Development and Validation of a Cultural Method for the Detection and Isolation of Salmonella in Cloves

A Fluorescent Biosensor for Sensitive Detection of Salmonella Typhimurium Using Low-Gradient Magnetic Field and Deep Learning via Faster Region-Based Convolutional Neural Network

In this study, a fluorescent biosensor was developed for the sensitive detection of Salmonella typhimurium using a low-gradient magnetic field and deep learning via faster region-based convolutional neural networks (R-CNN) to recognize the fluorescent spots on the bacterial cells. First, magnetic nanobeads (MNBs) coated with capture antibodies were used to separate target bacteria from the sample background, resulting in the formation of magnetic bacteria. Then, fluorescein isothiocyanate fluorescent microspheres (FITC-FMs) modified with detection antibodies were used to label the magnetic bacteria, resulting in the formation of fluorescent bacteria. After the fluorescent bacteria were attracted against the bottom of an ELISA well using a low-gradient magnetic field, resulting in the conversion from a three-dimensional (spatial) distribution of the fluorescent bacteria to a two-dimensional (planar) distribution, the images of the fluorescent bacteria were finally collected using a high-resolution fluorescence microscope and processed using the faster R-CNN algorithm to calculate the number of the fluorescent spots for the determination of target bacteria. Under the optimal conditions, this biosensor was able to quantitatively detect Salmonella typhimurium from 6.9 × 10¹ to 1.1 × 10³ CFU/mL within 2.5 h with the lower detection limit of 55 CFU/mL. The fluorescent biosensor has the potential to simultaneously detect multiple types of foodborne bacteria using MNBs coated with their capture antibodies and different fluorescent microspheres modified with their detection antibodies.

Development and validation of an improved method for the detection of Salmonella in cinnamon bark and oregano leaves using the adsorbent beta zeolite in the pre-enrichment media

The detection of Salmonella in spices is challenging due to the presence of antibacterial components. In this study, we evaluated the use of an adsorbent beta zeolite in pre-enrichment media to improve the recovery of Salmonella from cinnamon bark and oregano leaves. Samples (25 g) were spiked with varying levels of S. Montevideo or S. Senftenberg. After 2 weeks of stabilization at RT, betazeolite was added to cinnamon and oregano samples prior to the addition of 225 mL or 475 mL of pre-enrichment media, respectively. Detection sensitivity and rate of the test method were compared to the FDA Bacteriological Analytical Manual (BAM) method which requires the use of 2.5 L pre-enrichment broth. While Salmonella could not be detected in the test method using the reduced volume of pre-enrichment media alone, the addition of beta zeolite resulted in a positivity rate of 62% and 72.6% for cinnamon bark and oregano leaves respectively (all spike levels and both serovars combined). Furthermore, while there were differences in the LOD₅₀ compared to the BAM method, there was no significant difference in the minimum level of detection between the betazeolite and the BAM methods. Our results demonstrate that the use of betazeolite in the pre-enrichment media offers a method with reduced media volumes without compromising on the sensitivity or efficiency of Salmonella detection in cinnamon bark and oregano leaves.

On the Statistical Testing Methods for Single Laboratory Validation of Qualitative Microbiological Assays with an Unpaired Design

BACKGROUND

There exists several statistical methods for detecting a difference of detection rates between alternative and reference qualitative microbiological assays in a single laboratory validation study with an unpaired design.

OBJECTIVE

We compared performance of eight methods including Fisher's exact test, unequal variance two-sample t-test, Wilcoxon rank-sum test, z-test, and methods based on Wilson confidence intervals, complementary log-log regression, Firth's logistic regression, and ordinary logistic regression.

METHOD

We first compared the minimum detectable difference in the proportion of detections between the alternative and reference methods among these statistical methods for a varied number of test portions. We then compared power and size of test of these methods using simulated data.

RESULTS

Firth's logistic regression and the unequal variance two-sample t-test had the lowest minimum detectable difference and highest power. None of these statistical methods had an estimated size of test always within a 95% confidence interval of the nominal value 0.05 with small numbers of test portions (n = 12, 20, 30). Fisher's exact test, the Wilcoxon rank-sum test, and the z-test were conservative even with a moderately large number of test portions (n = 40), while Firth's logistic regression and the unequal variance two-sample t-test had a size of test closer to 0.05 than other methods.

CONCLUSIONS

Firth's logistic regression and the unequal variance two-sample t-test are better choices than other competing methods.

HIGHLIGHTS

We recommend the unequal variance two-sample t-test over Firth's logistic regression because the unequal variance two-sample t-test is better known and easier to use. We provide an example using real data.

A lab-on-chip device for the sample-in-result-out detection of viable Salmonella using loop-mediated isothermal amplification and real-time turbidity monitoring

Rapid screening of foodborne pathogens is key to prevent food poisoning. In this study, a lab-on-chip device was developed for rapid, automatic and sensitive detection of viable Salmonella typhimurium using loop-mediated isothermal amplification (LAMP) and smartphone real-time turbidity monitoring. First, magnetic nanoparticles (MNPs) coated with anti-Salmonella capture antibodies in propidium monoazide (PMA) were fully mixed with bacterial samples using two active magnetic stirring mixers at reverse rotating directions, and incubated in the serpentine channel with 470 nm blue light exposure, allowing specific formation of magnetic bacteria and sufficient PMA pretreatment of the DNA of dead bacteria. Then, the PMA-treated magnetic bacteria were separated in the separation chamber using the magnetic field and their genomic DNA templates were extracted using lysis buffer at 70 °C. Finally, the viable bacteria's DNA was amplified using LAMP in the detection chamber preloaded with the lyophilized LAMP reagents at 67.5 °C after blocking with paraffin oil to avoid aerosol cross contamination. Finally, the turbidity of the LAMP reaction system was monitored in a real-time manner for the quantitative detection of viable bacteria. The experimental results demonstrated that this device was able to automatically detect viable Salmonella as low as 14 CFU mL-1 in spiked chicken meat supernatants within 1.5 h. This device is very promising to provide a sample-in-result-out solution for the in-field detection of Salmonella and could be easily extended for other foodborne pathogens.

Effect of Talc as a Dry-Inoculation Carrier on Thermal Resistance of Enterococcus faecium NRRL B-2354 in Almond Meal

HIGHLIGHTS

E. faecium was more thermally resistant in dry- than in wet-inoculated almond meal. Presence of talc affected thermal resistance of E. faecium in almond meal. Use of dry inoculum carriers for thermal validation studies requires further work.

Challenges in Recovering Foodborne Pathogens from Low-Water-Activity Foods

There are numerous obstacles to the detection of foodborne pathogens in foods that exhibit a low water activity (a_w). These obstacles include the presence of antimicrobial compounds, particulates, PCR inhibitors, and fatty matrices. New approaches should be sought to increase the sensitivity of pathogen testing in low-a_w foods and to overcome the effects of various inhibitors and antimicrobials. The U.S. Food and Drug Administration and other laboratories are working toward this goal. This review will address these issues while delineating specific inhibitors and antimicrobials that impede testing of low-a_w foods. A review of relevant rapid and conventional testing methodologies for Salmonella in low-a_w foods will also be discussed.

A microfluidic biosensor for online and sensitive detection of Salmonella typhimurium using fluorescence labeling and smartphone video processing

Early screening of foodborne pathogens is a key to ensure food safety. In this study, we developed a microfluidic biosensor for online and sensitive detection of Salmonella based on immunomagnetic separation, fluorescence labeling and smartphone video processing. First, the immune magnetic nanoparticles were used to specifically separate and efficiently concentrate the target bacteria and the magnetic bacteria were formed. Then, the magnetic bacteria were labeled with the immune fluorescent microspheres and the fluorescent bacteria were formed. Finally, the fluorescent bacteria were continuously injected into the microfluidic chip on the smartphone-based fluorescent microscopic system, and the fluorescent spots were online counted using the smartphone App based on inter-frame difference algorithm to obtain the amount of the target bacteria. Under the optimal conditions, this proposed biosensor was able to quantitatively detect Salmonella typhimurium ranging from 1.4 × 10² to 1.4 × 10⁶ CFU/mL, and its lower detection limit was 58 CFU/mL. This biosensor could be extended for detection of multiple foodborne pathogens using different fluorescent materials.

Microbiological Safety of Dried Spices

Spices in the desiccated state provide an environment that allows the survival of many foodborne pathogens. Currently, the incidence of pathogen-positive spices imported into the United States is 1.9 times higher than for any other imported food. Correspondingly, imported spices have been associated with numerous foodborne outbreaks and multiple product recalls. Despite the association with recalls and outbreaks, the actual pathogen populations in spices, when found, are frequently extremely small. In addition to pathogenic bacterial species, toxigenic molds have been frequently recovered from spices, and aflatoxins have been found in as many as 58% of the spices sampled. The presence of toxigenic molds is especially problematic to the immunocompromised or those on immunosuppressive therapy and has been linked to gut aspergillosis. Numerous detection methods, including both traditional and advanced DNA regimes, are being tested to optimize recovery of pathogens from spices. Further, a number of new inactivation intervention methods to decontaminate spices are examined and discussed.

Evaluation of Three Real-Time PCR Methods for Detection of Salmonella from Cloves

The purpose of the study was to evaluate three real-time PCR platforms for rapid detection of Salmonella from cloves and to compare three different DNA extraction methods. Six trials were conducted with two clove cultivars, Ceylon and Madagascar, and three Salmonella serotypes, Montevideo, Typhimurium, and Weltevreden. Each trial consisted of 20 test portions. The preenrichment cultures were used to perform PCR for comparison of the effectiveness of U.S. Food and Drug Administration, Pacific Regional Laboratory Southwest (FDA-PRLSW), Applied Biosystems Inc. (ABI) MicroSEQ, and GeneDisc platforms for detection of Salmonella. Three DNA extraction methods were used: standard extraction method for each PCR platform, boil preparation, and LyseNow food pathogen DNA extraction cards. The results from real-time PCR correlated well with FDA Bacteriological Analytical Manual culture assay results, with a wide range of cycle threshold (C_T) values among the three PCR platforms for intended positive samples. The mean C_T values for MicroSEQ (16.36 ± 2.78) were significantly lower than for PRLSW (20.37 ± 3.45) and GeneDisc (23.88 ± 2.90) (P < 0.0001). Pairwise comparisons between PCR platforms using different DNA extraction methods indicate that the C_T values are inversely proportional to the relative DNA quantity (RDQ) yields by different platform-extraction combinations. The pairing of MicroSEQ and boil preparation generated the highest RDQ of 120 and the lowest average C_T value of 14.48, whereas the pairing of GeneDisc and LyseNow generated the lowest RDQ of 0.18 and the highest average C_T of 25.97. Boil preparation yielded higher RDQ than the other extraction methods for all three PCR platforms. Although the MicroSEQ platform generated the lowest C_T values, its sensitivity was compromised by narrow separations between the positive and negative samples. The PRLSW platform generated the best segregation between positive and negative groups and is less likely to produce false results. In conclusion, FDA-PRLSW was the most efficient PCR assay for Salmonella detection, and boil preparation was the best method for DNA extraction.