Competitive annealing mediated isothermal amplification (CAMP) for rapid and simple detection of Listeria monocytogenes in milk

A portable CRISPR-Cas12a triggered photothermal biosensor for sensitive and visual detection of Staphylococcus aureus and Listeria monocytogenes

The detection of foodborne pathogens is crucial for ensuring the maintenance of food safety. In the present study, a portable CRISPR-Cas12a triggered photothermal biosensor integrating branch hybrid chain reaction (bHCR) and DNA metallization strategy for sensitive and visual detection of foodborne pathogens was proposed. The sheared probes were utilized to block the locker probes, which enabled preventing the assembly of bHCR in the absence of target bacteria, while target bacteria can activate the cleavage of sheared probes through CRISPR-Cas12a. Therefore, the locker probes functioned as initiating chains, triggering the formation of the branching double-stranded DNA consisting of H1, H2, and H3. The silver particles, which were in situ deposited on the DNA structure, functioned as a signal factor for conducting photothermal detection. Staphylococcus aureus and Listeria monocytogenes were selected as the foodborne pathogens to verify the analytical performance of this CRISPR-Cas12a triggered photothermal sensor platform. The sensor exhibited a sensitive detection with a low detection limit of 1 CFU/mL, while the concentration ranged from 100 to 108 CFU/mL. Furthermore, this method could efficiently detect target bacteria in multiple food samples. The findings demonstrate that this strategy can serve as a valuable reference for the development of a portable platform enabling quantitative analysis, visualization, and highly sensitive detection of foodborne bacteria.

Development of a Novel Phagomagnetic-Assisted Isothermal DNA Amplification System for Endpoint Electrochemical Detection of Listeria monocytogenes

The hitherto implemented Listeria monocytogenes detection techniques are cumbersome or require expensive non-portable instrumentation, hindering their transposition into on-time surveillance systems. The current work proposes a novel integrated system resorting to loop-mediated isothermal amplification (LAMP), assisted by a bacteriophage P100-magnetic platform, coupled to an endpoint electrochemical technique, towards L. monocytogenes expeditious detection. Molybdophosphate-based optimization of the bacterial phagomagnetic separation protocol allowed the determination of the optimal parameters for its execution (pH 7, 25 °C, 32 µg of magnetic particles; 60.6% of specific capture efficiency). The novel LAMP method targeting prfA was highly specific, accomplishing 100% inclusivity (for 61 L. monocytogenes strains) and 100% exclusivity (towards 42 non-target Gram-positive and Gram-negative bacteria). As a proof-of-concept, the developed scheme was successfully validated in pasteurized milk spiked with L. monocytogenes. The phagomagnetic-based approach succeeded in the selective bacterial capture and ensuing lysis, triggering Listeria DNA leakage, which was efficiently LAMP amplified. Methylene blue-based electrochemical detection of LAMP amplicons was accomplished in 20 min with remarkable analytical sensitivity (1 CFU mL^-1). Hence, the combined system presented an outstanding performance and robustness, providing a 2.5 h-swift, portable, cost-efficient detection scheme for decentralized on-field application.

Visual detection of Cronobacter sakazakii on a microfluidic chip fabricated by a 3D molding method

Cronobacter sakazakii (C. sakazakii) is a pathogenic bacterium associated with life-threatening neonatal infections that have been linked to contaminated powdered infant formula (PIF). Most C. sakazakii testing is still limited in microbiology laboratories due to the need for sophisticated equipment and professional technicians. Microfluidic chips combined with isothermal amplification analysis are shown to be one of the most attractive microbiological on-site detection platforms. In this study, PDMS microfluidic chips were fabricated by a simple 3D molding method and sealed with "PDMS glue". The chip consisted of an inlet, a microchannel, six reaction wells, and six vent holes. And based on the 16S rRNA and ITS genes of C. sakazakii, we have successfully proposed a multiplex competitive annealing mediated isothermal amplification (mCAMP) assay on the microfluidic chip for the visual detection of C. sakazakii in PIF samples. The primers were fixed in the reaction wells of the chip before detection, which can be preserved for 60 days at 4 °C. The results showed that the established mCAMP assay had high specificity, and the limit of detection was 2.2 × 10³ CFU g^-1. With enrichment culture, even if the initial inoculation level is 1 CFU g^-1, the mCAMP assay can still detect the presence of C. sakazakii in spiked PIF samples. The test results are visible to the naked eye, which is suitable for rapid analysis in resource-limited settings.