Name: loop-mediated isothermal amplification for screening salmonella in animal food and confirming salmonella from culture isolation
Uploaded: 2020-05-20T14:30:00
Duration: 8 min 24 s
Description: Watch this Scientific Journal Video about Loop-Mediated Isothermal Amplification for Screening Salmonella in Animal Food and Confirming Salmonella from Culture Isolation at JoVE.com

The authors thank members of the FDA&#8217;s Microbiology Methods Validation Subcommittee (MMVS) and Bacteriological Analytical Manual (BAM) Council for critically reviewing Salmonella LAMP method validation studies.

<ol>
	<li>Notomi, T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Loop-mediated+isothermal+amplification+of+DNA.">Loop-mediated isothermal amplification of DNA.</a> Nucleic Acids Research. 28 (12), 63 (2000).</li><li>Kaneko, H., Kawana, T., Fukushima, E., Suzutani, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Tolerance+of+loop-mediated+isothermal+amplification+to+a+culture+medium+and+biological+substances.">Tolerance of loop-mediated isothermal amplification to a culture medium and biological substances.</a> Journal of Biochemical and Biophysical Methods. 70 (3), 499-501 (2007).</li><li>Francois, P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Robustness+of+a+loop-mediated+isothermal+amplification+reaction+for+diagnostic+applications.">Robustness of a loop-mediated isothermal amplification reaction for diagnostic applications.</a> FEMS Immunology and Medical Microbiology. 62 (1), 41-48 (2011).</li><li>Yang, Q., Wang, F., Prinyawiwatkul, W., Ge, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Robustness+of+Salmonella+loop-mediated+isothermal+amplification+assays+for+food+applications.">Robustness of Salmonella loop-mediated isothermal amplification assays for food applications.</a> Journal of Applied Microbiology. 116 (1), 81-88 (2014).</li><li>Nagamine, K., Watanabe, K., Ohtsuka, K., Hase, T., Notomi, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Loop-mediated+isothermal+amplification+reaction+using+a+nondenatured+template.">Loop-mediated isothermal amplification reaction using a nondenatured template.</a> Clinical Chemistry. 47 (9), 1742-1743 (2001).</li><li>Zhang, X., Lowe, S. B., Gooding, J. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Brief+review+of+monitoring+methods+for+loop-mediated+isothermal+amplification+(LAMP).">Brief review of monitoring methods for loop-mediated isothermal amplification (LAMP).</a> Biosensors &amp; Bioelectronics. 61, 491-499 (2014).</li><li>Nagamine, K., Hase, T., Notomi, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Accelerated+reaction+by+loop-mediated+isothermal+amplification+using+loop+primers.">Accelerated reaction by loop-mediated isothermal amplification using loop primers.</a> Molecular and Cellular Probes. 16 (3), 223-229 (2002).</li><li>Yang, Q., Domesle, K. J., Ge, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Loop-mediated+isothermal+amplification+for+Salmonella+detection+in+food+and+feed:+Current+applications+and+future+directions.">Loop-mediated isothermal amplification for Salmonella detection in food and feed: Current applications and future directions.</a> Foodborne Pathogens and Disease. 15 (6), 309-331 (2018).</li><li>Mori, Y., Notomi, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Loop-mediated+isothermal+amplification+(LAMP):+Expansion+of+its+practical+application+as+a+tool+to+achieve+universal+health+coverage.">Loop-mediated isothermal amplification (LAMP): Expansion of its practical application as a tool to achieve universal health coverage.</a> Journal of Infection and Chemotherapy. 26 (1), 13-17 (2020).</li><li>Mansour, S. M., Ali, H., Chase, C. C., Cepica, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Loop-mediated+isothermal+amplification+for+diagnosis+of+18+World+Organization+for+Animal+Health+(OIE)+notifiable+viral+diseases+of+ruminants,+swine+and+poultry.">Loop-mediated isothermal amplification for diagnosis of 18 World Organization for Animal Health (OIE) notifiable viral diseases of ruminants, swine and poultry.</a> Animal Health Research Reviews. 16 (2), 89-106 (2015).</li><li>Kumar, Y., Bansal, S., Jaiswal, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Loop-mediated+isothermal+amplification+(LAMP):+A+rapid+and+sensitive+tool+for+quality+assessment+of+meat+products.">Loop-mediated isothermal amplification (LAMP): A rapid and sensitive tool for quality assessment of meat products.</a> Comprehensive Reviews in Food Science and Food Safety. 16 (6), 1359-1378 (2017).</li><li>Kundapur, R. R., Nema, V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Loop-mediated+isothermal+amplification:+Beyond+microbial+identification.">Loop-mediated isothermal amplification: Beyond microbial identification.</a> Cogent Biology. 2, 1137110 (2016).</li><li>. Compliance Policy Guide Sec. 690.800 Salmonella in Food for Animals Available from: <a target="_blank" href="https://www.fda.gov/downloads/iceci/compliancemanuals/compliancepolicyguidancemanual/ucm361105.pdf">https://www.fda.gov/downloads/iceci/compliancemanuals/compliancepolicyguidancemanual/ucm361105.pdf</a> (2013)</li><li>Bird, P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Evaluation+of+the+3M+molecular+detection+assay+(MDA)+2+-+Salmonella+for+the+detection+of+Salmonella+spp.+in+select+foods+and+environmental+surfaces:+collaborative+study,+first+action+2016.01.">Evaluation of the 3M molecular detection assay (MDA) 2 - Salmonella for the detection of Salmonella spp. in select foods and environmental surfaces: collaborative study, first action 2016.01.</a> Journal of AOAC International. 99 (4), 980-997 (2016).</li><li>D'Agostino, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Validation+of+a+loop-mediated+amplification/ISO+6579-based+method+for+analysing+soya+meal+for+the+presence+of+Salmonella+enterica.">Validation of a loop-mediated amplification/ISO 6579-based method for analysing soya meal for the presence of Salmonella enterica.</a> Food Analytical Methods. 9 (11), 2979-2985 (2016).</li><li>Ge, B., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Multi-laboratory+validation+of+a+loop-mediated+isothermal+amplification+method+for+screening+Salmonella+in+animal+food.">Multi-laboratory validation of a loop-mediated isothermal amplification method for screening Salmonella in animal food.</a> Frontiers in Microbiology. 10, 562 (2019).</li><li>D'Agostino, M., Diez-Valcarce, M., Robles, S., Losilla-Garcia, B., Cook, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+loop-mediated+isothermal+amplification-based+method+for+analysing+animal+feed+for+the+presence+of+Salmonella.">A loop-mediated isothermal amplification-based method for analysing animal feed for the presence of Salmonella.</a> Food Analytical Methods. 8 (10), 2409-2416 (2015).</li><li>Galan, J. E., Ginocchio, C., Costeas, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Molecular+and+functional+characterization+of+the+Salmonella+invasion+gene+invA:+homology+of+InvA+to+members+of+a+new+protein+family.">Molecular and functional characterization of the Salmonella invasion gene invA: homology of InvA to members of a new protein family.</a> Journal of Bacteriology. 174 (13), 4338-4349 (1992).</li><li>Chen, S., Wang, F., Beaulieu, J. C., Stein, R. E., Ge, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rapid+detection+of+viable+salmonellae+in+produce+by+coupling+propidium+monoazide+with+loop-mediated+isothermal+amplification.">Rapid detection of viable salmonellae in produce by coupling propidium monoazide with loop-mediated isothermal amplification.</a> Applied and Environmental Microbiology. 77 (12), 4008-4016 (2011).</li><li>Yang, Q., Chen, S., Ge, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Detecting+Salmonella+serovars+in+shell+eggs+by+loop-mediated+isothermal+amplification.">Detecting Salmonella serovars in shell eggs by loop-mediated isothermal amplification.</a> Journal of Food Protection. 76 (10), 1790-1796 (2013).</li><li>Yang, Q., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Evaluation+of+loop-mediated+isothermal+amplification+for+the+rapid,+reliable,+and+robust+detection+of+Salmonella+in+produce.">Evaluation of loop-mediated isothermal amplification for the rapid, reliable, and robust detection of Salmonella in produce.</a> Food Microbiology. 46, 485-493 (2015).</li><li>Yang, Q., Domesle, K. J., Wang, F., Ge, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rapid+detection+of+Salmonella+in+food+and+feed+by+coupling+loop-mediated+isothermal+amplification+with+bioluminescent+assay+in+real-time.">Rapid detection of Salmonella in food and feed by coupling loop-mediated isothermal amplification with bioluminescent assay in real-time.</a> BMC Microbiology. 16 (1), 112 (2016).</li><li>Domesle, K. J., Yang, Q., Hammack, T. S., Ge, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Validation+of+a+Salmonella+loop-mediated+isothermal+amplification+assay+in+animal+food.">Validation of a Salmonella loop-mediated isothermal amplification assay in animal food.</a> International Journal of Food Microbiology. 264, 63-76 (2018).</li><li>Bustin, S. A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+MIQE+guidelines:+minimum+information+for+publication+of+quantitative+real-time+PCR+experiments.">The MIQE guidelines: minimum information for publication of quantitative real-time PCR experiments.</a> Clinical Chemistry. 55 (4), 611-622 (2009).</li></ol>

The authors declare that they have no competing financial interests. The views expressed in this manuscript are those of the authors and do not necessarily reflect the official policy of the Department of Health and Human Services, the U.S. Food and Drug Administration, or the U.S. Government. Reference to any commercial materials, equipment, or process does not in any way constitute approval, endorsement, or recommendation by the Food and Drug Administration.

We have presented here a simple, rapid, specific, and sensitive LAMP method for screening and confirming Salmonella in animal food and pure culture, respectively. With the convenience of an isothermal master mix that contains four key reagents, and a ready-to-use, in-house prepared primer mix, assembling a LAMP reaction requires only a few pipetting steps (Figure 1). The total run time including amplification and anneal phases is less than 38 min (Figure 2A,B and Figure 3A,B). Positive results are monitored via real-time fluorescence (Figure 2C and Figure 3C,D) and can be detected as early as 5 min26. The anneal phase serves as an extra confirmation of LAMP specificity since only samples with correct Tm (around 90 &#176;C) are reported as positive (Figure 2D,E and Figure 3E&#8722;G). Sensitivities of 1 Salmonella cell in pure culture and &#60; 1 CFU/25 g in animal food have been reported previously26.
As LAMP is quite effective and generates a large quantity of DNA1, it is critical that best laboratory practices are used to prevent cross-contamination, which may include physically separating the areas for preparing the LAMP master mix and adding DNA templates, avoiding generating aerosols, using filter pipette tips, changing gloves often, and refraining from opening LAMP reaction tubes post-amplification.
The specificity of this Salmonella LAMP method was previously tested using 300 bacterial strains (247 Salmonella of 185 serovars and 53 non-Salmonella) and demonstrated to be 100% specific26. Notably, significant differences in Tmax were observed between the two Salmonella species, S. enterica and Salmonella bongori, and among S. enterica subspecies, especially subsp. arizonae (IIIa)26. Nonetheless, these were still valid positive results per the rules for interpreting LAMP results. In our multi-laboratory collaborative study in dry dog food which involved 14 analysts19, samples having inconsistent results in duplicate LAMP runs were occasionally observed. These usually involved samples with delayed positive results (Tmax &#62; 15 min). Repeating both runs independently usually resolved the issue. More rarely, we observed samples with correct Tm but no or irregular Tmax values (&#60; 5 min). This was usually caused by air bubbles in the reaction tube.
Throughout the lifecycle of LAMP method development, evaluation, precollaborative study, and multi-laboratory validation, we have observed high tolerance of LAMP to inhibitors in various animal food or food matrices and culture media4,19,22,23,24, highlighting the robustness of the method and collaborating numerous other studies on a global scale8. This is superior compared to PCR or real-time PCR, which usually requires an internal amplification control to ensure that negative results are not due to matrix inhibition28. Further, LAMP demonstrated similar (or superior) specificity and sensitivity compared to PCR or real-time PCR in the vast majority of studies8. The cost of LAMP reagents is at about $1 per reaction. The LAMP instruments used in this protocol are small, low-maintenance, and portable. They can handle any isothermal amplification method that employs target detection by fluorescence measurement, LAMP included. Using the LAMP software, comprehensive reports can be generated in multiple format (pdf, text, and image).
Method validation is a critical step before a new method can be adopted for routine use. It is noteworthy that the LAMP protocol reported here has successfully completed multi-laboratory validation19. With the recent incorporation of this LAMP protocol into the U.S. FDA&#8217;s BAM Chapter 5 Salmonella27, it is expected that the method will gain much wider use, both as a rapid screening method in animal food and as a reliable confirmation method for presumptive Salmonella isolates from all food categories.

Loop-mediated isothermal amplification (LAMP) is a novel isothermal nucleic acid amplification test (iNAAT) invented in 2000 by a group of Japanese scientists1. Through the formation of a target-specific stem-loop DNA structure during initial steps, LAMP uses a strand-displacing DNA polymerase to efficiently amplify this starting material quasi-exponentially, resulting in 109 copies of target in less than 1 h1. Compared to polymerase chain reaction (PCR), a widely used NAAT, LAMP possesses several advantages. First, LAMP reactions are carried out under isothermal conditions. This obviates the need for a sophisticated thermal cycling instrument. Second, LAMP is highly tolerant to culture media and biological substances2 with robustness demonstrated for both clinical and food applications3,4. This simplifies sample preparation and minimizes false negative results5. Third, LAMP is amenable to multiple detection platforms, such as turbidity, colorimetry, bioluminescence, fluorescence, and microfluidics6. Fourth, LAMP is highly specific as it uses four to six specially designed primers to target six to eight specific regions1,7. Fifth, LAMP is ultrasensitive and numerous studies have reported its superior sensitivity to PCR or real-time PCR8. Finally, LAMP is faster with many assays now adopting a 30 min standard run time while PCR-type assays usually take 1&#8722;2 h8.
These attractive features fueled the application of LAMP in broad pathogen detection areas, including in vitro diagnostics9, animal disease diagnostics10, and food and environmental testing11. Notably, a TB-LAMP (LAMP for Mycobacterium tuberculosis) has been recommended by WHO as a valid replacement test for sputum-smear microscopy for pulmonary tuberculosis diagnoses in peripheral settings12. LAMP application also expands beyond microbial identification to include the detection of allergens, animal species, drug resistance, genetically modified organisms, and pesticides13.
Nontyphoidal Salmonella is a zoonotic pathogen of substantial food safety and public health concern worldwide14. It has also been identified as an important microbial hazard in food for animals (i.e., animal food)15,16. To prevent Salmonella illnesses/outbreaks from contaminated human food and animal food, it is imperative to have rapid, reliable, and robust methods for testing Salmonella in a variety of matrices. In the past decade, considerable efforts have been made internationally on the development and application of Salmonella LAMP assays in a wide array of food matrices, as recently summarized in an extensive review8. Several Salmonella LAMP assays, including the one presented here, have successfully completed multi-laboratory validation following well-established international guidelines17,18,19,20.
Our Salmonella LAMP assay specifically targets the Salmonella invasion gene invA (GenBank accession number M90846)21 and is rapid, reliable, and robust in multiple food matrices4,22,23,24,25,26. The method has been validated in six animal food matrices in a precollaborative study26 and in dry dog food in a multi-laboratory collaborative study19. As a result, the Salmonella LAMP method presented here has recently been incorporated into the U.S. Food and Drug Administration (FDA)&#8217;s Bacteriological Analytical Manual (BAM) Chapter 5 Salmonella27 to serve two purposes, one as a rapid screening method for the presence of Salmonella in animal food and two as a reliable confirmation method for presumptive Salmonella isolated from all foods.

<table>
	<tbody>
		<tr>
			<td>Brain heart infusion (BHI) broth</td>
			<td>BD Diagnostic Systems, Sparks, MD</td>
			<td>299070</td>
			<td>Liquid growth medium used in the cultivation of Salmonella.</td>
		</tr>
		<tr>
			<td>Buffered peptone water (BPW)</td>
			<td>BD Diagnostic Systems, Sparks, MD</td>
			<td>218105</td>
			<td>Preenrichment medium for the recovery of Salmonella from animal food samples.</td>
		</tr>
		<tr>
			<td>DNA AWAY</td>
			<td>Thermo Fisher Scientific, Waltham, MA</td>
			<td>7010</td>
			<td>Eliminates unwanted DNA and DNase from laboratory bench, glassware, and plasticware without affecting subsequent DNA samples.</td>
		</tr>
		<tr>
			<td>Genie Explorer software</td>
			<td>OptiGene Ltd., West Sussex, United Kingdom</td>
			<td>Version 2.0.6.3</td>
			<td>Supports remote operation of Genie instruments including LAMP runs and data analysis.</td>
		</tr>
		<tr>
			<td>Genie II or Genie III (LAMP instrument)</td>
			<td>OptiGene Ltd., West Sussex, United Kingdom</td>
			<td>GEN2-02 or GEN3-02</td>
			<td>A small instrument capable of temperature control up to 100 &#176;C with &#177; 0.1 &#176;C accuracy and simultaneous fluorescence detection via the FAM channel. Genie II has 2 blocks (A and B) with 8 samples in each block. Genie III has a single block that accommodates 8 samples.</td>
		</tr>
		<tr>
			<td>Genie strip</td>
			<td>OptiGene Ltd., West Sussex, United Kingdom</td>
			<td>OP-0008</td>
			<td>8-well microtube strips with integral locking caps and a working volume of 10 to 150 &#181;l.</td>
		</tr>
		<tr>
			<td>Genie strip holder</td>
			<td>OptiGene Ltd., West Sussex, United Kingdom</td>
			<td>GBLOCK</td>
			<td>Used to hold Genie strips when setting up a LAMP reaction, the aluminum holder can also be used as a cool block.</td>
		</tr>
		<tr>
			<td>Hydrochloric acid (HCl) solution, 1 N</td>
			<td>Thermo Fisher Scientific, Waltham, MA</td>
			<td>SA48-500</td>
			<td>Adjusts pH of animal food samples after adding BPW and prior to overnight enrichment.</td>
		</tr>
		<tr>
			<td>Heat block</td>
			<td>Thermo Fisher Scientific, Waltham, MA</td>
			<td>88-860-022</td>
			<td>Heats samples at 100 &#177; 1 oC for DNA extraction.</td>
		</tr>
		<tr>
			<td>Incubator</td>
			<td>Thermo Fisher Scientific, Waltham, MA</td>
			<td>3960</td>
			<td>Standard laboratory incubator.</td>
		</tr>
		<tr>
			<td>ISO-001 isothermal master mix</td>
			<td>OptiGene Ltd., West Sussex, United Kingdom</td>
			<td>ISO-001</td>
			<td>An optimized master mix to simplify the assembly of a LAMP reaction, containing a strand-displacing GspSSD DNA polymerase large fragment from Geobacillus spp., thermostable inorganic pyrophosphatase, reaction buffer, MgSO4, dNTPs, and a double-stranded DNA binding dye (FAM detection channel).</td>
		</tr>
		<tr>
			<td>Isopropanol</td>
			<td>Thermo Fisher Scientific, Waltham, MA</td>
			<td>A416</td>
			<td>Disinfects work surfaces.</td>
		</tr>
		<tr>
			<td>LAMP primers</td>
			<td>Integrated DNA Technologies Inc., Coralville, IA</td>
			<td>Custom</td>
			<td>LAMP primers with detailed information in Table 1.</td>
		</tr>
		<tr>
			<td>Microcentrifuge</td>
			<td>Eppendorf North America, Hauppauge, NY</td>
			<td>22620207</td>
			<td>MiniSpin plus personal microcentrifuge.</td>
		</tr>
		<tr>
			<td>Microcentrifuge tubes</td>
			<td>Thermo Fisher Scientific, Waltham, MA</td>
			<td>05-408-129</td>
			<td>Standard microcentrifuge tubes.</td>
		</tr>
		<tr>
			<td>Molecular grade water</td>
			<td>Thermo Fisher Scientific, Waltham, MA</td>
			<td>AM9938</td>
			<td>Used in making primer stocks, primer mix, and LAMP reaction mix.</td>
		</tr>
		<tr>
			<td>Sodium hydroxide (NaOH) solution, 1 N</td>
			<td>Thermo Fisher Scientific, Waltham, MA</td>
			<td>SS266-1</td>
			<td>Adjusts pH of animal food samples after adding BPW and prior to overnight enrichment.</td>
		</tr>
		<tr>
			<td>Nonselective agar (e.g., blood agar, nutrient agar, and trypticase soy agar)</td>
			<td>Thermo Fisher Scientific, Waltham, MA</td>
			<td>R01202</td>
			<td>Solid growth medium used in the cultivation of Salmonella.</td>
		</tr>
		<tr>
			<td>Peptone water</td>
			<td>BD Diagnostic Systems, Sparks, MD</td>
			<td>218071</td>
			<td>Dilutes overnight Salmonella cultures to make positive control DNA.</td>
		</tr>
		<tr>
			<td>Pipettes and tips</td>
			<td>Mettler-Toledo Rainin LLC, Oakland CA</td>
			<td>Pipet Lite LTS series</td>
			<td>Standard laboratory pipettes and tips.</td>
		</tr>
		<tr>
			<td>PrepMan Ultra sample preparation reagent</td>
			<td>Thermo Fisher Scientific, Waltham, MA</td>
			<td>4318930</td>
			<td>A simple kit used for the rapid preparation of DNA templates for use in a LAMP reaction.</td>
		</tr>
		<tr>
			<td>Salmonella reference strain LT2</td>
			<td>ATCC, Manassas, VA</td>
			<td>700720</td>
			<td>Salmonella reference strain used as positive control.</td>
		</tr>
		<tr>
			<td>Trypticase soy broth (TSB)</td>
			<td>BD Diagnostic Systems, Sparks, MD</td>
			<td>211768</td>
			<td>Liquid growth medium used in the cultivation of Salmonella.</td>
		</tr>
		<tr>
			<td>Vortex mixer</td>
			<td>Scientific Industries, Inc., Bohemia, NY</td>
			<td>SI-0236</td>
			<td>Standard laboratory vortex mixer.</td>
		</tr>
		<tr>
			<td>Whirl-pak filter bag</td>
			<td>Nasco Sampling Brand, Fort Atkinson, WI</td>
			<td>B01318</td>
			<td>Filter bags to hold animal food samples for preenrichment.</td>
		</tr>
	</tbody>
</table>

loop-mediated isothermal amplification for screening salmonella in animal food and confirming salmonella from culture isolation

Loop-mediated isothermal amplification (LAMP) has emerged as a powerful nucleic acid amplification test for the rapid detection of numerous bacterial, fungal, parasitic, and viral agents. Salmonella is a bacterial pathogen of worldwide food safety concern, including food for animals. Presented here is a multi-laboratory-validated Salmonella LAMP protocol that can be used to rapidly screen animal food for the presence of Salmonella contamination and can also be used to confirm presumptive Salmonella isolates recovered from all food categories. The LAMP assay specifically targets the Salmonella invasion gene (invA) and is rapid, sensitive, and highly specific. Template DNAs are prepared from enrichment broths of animal food or pure cultures of presumptive Salmonella isolates. The LAMP reagent mixture is prepared by combining an isothermal master mix, primers, DNA template, and water. The LAMP assay runs at a constant temperature of 65 &#176;C for 30 min. Positive results are monitored via real-time fluorescence and can be detected as early as 5 min. The LAMP assay exhibits high tolerance to inhibitors in animal food or culture medium, serving as a rapid, reliable, robust, cost-effective, and user-friendly method for screening and confirming Salmonella. The LAMP method has recently been incorporated into the U.S. Food and Drug Administration&#8217;s Bacteriological Analytical Manual (BAM) Chapter 5.

Figure 2 and Figure 3 show representative LAMP graphs/tables displayed on both platforms. In this LAMP run, samples S1 to S6 are 10-fold serial dilutions of S. enterica serovar Infantis ATCC 51741 ranging from 1.1 x 106 CFU to 11 CFU per reaction. Positive control is S. enterica serovar Typhimurium ATCC 19585 (LT2) at 1.7 x 104 CFU per reaction and NTC is molecular grade water.
As shown in Figure 2E and Figure 3G, both NTC and PC wells are valid controls. The NTC well has blank Tmax while Tm is &#60; 83 &#176;C on the LAMP instrument panel and blank in the LAMP software, suggesting a negative result. The PC well has Tmax of 7 min 45 sec and Tm of ~ 90 &#176;C on both platforms, suggesting a positive result. Samples S1 to S6 have Tmax between 6 min 30 sec&#160;and 12 min 15 sec, all being Salmonella-positive.&#160;
Following duplicate runs of the same set of samples, the final LAMP results are reported for these samples. This representative LAMP run shows that LAMP successfully detects Salmonella with a wide range of concentrations in the samples.&#160;
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/61239/61239fig2v2.jpg" /> 
Figure 2: Representative LAMP results displayed on the LAMP instrument panel.&#160;(A) The Profile tab shows the programmed temperature profile. (B) The Temperature tab shows actual temperatures in the sample wells as LAMP reaction proceeds. (C) The Amplification tab shows fluorescence readings during LAMP amplification. (D) The Anneal tab shows changes in fluorescence (derivative) during the anneal phase. (E) The Results tab shows a tabular view of the LAMP results. <a href="https://www.jove.com/files/ftp_upload/61239/61239fig2v2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/61239/61239fig3v2.jpg" /> 
Figure 3: Representative LAMP results viewed in the LAMP software.&#160;(A) The Profile tab shows the programmed temperature profile. (B) The Temperature tab shows actual temperatures in the sample wells as LAMP reaction proceeds. (C) The Amplification tab shows fluorescence readings during LAMP amplification. (D) The Amplification Rate tab shows changes in fluorescence (fluorescence ratio) during LAMP amplification. (E) The Anneal tab shows fluorescence readings during the anneal phase. (F) The Anneal Derivative tab shows changes in fluorescence (derivative) during the anneal phase. (G) The Result tab shows a tabular view of the LAMP results. <a href="https://www.jove.com/files/ftp_upload/61239/61239fig3v2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Loop-Mediated Isothermal Amplification for Screening Salmonella in Animal Food and Confirming Salmonella from Culture Isolation at JoVE.com

Loop-Mediated Isothermal Amplification for Screening Salmonella in Animal Food and Confirming Salmonella from Culture Isolation

Loop-mediated isothermal amplification (LAMP) is an isothermal nucleic acid amplification test (iNAAT) that has attracted broad interest in the pathogen detection field. Here, we present a multi-laboratory-validated Salmonella LAMP protocol as a rapid, reliable, and robust method for screening Salmonella in animal food and confirming presumptive Salmonella from culture isolation.

Loop-Mediated Isothermal Amplification for Screening Salmonella in Animal Food and Confirming Salmonella from Culture Isolation

Loop-mediated isothermal amplification (LAMP) has emerged as a powerful nucleic acid amplification test for the rapid detection of numerous bacterial, ...

This Salmonella LAMP method has been incorporated within the FDA's Bacteriological Analytical Manual, both as a screening method in animal food and a confirmation method for any presumptive Salmonella isolates. The method is rapid, reliable, and robust. Compared to PCR, its main advantages include rapid testing, simple instrumentation, a low false-negative rate, and flexible workflow.Rapid screening for pathogens such as salmonella plays an important role in quickly detecting a potentially contaminated product and can prevent illnesses or outbreaks in humans and animals. Begin by preparing DNA templates for the LAMP assay. There are two types of samples used to make DNA templates.To prepare templates from animal food samples, aseptically weigh 25 grams of food sample into a sterile filter bag. Add 225 milliliters of sterile buffered Peptone water to the bag and incubate it at 35 degrees Celsius for 24 hours. After the incubation, transfer one milliliter from the filtered side of the bag to a micro centrifuge tube.Centrifuge the tube at 900 times G for one minute. Then transfer the supernatant to a new micro centrifuge tube. Centrifuge the sample at 16, 000 times G for two minutes.And to discard the supernatant, resuspend the pellet in 100 microliters of the sample preparation reagent and heat it at 100 degrees Celsius for 10 minutes in a dry heat block. Then, cool the sample to room temperature and store it at negative 20 degrees Celsius. The second type of sample for this LAMP assay comes directly from presumptive salmonella cultures.To prepare DNA templates, transfer 500 microliters of an overnight salmonella culture to a micro centrifuge tube, and heat it in a 100 degrees Celsius heat block for 10 minutes. Then, cool the sample to room temperature and store it at negative 20 degrees Celsius. To prevent cross-contamination, physically separate the areas used for preparing the LAMP Master Mix and adding the DNA templates.Clean the work surfaces with isopropanol and a DNA and DNA's degrading solution. Then, clean the pipettes and tube strip holders. Thaw the isothermal Master Mix, 10X Primer Mix, molecular grade water, positive control DNA, and DNA templates at room temperature.Turn on the LAMP instrument, and enter the relevant sample information. Prepare the LAMP Master Mix according to manuscript directions, vortex and centrifuge it briefly, and distribute 23 microliters of the Master Mix into each well of a tube strip. Vortex all DNA templates and centrifuge them briefly.Then, add two microliters of DNA template to the appropriate well, and cap the well tightly. Remove the tube strip from the holder, and flick it to ensure all reagents have pooled at the bottom of the tube. Load the tube strip into the LAMP instrument, and start the LAMP run.While the LAMP reaction is in progress, tap the Temperature, Amplification, and Anneal tabs to see the dynamic changes of various parameters during the LAMP run. The LAMP results can be viewed on the LAMP instrument panel in real time, or using LAMP software. To interpret the results on the instrument panel, open the LAMP run of interest.Observe the five tabs associated with each run. The Profile and Temperature tabs show programmed and actual temperatures in the sample wells as the LAMP reaction proceeds. The Amplification and Anneal tabs show fluorescence readings and changes in fluorescence during the Amplification and Anneal phases respectively.The Results tab shows the tabular view of the LAMP results that can be interpreted according to manuscripts directions. The second way to view LAMP results is using the LAMP software. To interpret the results with the software, click on the Computer icon on the left panel and navigate to the file location to load the LAMP run of interest.Observe the seven tabs associated with each run. The Profile and Temperature tabs show programmed and actual temperatures in the sample wells as the LAMP reaction proceeds. The Amplification and Amplification Rate tabs show fluorescence readings and changes in fluorescence during the Amplification phase.The Anneal and Anneal Derivative tabs show fluorescence readings and changes in fluorescence during the Anneal phase. The Results tab shows a tabular view of the LAMP results. There are four columns titled Graph Name, Well Number, Well Name, and Peak Value.The Amp Time and Anneal Derivatives of all eight samples are shown. The results can be interpreted according to manuscripts directions. The LAMP instrument panel and LAMP software can be used to display results of the assay.In this LAMP run, samples S1 through S6 are tenfold serial dilutions of Salmonella enterica serovar Infantis ATCC 51741, ranging from 1.1 million to 11 colony forming units per reaction. The positive control, or PC, is Salmonella enterica Serovar Typhimurium LT2 at 17, 000 colony forming units per reaction, and the no template control, or NTC, is molecular grade water. The NTC well has a blank Tmax and an Aneal temperature of approximately 81 degrees Celsius on the LAMP instrument panel, and a blank Tmax and a blank Anneal temperature in the LAMP software.The positive control well has a Tmax of seven minutes and 45 seconds and Annealing temperature of approximately 90 degrees Celsius on both platforms. Samples S1 to S6 have Tmax between six minutes and 30 seconds and 12 minutes and 15 seconds, and Annealing temperature of approximately 90 degrees Celsius, all indicating a positive detection. LAMP is very effective and generates a large quantity of DNA.So it's critical that best laboratory practices are used to prevent cross-contamination. Use similar practices when making your DNA templates, as enrichments from contaminated animal food can have high levels of salmonella. Animal food sample screening positive with LAMP should be confirmed by culture isolation following the procedures in the FDA's Bacteriological Analytical Manual.Negative samples can be reported as such. The incorporation of this LAMP method into the FDA's Bacteriological Analytical Manual paves away for wider application of this rapid, robust and user-friendly technology in food safety testing.

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