Rapid Screening for Salmonella in Raw Pet Food by Loop-Mediated Isothermal Amplification

ABSTRACT

Raw pet food, composed of raw meat and vegetables, has increased in popularity in recent years. Multiple surveys and frequent recalls indicate that this commodity has a high risk of contamination with Salmonella and other foodborne pathogens. Improved screening methods are needed to meet the growing demand for testing. This matrix verification study aimed to apply a Salmonella loop-mediated isothermal amplification (LAMP) method, recently completed multilaboratory validation in dry dog food, in several raw pet food matrices, following the U.S. Food and Drug Administration (FDA)'s method validation guidelines. Five types of raw pet food, consisting of freeze-dried beef and chicken treats and frozen beef, pork, and turkey complete foods, were evaluated. For each matrix, two sets of ten 25-g test portions (seven inoculated with ≤30 cells of Salmonella Typhimurium and three uninoculated controls) were examined. One set was preenriched in buffered peptone water and the other one was preenriched in lactose broth, which was followed by LAMP screening using two isothermal master mixes (ISO-001 and ISO-004). All results were confirmed by culture as specified in the FDA Bacteriological Analytical Manual (BAM). The LAMP method accurately detected Salmonella in all inoculated test portions of the five raw pet food samples, regardless of the preenrichment broth used. Positive results could be obtained within 4 min of the LAMP run using the ISO-004 master mix. All uninoculated controls tested negative using LAMP or BAM. In addition, one turkey-based complete pet food sample was found to be already contaminated with three Salmonella serovars harboring multiple antimicrobial resistance genes. The Salmonella LAMP method offers a rapid, reliable, and robust tool for routine screening of Salmonella in raw pet food, which will help better ensure product safety and protect public health.

HIGHLIGHTS

Loop-Mediated Isothermal Amplification (LAMP) as a Rapid, Affordable and Effective Tool to Involve Students in Undergraduate Research

Undergraduate research (UR) is a high-impact practice (HIP) to engage undergraduate student in science, technology, engineering and mathematics (STEM), especially from underrepresented groups. UR experiences (UREs) can be integrated into the classroom, making authentic research experiences inclusive and available to all students. However, developing UR pedagogy can be challenging for faculty in resource-limited labs, such as community colleges and small liberal arts colleges. Often molecular biology research methods are expensive, time-consuming and need equipment not readily available or affordable in small schools. Polymerase chain reaction (PCR) is one of the most commonly used techniques in research labs and many UREs. We have investigated loop-mediated isothermal amplification (LAMP) as an inexpensive, accessible alternative to PCR for DNA amplification enabling the identification of microorganisms in the context of UREs. LAMP does not require expensive instrumentation or reagents and uses equipment commonly found in teaching labs. By performing the technique, students learn several key scientific skills that will be useful in their undergraduate or graduate STEM careers. We designed guided independent research experiences for several undergraduates that included the use of LAMP. Students successfully applied the technique to culture samples of common environmental bacteria, including Escherichia coli, Salmonella spp., Staphylococcus aureus, and Enterococcus, and were in addition, able to detect both Salmonella and Enterococcus in directly sampled environmental waters. To highlight the accessibility and affordability of this URE, a simple boiling method was used for DNA preparation from environmental samples. Student response data show positive attitudes toward UR when LAMP is utilized as a research tool to tackle relevant biological questions. The feasibility of using simplified LAMP in UREs points to a potential, more expanded application to public engagement with science and broader and more inclusive interactions with the research community.

Modern Approach to Medical Diagnostics - the Use of Separation Techniques in Microorganisms Detection

Background:

Analytical chemistry and biotechnology as an interdisciplinary fields of science have been developed during many years and are experiencing significant growth, to cover a wide range of microorganisms separation techniques and methods, utilized for medical therapeutic and diagnostic purposes. Currently scientific reports contribute by introducing electrophoretical and immunological methods and formation of devices applied in food protection (avoiding epidemiological diseases) and healthcare (safety ensuring in hospitals).

Methods:

Electrophoretic as well as nucleic-acid-based or specific immunological methods have contributed tremendously to the advance of analyses in recent three decades, particularly in relation to bacteria, viruses and fungi identifications, especially in medical in vitro diagnostics, as well as in environmental or food protection.

Results:

The paper presents the pathogen detection competitiveness of these methods against conventional ones, which are still too time consuming and also labor intensive. The review is presented in several parts following the current trends in improved pathogens separation and detection methods and their subsequent use in medical diagnosis.

Discussion:

Part one, consists of elemental knowledge about microorganisms as an introduction to their characterization: descriptions of divisions, sizes, membranes (cells) components. Second section includes the development, new technological and practical solution descriptions used in electrophoretical procedures during microbes analyses, with special attention paid to bio-samples analyses like blood, urine, lymph or wastewater. Third part covers biomolecular areas that have created a basis needed to identify the progress, limitations and challenges of nucleic-acid-based and immunological techniques discussed to emphasize the advantages of new separative techniques in selective fractionating of microorganisms.

Loop-mediated isothermal amplification: rapid molecular detection of virulence genes associated with avian pathogenic Escherichia coli in poultry

Infections with pathogenic Escherichia coli can lead to different animal- and human-associated diseases. E. coli infections are common in intensive poultry farming, and important economic losses can be expected during infections with avian pathogenic E. coli (APEC) strains followed by colibacillosis. Loop-mediated isothermal amplification (LAMP) assays were developed for rapid detection of 3 APEC-associated virulence genes: sitA, traT, and ompT. All 3 LAMP assays are shown to be specific, repeatable, and reproducible. High sensitivities of the assays are shown, where as few as 1,000 bacterial cells/mL can be detected in different matrices. On-site applicability of this LAMP method is demonstrated through testing of different sample types, from animal swabs and tissues, and from environmental samples collected from 6 commercial poultry farms. All 3 virulence genes were detected at high rates (above 85%) in samples from layer and broiler chickens with clinical signs and, interestingly, high prevalence of those genes was detected also in samples collected from clinically healthy broiler flock (above 75%) while lower prevalence was observed in remaining 3 clinically healthy chicken flocks (less than 75%). Importantly, these virulence genes were detected in almost all of the air samples from 11 randomly selected poultry houses, indicating air as an important route of E. coli spread. Three LAMP assays that target APEC-associated virulence genes are shown to be sensitive and robust and are therefore applicable for rapid on-site testing of various sample types, from animal swabs to air. This on-site LAMP testing protocol offers rapid diagnostics, with results obtained in <35 min, and it can be applied to other important microorganisms to allow the required prompt measures to be taken.

Development of a loop-mediated isothermal amplification assay for the detection of chicken anemia virus

Chicken anemia virus (CAV) infection has been reported in various poultry industries worldwide. Since CAV infection is becoming increasingly prevalent, especially in local chickens of China, rapid CAV detection has become essential. The conventional diagnostic methods are time consuming and need special expertise. Therefore, in this study, we developed a specific and sensitive loop-mediated isothermal amplification (LAMP) assay for CAV detection by using multiple sequence alignment of VP2. This assay was performed at 61°C for 1 h, and there was no non-specific reaction to common avian disease viruses. The detection limit was 65 copies of viral DNA; thus, this assay showed similar sensitivity to quantitative polymerase chain reaction (qPCR) but it was more sensitive than conventional PCR. Moreover, this assay was performed using clinical samples. The LAMP assay results were 83.6% correlated to the PCR results of the clinical samples, indicating that this method is an effective tool for the rapid detection of CAV.

Loop-Mediated Isothermal Amplification for Salmonella Detection in Food and Feed: Current Applications and Future Directions

Loop-mediated isothermal amplification (LAMP) has become a powerful alternative to polymerase chain reaction (PCR) for pathogen detection in clinical specimens and food matrices. Nontyphoidal Salmonella is a zoonotic pathogen of significant food and feed safety concern worldwide. The first study employing LAMP for the rapid detection of Salmonella was reported in 2005, 5 years after the invention of the LAMP technology in Japan. This review provides an overview of international efforts in the past decade on the development and application of Salmonella LAMP assays in a wide array of food and feed matrices. Recent progress in assay design, platform development, commercial application, and method validation is reviewed. Future perspectives toward more practical and wider applications of Salmonella LAMP assays in food and feed testing are discussed.

Loop-Mediated Isothermal Amplification of the sefA Gene for Rapid Detection of Salmonella Enteritidis and Salmonella Gallinarum in Chickens

Salmonella spp. pose a threat to both human and animal health, with more than 2600 serovars having been reported to date. Salmonella serovars are usually identified by slide agglutination tests, which are labor intensive and time consuming. In an attempt to develop a more rapid screening method for the major poultry Salmonella serovars, we developed a loop-mediated isothermal amplification (LAMP) assay, which directly detected the sefA gene, a fimbrial operon gene existing in several specific serovars of Salmonella enterica including the major poultry serovars, namely Salmonella enterica serovar Enteritidis (Salmonella Enteritidis) and Salmonella enterica serovar Gallinarum (Salmonella Gallinarum). With the 177 bacterial strains we tested, positive reactions were only observed with 85 strains of serovar Salmonella Enteritidis and Salmonella Gallinarum. The detection limit of the LAMP assay was 4 CFU/reaction with genomic DNAs of Salmonella Enteritidis (ATCC 13076) from pure culture and 400 CFU/ reaction with DNA extracted from spiked chicken feces. The LAMP assay was more sensitive than conventional culture, especially without enrichment, in detecting Salmonella Enteritidis (CMCC 50041) in the spiked fecal samples. The results show the sefA LAMP method is a rapid, sensitive, specific, and practical method for directly detection of Salmonella Enteritidis and Salmonella Gallinarum in chickens. The sefA LAMP assay can potentially serve as new on-site diagnostics in the poultry industry.

Detection of Salmonella spp. by a loop-mediated isothermal amplification (LAMP) method targeting bcfD gene

In this study, we developed and validated a loop-mediated isothermal amplification (LAMP) assay for Salmonella detection targeting bcfD gene, a conserved fimbrial operon gene existing in Salmonella. The Salmonella LAMP assay we developed successfully amplified 44 Salmonella strains (14 standard strains and 30 clinical isolates), but none of 9 non-Salmonella standard strains (Proteus mirabilis, Listeria monocytogenes, Escherichia coli, Pseudomonas aeruginosa, Shigella flexneri, Shigella sonnei, Klebsiella pneumoniae, Campylobacter jejuni and Vibrio parahemolyticus). The detection limit was 5 CFU of Salmonella pure culture or 200 CFU of artificially spiked faeces per reaction system (equivalent to 5000 CFU g(-1) of faeces), and this method could directly detect Salmonella in chicken faeces free of pre-enrichment in a reaction time of 25 min. Our experiments show that the LAMP method we developed is a rapid, sensitive, specific and practical method for Salmonella detection. The Salmonella LAMP assay can potentially serve as new on-site diagnostics in the food and agricultural industries.

SIGNIFICANCE AND IMPACT OF THE STUDY

A loop-mediated isothermal amplification (LAMP) assay was established to detect Salmonella targeting bcfD gene, a conserved fimbrial operon gene. The detection limit was 5 CFU of Salmonella pure culture or 200 CFU of artificially spiked faeces per reaction system (equivalent to 5000 CFU g(-1) of faeces), and this method could directly detect Salmonella in chicken faeces free of pre-enrichment in a reaction time of 25 min. The Salmonella LAMP assay is a rapid, sensitive, specific and practical method for Salmonella detection and can potentially serve as new on-site diagnostics in the food and agricultural industries.