmec] remnants) and 98.1% of the MRSA strains (n = 160, including 10 mecC-positive MRSA) were accurately categorized.

Vancomycin tolerant, methicillin-resistant Staphylococcus aureus reveals the effects of vancomycin on cell wall thickening

Methicillin-resistant Staphylococcus aureus (MRSA) is an important opportunistic pathogen that causes both healthcare- and community-acquired infections. An increase in the incidence of these infections may lead to a substantial change in the rate of vancomycin usage. Incidence of reduced susceptibility to vancomycin has been increasing worldwide for the last few years, conferring different levels of resistance to vancomycin as well as producing changes in the cell wall structure. The aim of the present study was to determine the effect of vancomycin on cell wall thickening in clinical isolates of vancomycin-tolerant (VT) MRSA obtained from pediatric patients. From a collection of 100 MRSA clinical isolates from pediatric patients, 12% (12/100) were characterized as VT-MRSA, and from them, 41.66% (5/12) exhibited the heterogeneous vancomycin-intermediate S. aureus (hVISA) phenotype. Multiplex-PCR assays revealed 66.66% (8/12), 25% (3/12), and 8.33% (1/12) of the VT-MRSA isolates were associated with agr group II, I, and III polymorphisms, respectively; the II-mec gene was amplified from 83.3% (10/12) of the isolates, and the mecIVa gene was amplified from 16.66% (2/12) of the isolates. Pulsed field electrophoresis (PFGE) fingerprint analysis showed 62% similarity among the VT-MRSA isolates. Thin transverse sections analyzed by transmission electron microscopy (TEM) revealed an average increase of 24 nm (105.55%) in the cell wall thickness of VT-MRSA compared with untreated VT-MRSA isolates. In summary, these data revealed that the thickened cell walls of VT-MRSA clinical isolates with agr type II and SCCmec group II polymorphisms are associated with an adaptive resistance to vancomycin.

Genome-wide analysis reveals two novel mosaic regions containing an ACME with an identical DNA sequence in the MRSA ST398-t011 and MSSA ST8-t008 isolates

OBJECTIVES

The presence of the arginine catabolic mobile element (ACME) in Staphylococcus aureus has been reported to enhance the colonization of the human host. The aim of this study was to determine the genetic organization of composite islands harbouring ACME.

METHODS

Two ACME-positive S. aureus isolates obtained during two different surveys conducted in the Netherlands and Poland were characterized in this study. The isolates were analysed by spa typing, DNA microarrays and whole-genome sequencing.

RESULTS

The two isolates harboured a truncated yet fully functional ACME type II with an identical nucleotide sequence, but differed in their adjacent mobile genetic elements. The first strain was a livestock-associated ST398-t011 MRSA, which had a staphylococcal cassette chromosome mec (SCCmec) composite island composed of SCCpls adjacent to orfX followed by ACME type II and SCCmec type IVa. The second ACME-positive isolate was an ST8-t008 MSSA. Its composite island showed an SCC-like element carrying the ccrC gene followed by ACME II.

CONCLUSIONS

This is the first report of an ACME in a livestock-associated MRSA ST398. It is also the first presentation of an ACME composite island structure in an MSSA isolate. Our findings indicate an extensive mosaicism of composite islands in S. aureus, which has implications for the transmissibility among humans and thus for public health.

Early detection of dengue virus by use of reverse transcription-recombinase polymerase amplification

A method for the rapid diagnosis of early dengue virus (DENV) infection is highly needed. Here, a prototype reverse transcription-recombinase polymerase amplification (RT-RPA) assay was developed. The assay detected DENV RNA in <20 min without the need for thermocycling amplification. The assay enabled the detection of as few as 10 copies of DENV RNA. The designed RT-RPA primers and exo probe detected the DENV genome of at least 12 genotypes of DENV circulating globally without cross-reacting with other arboviruses. We assessed the diagnostic performance of the RT-RPA assay for the detection of DENV RNA in 203 serum samples of patients with clinically suspected dengue. The sera were simultaneously tested for DENV using a reverse transcription-loop-mediated isothermal amplification (RT-LAMP) assay, quantitative RT-PCR (qRT-PCR), and IgM- and IgG-capture enzyme-linked immunosorbent assays (ELISA). Acute DENV infection was confirmed in 130 samples and 61 of the samples (46.9%) were classified as viremic with qRT-PCR. The RT-RPA assay showed good concordance (κ of ≥0.723) with the RT-LAMP and qRT-PCR assays in detecting the dengue viremic samples. When used in combination with ELISA, both the RT-RPA and RT-LAMP assays increased the detection of acute DENV infection to ≥95.7% (≥45/47) in samples obtained within 5 days of illness. The results from the study suggest that the RT-RPA assay is the most rapid molecular diagnostic tool available for the detection of DENV. Hence, it is possible to use the RT-RPA assay in a laboratory to complement routine serology testing for dengue.

Influence of sequence mismatches on the specificity of recombinase polymerase amplification technology

Recombinase polymerase amplification (RPA) technology relies on three major proteins, recombinase proteins, single-strand binding proteins, and polymerases, to specifically amplify nucleic acid sequences in an isothermal format. The performance of RPA with respect to sequence mismatches of closely-related non-target molecules is not well documented and the influence of the number and distribution of mismatches in DNA sequences on RPA amplification reaction is not well understood. We investigated the specificity of RPA by testing closely-related species bearing naturally occurring mismatches for the tuf gene sequence of Pseudomonas aeruginosa and/or Mycobacterium tuberculosis and for the cfb gene sequence of Streptococcus agalactiae. In addition, the impact of the number and distribution of mismatches on RPA efficiency was assessed by synthetically generating 14 types of mismatched forward primers for detecting five bacterial species of high diagnostic relevance such as Clostridium difficile, Staphylococcus aureus, S. agalactiae, P. aeruginosa, and M. tuberculosis as well as Bacillus atropheus subsp. globigii for which we use the spores as internal control in diagnostic assays. A total of 87 mismatched primers were tested in this study. We observed that target specific RPA primers with mismatches (n > 1) at their 3'extrimity hampered RPA reaction. In addition, 3 mismatches covering both extremities and the center of the primer sequence negatively affected RPA yield. We demonstrated that the specificity of RPA was multifactorial. Therefore its application in clinical settings must be selected and validated a priori. We recommend that the selection of a target gene must consider the presence of closely-related non-target genes. It is advisable to choose target regions with a high number of mismatches (≥36%, relative to the size of amplicon) with respect to closely-related species and the best case scenario would be by choosing a unique target gene.