Studies on PCR-Based Rapid Detection Systems for Salmonella spp.

1.1 Using molecular biological methods to identified Salmonella spp. As the industrialization had made food supply to exceed demand, more and more consumers were interested in ‘Delicious food’ than ‘Good quality food’ today and they ask for not only taste but also safety. Microbiological safety is one of the most critical factors for ensuring safe food supply. Fig 1. is a flow chart of traditional detection procedure for Salmonella spp., showing that it takes about one week. Therefore, a rapid screening method using PCR is the basis of most molecular diagnostic laboratories. As the field of molecular pathology becomes more accessible to practicing pathologists, a working knowledge of PCR techniques is necessary related to biological safety of food is very essential in food industry. Salmonella spp. are an important cause of food-borne infections throughout the world, and the availability of rapid and simple detection techniques is critical for the food industry.


Using molecular biological methods to identified Salmonella spp.
As the industrialization had made food supply to exceed demand, more and more consumers were interested in 'Delicious food' than 'Good quality food' today and they ask for not only taste but also safety. Microbiological safety is one of the most critical factors for ensuring safe food supply. Fig 1. is a flow chart of traditional detection procedure for Salmonella spp., showing that it takes about one week. Therefore, a rapid screening method using PCR is the basis of most molecular diagnostic laboratories. As the field of molecular pathology becomes more accessible to practicing pathologists, a working knowledge of PCR techniques is necessary related to biological safety of food is very essential in food industry. Salmonella spp. are an important cause of food-borne infections throughout the world, and the availability of rapid and simple detection techniques is critical for the food industry. www.intechopen.com Present commercial detection system for Salmonella spp. can be classified into four categories. The first, traditional method which uses culture medium and observe colony morphology formed on it. This requires at least four days and experienced skill to perform biological tests, but it is the only common method authorized throughout the world for now. The second, Enzyme-Linked Immuno-Sorbent Assay (ELISA) detects certain bacteria using immune reaction between antibody and antigen specific for them. This method is easy to use because it makes color change or forms lines but it can be applied only for those which has specific toxin protein and requires more than 10 6 CFU / ml for detection which needs 16 hours of incubation. The third, Adenosine triphosphate (ATP) detection kit detects level of bacterial contamination by the amount of ATP in sample. This method can not be used for identification of bacteria because it can only tell including the total amount of ATP from food. This is usually used for comparing hygiene level before and after washing. The fourth, genetic method which is based on PCR is highly specific and sensitive enough to detect 100 CFU / ml of bacteria, but at the same time it can detect even the dead cells after processing or cooking food because of the high sensitivity.

Advanced PCR technologies 1.2.1 Multiplex PCR
Multiplex PCR can amplify two or more amplicons in a single PCR reaction. For multiplex PCR, each primer set is designed to amplify its target gene and make a PCR product of certain size to the target gene. To perform a multiplex PCR, the concentration of primers, Mg 2+ , free dNTPs and polymerase must be optimized to allow synthesis of the genes of interest, And also the PCR reaction temperature parameters must be optimized to the best average for amplicon production for all primer sets. This technique saves time and labor more than one target DNA sequence can be detected in each reaction, It might not be optimal if the PCR products are limited in certain sizes and agarose gel staining with ethidium bromide (John Maurer, 2006). Therefore, it is possible to detect multiple pathogens in a sample with a single PCR test (Panicker et al., 2004)

Real-Time PCR
Real-Time PCR technology is based on the ability of detection and quantification of PCR products, or amplicons, as the reaction cycles progress. Higuchi and colleagues introduced this technology (Higuchi et al., 1993) and it became possible by including of a fluorescent dye that binds to the amplicon as it is made (Fig. 2. A). Initially, a fluorescent dye, SYBR green I (A), was used to detect the amplicons. SYBR green I binds the double stranded, DNA amplicon and fluorescences upon illumination with UV light. In TaqMan PCR (B), the oligoprobe contains a fluorescent marker and chemical group that quenches fluorescent of oligoprobe until the dye is liberated by 3' exonuclease activity of the Taq DNA polymerase (Source http://cafe.naver.com/solgent.cafe?iframe_url= /ArticleRead.nhn%3Farticleid=38&) In TanMan PCR, an intact, "internal" fluorogenic oligoprobe binds to target DNA sequence, internal to the PCR primer binding sites. This oligoprobe possesses a reporter dye that will fluorescence and a suppressor dye known as quencher that prevent fluorescent activity via Fluorescence Resonance Energy Transfer (FRET). After each PCR cycle, when the doublestranded DNA products are made, a measure of fluorescence is taken after the fluorogenic probe is hydrolytically cleaved from the DNA structure by exonuclease activity of the Thermus aquaticus DNA polymerase (Heid et al., 1996;Holland et al., 1991). Once cleaved, the probe's fluorescent activity is no longer suppressed (Fig. 2. B). FAM (6-Carboxyfluorescein) and TAMRA(6-Carboxy-Tetramethyl-Rhodamin) are most frequently used as reporter and quencher, respectively. This PCR is often referred to as 5' exonuclase-based, real-time PCR or TaqMan PCR (Mullah et al., 1998).

Isothermal PCR
Recently, Jung et al. (2010) developed a new highly sensitive and specific isothermal amplification and detection system called isothermal target and probe amplification (iTPA) by employing DNA-RNA-DNA chimeric primers and a FRET (fluorescence Resonance Energy Transfer) probe. The iTPA method is based on a combination of novel isothermal chain amplification (ICA) and FRET cycling probe technology (CPT) (Fig. 3).

B)
www.intechopen.com In the ICA method, which relies on the strand displacement activity of DNA polymerase and the RNA-degrading activity of RNase H, two displacement events occur in the presence of four specially designed primers that lead to high specificity for the target sequence. In the CPT method, a DNA-RNA-DNA chimeric probe is hybridized with the target DNA, and the RNA region of the duplex is specifically cleaved by RNase H. The cleaved probe fragment is disassociated from the target DNA and another intact probe is again hybridized and then cleaved. In this cycling event, a single target DNA molecule results in a large number of cleaved probe fragments, which can be designed to generate fluorescence signals ).

Various PCR methods approaches for the detection of Salmonella spp.
2.1 Rapid and simultaneous detection of five pathogenic bacteria by a novel multiplex PCR assay: Salmonella spp., Escherichia coli O157:H7, Listeria monocytogenes, Staphylococcus aureus and Vibrio parahaemolyticus According to Centers for Disease Control (CDC), about 5 millions food mediated diseases are killing 4,000 people every year. Salmonella was the most frequently found pathogenic bacteria in food poisoning : 1 ~ 4 millions of people were infected, 2,000 (0.1%) of them were dead. Salmonella is an important pathogen associated with bacterial foodborne outbreaks in the United states, accounting for 24% of all food outbreaks and 18% of produce-related outbreaks between 1990 and 2009 (Center for Science in the Public Interest, 2009). An outbreak in 2009 associated with Salmonella-contaminated peanut butter and peanut containing products caused nine deaths in 46 states as of 17 March 2009. This outbreak led to the largest recall of food items in the United States resulting in over 2100 products being voluntarily recalled by more than 200 companies (FDA, 2009). Recently, more than 500 million eggs were recalled after dangerous levels of Salmonella were detected in the eggs from two Iowa producers, who distributed the eggs in 14 US states. Nearly 2000 illnesses were reported between May and July 2010 (CDC, 2010). Food poisoning by E. coli O157:H7 broke out in 10000 people, 300 of them were dead. As for Listeria monocytogenes, 1500 people were infected and 400 were dead. This shows that stock farm products which were contaminated by these four bacteria (E. coli O157:H7, Salmonella spp., Listeria monocytogene and Staphylococcus aureus) is seriously threatening consumer's health. In korea, 50% of food poisoning are caused by meat or processed meat products, and Salmonella strains (50%), S. aureus (20%) are two major sources. Different molecular targets have been used to characterize the presence of food-borne pathogenic bacteria. In this study, genes encoding the virulence determinants and their expression regulator have been used to characterize numerous bacteria. A molecular test based on the detection of shiga-like toxin (verotoxin type II), femA (cytoplasmic protein), toxR (trans-membrane DNA binding protein), iap (invasive associative protein), and invA (invasion protein A) genes has been applied for identification of E. coli O157:H7 (Jinneman et al., 2003;Kaneko et al., 2001;Karpman et al., 1998;Schmidt et al., 1995;Wang et al., 2002), Staphylococcus aureus (Mehrotra et al., 2000), Vibrio parahaemolyticus (Karpman et al., 1998;Cabrera-Garcia et al., 2004), Listeria monocytogenes (Bubert et al., 1992;Bubert et al., 1999;Volokhov et al., 2002), and Salmonella spp. (Chiu et al., 1996). To our knowledge, there is not a single acceptable method which is available to detect these five food-borne pathogenic bacteria simultaneously in food samples. The objective of the present work, therefore, was to establish a multiplex PCR assay method to detect the specific bacterial genus simultaneously and to analyze their distribution in contaminated foods. Our results indicate, that this method is rapid and specific for the simultaneous detection of E. coli O157:H7, Staphylococcus aureus, Vibrio parahaemolyticus, Listeria monocytogenes and Salmonella spp.

Materials & methods [bacterial strains]
Bacterial strains were obtained from the American Type Culture Collection (ATCC; Manassas, Va.), the Korean Collection for Type Culture (KCTC; Daejeon, South Korea), and the Korean Culture Center of Microorganisms (KCCM; Seoul, South Korea), Also the strains isolated from various food samples were used in this study (Table 1).
All bacterial strains were grown on Luria-Bertani broth (LB; Bactopeptone 10 g, Yeast extract 5 g, and NaCl 10 g, each per Liter) at 37°C. All Vibrio species were grown in LB broth with supplementary 2% sodium chloride. Cultures were grown in LB, and a population of visible microorganisms was obtained by plating 10-fold serial dilutions of broth cultures on to plate count agar (Difco, Sparks, USA) and incubating the plates at 37°C for 16 hours. At each sampling dilution ratio, all bacterial cultures were mixed, and 100 l (approximately 10 7 CFU) of the suspension was used as DNA templates for PCR.

Strain
Source

[Enrichment procedures for detection of food-borne microorganisms]
All food-borne pathogens were grown for 16 hours in LB broth at 37°C in a shaking water bath. Cells were diluted from 1:10 to 1:10 8 in 10 ml of Luria-Bertani broth and manipulated as described above to make approximate cell count from 10 to 10 8 CFU / ml. In each dilution ratio, single enrichment broth samples (1 ml) were collected into 1.5 ml micro-centrifuge tubes and used for DNA extraction (Fig. 1).

[Extraction and preparation of DNA templates for PCR assay]
Individual samples (1 ml) were centrifuged at 10,000 X g for 3 min. The cell pellets were resuspended in RNase free water (100 l) and placed in a 100°C heating block for 20 min. The samples were cooled for 2 min at room temperature and centrifuged at 16,000 X g for 5 min. The supernatant fluids (5 l) were used to make 25 l of a multiplex PCR reaction mixture, which included 5 l of 5 X reaction buffer (2.5 mM MgCl 2 and 0.8 mM concentration of each dNTP), 4 l of the primer mixtures of the five food-borne bacteria, 1 l of Super Taq plus polymerase (Rexgene Biotech., Cheongwon, South Korea), and 10 l of DNase free water in a single tube. The multiplex PCR was run for 35 cycles on a Tpersonal cycler (Whatman Biometra, Goettingen, Germany) under the following conditions : denaturation at 94°C for 30 sec, primer annealing at 60°C for 30 sec, and extension at 72°C for 30 sec. The final cycle included an additional 5 min of extension time at 72°C. A 5 l aliquot of the reaction mixture was then electrophoresis on a 2% agarose gel electrophoresis in 0.5 X Tris-borate buffer at 100 V for 25 min. The amplification products were stained with ethidium bromide and visualized by UV trans-illumination.

[Oligonucleotides]
The oligonucleotide primers designed with Primer 3.0 software (Whitehead Institute, Cambridge, Mass.) were based on sequences obtained from Genbank and were used to amplify chromosomal DNA for the five food-borne pathogens (

[Specificity of the primer pairs and the multiplex PCR]
To evaluate the specificity of each oligonucleotide primer pair for its target gene, a PCR assay was carried out by testing all the reference strains reported in Table 2.1. The multiplex PCR was developed specifically and efficiently using amplified reactions and the same PCR program. The reaction was performed in a total volume of 25 l that contained 5 to 15 l (50 ng) of template.

[Food sample processing and multiplex PCR assay]
A sample of ham (CJ, Seoul, South Korea) from the Korea Food & Drug Administration was used for all tests. Equal concentration of the bacteria were used for inoculation of the ham. E. coli O157:H7, Staphylococcus aureus, Listeria monocytogenes, V. parahaemolyticus and Salmonella typhimurium were inoculated either single or as two or three species simultaneously. Media bottles (500 ml) containing 25 g of crushed ham were inoculated with bacteria at 100 CFU of each species alone or with 2 X 10 3 CFU for inoculation of the three species together. inoculated ham was vigorously mixed by shaking for about 30sec to distribute the bacteria. After inoculation, 225 ml of freshly made LB broth was added to each bottle containing ham. To suspend the bacteria, the bottles were shaken for 10 min at 200 rpm and then incubated at 37°C for 16 hours (Kim et al., 2006). Raw pork was also processed as described method above. The five bacterial species were inoculated simultaneously in raw pork. Water and milk were directly inoculated with five strains; 1 ml of medium containing each strain was added to 9 ml of water and milk and diluted 10 times from 1:10 to 1:10 8 .

[Multiplex PCR assay]
Five PCR products of different sizes were amplified simultaneously from five food-borne pathogenic bacteria with the multiplex PCR assay used in this study (Fig. 2). For all of the bacteria tested, the optical density (absorbance value) at 600 nm was 0.010 and 0.080. The different sizes of the amplification products allowed rapid and specific discrimination of Vibrio parahaemolyticus, Salmonella spp., Staphylococcus aureus, E. coli O157:H7 and L. monocytogenes. The annealing temperature, extension time, and primer concentrations used in this multiplex PCR assay were optimized. The PCR products were separated by agarose gel electrophoresis, and the negative controls used with the multiplex PCR produced negative results. Using the multiplex primers, another single amplification was conducted to confirm the chromosomal DNA from samples contaminated with single specific pathogenic bacteria. In the multiplex PCR with mixed DNA samples, five different bands of specific sizes corresponding to the target genes (Table 2) were detected simultaneously after amplification of the contents of a single tube (Fig. 2 [Specificity and sensitivity for selected primer sets] The sensitivity and specificity of the PCR assay were evaluated with 67 food-borne pathogenic bacteria (Table 1). Fig. 3 shows the result of amplification from a representative sample of Salmonella spp. The multiplex primer is highly specific for the five pathogenic bacteria target sequence; all Salmonella serovars tested produced amplicons of the expected size (678 bp) without spurious priming and without cross-reactivity with non-Salmonella species. Results for the other four bacterial species also highly specific (data not shown). Fig.  4 illustrates the detection sensitivities of the multiplex PCR assay, which were evaluated using whole cell cultures of S. choleraesuis KCCM41035 and S. bongori KCCM41758, cell cultures diluted 10-fold from 1:10 to 1:10 8 were tested. Based on these results, the multiplex PCR assay detection limits were approximately 10 5 CFU / ml. Detection results for the other four bacteria with this assay were similar (data not shown).  non-autoclaved ham incubated with two or three organisms together, the sensitivity was the same as that achieved when the pathogens were evaluated singly (Fig. 5). Without enrichment of the bacterial culture, the detection limits after inoculation of non-autoclaved ham with E. coli O157:H7, S. aureus and L. monocytogenes together were 20,000 cells, respectively (data not shown This multiplex PCR assay offers the advantages of significantly short processing time and saving cost. Only one composite DNA sample is required rather than separate samples for each target gene to be analyzed (Kim et al., 2006). To test the efficacy of this PCR assay for detecting pathogenic bacteria in food, Salmonella typhimurium ATCC19585 (10 CFU / g of food) was inoculated into samples of selected foods (milk, raw pork and raw chicken) that had been previously screened for detectable pathogenic microbial contamination. The inoculated samples were then incubated aerobically at 37°C for 8 hours (enrichment culture step). The PCR assay detected bacterial cells in all inoculated samples. However, when a 5 hours instead of 8 hours enrichment culture step was used, no bacteria were detected (data not shown). Thus, our PCR assay requires at least 8 hours of enrichment to detect the added pathogenic bacteria in these foods with a detection sensitivity ranging from 10 to 100 CFU / g. Therefore, the enrichment step is required in this PCR protocol for detection of food-borne pathogenic bacteria. The five specific primer sets tested for Vibrio parahaemolyticus, Salmonella spp., Staphylococcus aureus, E. coli O157:H7 and L. monocytogenes can be used specifically and simultaneously. These five food pathogens were clearly detected from both culture medium artificially inoculated water, milk and raw pork. Thus, the protocol developed in this study could have important application for the rapid and simultaneous detection and identification of up to five food-borne pathogenic bacteria in many foods. This simple method is expected to enable rapid risk assessment of pathogen contamination of foods at a low cost. The cost of detection could be reduced from the $ 50 (approximately $ 10 per pathogen) for the traditional method to less than $ 1 for this multiplex PCR method.

The development of rapid real-time PCR detection system for Salmonella spp.
Previously, we developed multiplex conventional PCR assay from the conventional PCR methods . Conventional PCR methods for the detection of food-borne bacterial pathogens are time consuming and insensitive that it can not provide adequate screening of samples for the presence of potential pathogens. With the advent of automated real-time PCR suspected food-borne contaminants can be detected in less than an hour. This technique, using TaqMan PCR, has been successfully adapted for the detection of pathogenic bacteria, including Salmonella enterica, Listeria monocytogenes, Escherichia coli O157:H7, and Yersinia pestis (Bassler et al, 1995;Bellin et al, 2001;Higgins et al, 1998;Hoorfar et al, 2000;Jothikumar et al, 2002;Knutsson et al, 2002;Oberst et al, 1998;Sharma et al, 1999). Can there be a better method which has the same sensitivity with nested PCR and can be performed with one PCR reaction? It will be more effective if there is more sensitive optical instrument and staining dye which can detect very small amount of product than naked eyes and EtBr. Micro PCR, which was developed for this purpose, uses real-time PCR machine as a detector and SYBR Green reagent as a staining dye. Real-Time PCR is currently used for the diagnosis of Escherichia coli strain O157:H7 (Ibekwe et al, 2002) and Plesimonas shigelloides (Loh et al, 2001) in stool specimens. To develop micro PCR, following factors were studied. First, selection of specific primers; primer size (17~25 mer), hybridization ability, secondary structure within primer, GC content (40~60%), melting temperature (Tm) (55~65°C). Second, factors affecting Tm; product size, GC contents of product. Third, effect of commercial SYBR Green reagent; Takara, A&B, Qiagen and in house reagent. Forth, product size; 60, 100, 200, 300, 400, 500, 600 and 700bp Fifth, condition of PCR. To enable s i m u l t a n e o u s d e t e c t i o n , e a c h P C R p r o d u c t s w e r e d e s i g n e d t o h a v e d i f f e r e n t m e l t i n g temperature, at least 2°C apart from each other (Fig. 1). Fig. 9. Strategy for the development of SYBR Green I real-time PCR detection system.

[Oligonucleotides ]
The oligonucleotide primers were designed by Primer version 3.0 software (Whitehead Institute), referring to Genbank in order to amplify a chromosomal DNA of Salmonella spp. (

[Detection studies with diversity food samples]
Before inoculating into food, five bacterial strains were incubated in 5 ml LB broth for overnight. 100 l of each culture broth was inoculated in 25 g of each food material. These food samples were then mixed with 225 ml of LB broth and incubated for overnight. Food segments in sample solution must be removed before assay because PCR can be inhibited by them. Among the 10 samples, water contaminated with Salmonella spp. were analyzed without any pre-treatment, other samples were filtered through gauze before assay. 1 ml of each prepared sample solution was transferred to 1.5 ml e-tube and centrifuged at 12,000 rpm for 10 min. The supernatant was removed and the pellet was re-suspended in 500 ml deionized water. Centrifugation and re-suspension in deionized water was performed one more time for exact assay. 150 l of Deionized water and 50 l of 10% chelex resin was added to the pellet and mixed throughly. The solution was heated at 100°C for 10 min, and centrifuged at 12,000 rpm for 10 min. 5 l aliquot of this solution was taken for SYBR Green I assay. PCR using ABI 7500 (Perkin-elmer, USA) was cycled 35 times as follows: 30 sec denaturation at 94°C, 30 seconds annealing at 60°C, and 30 seconds polymerization at 72°C. The products of real-time PCR were run on 2% agarose gel electrophoresis and melting curves were acquired on the SYBR channel using a ramping rate of 1°C / 30 seconds for 60 ~ 94°C.

[Primers design and specificity]
Fig. 2 is the result of PCR reaction performed with various primer sets which are designed for Salmonella spp. Specific PCR reaction was observed with primer sets of which product sizes are 60 bp, 284 bp and 678 bp, respectively. However, primer sets of 137 bp, 330 bp and 551 bp showed non-specific products in the place of negative control, meaning that these primers are not available. As for the primers of 424 bp, bacterial DNA was not amplified. Therefore, primer sets of 60 bp, 284 bp, and 678 bp were selected.

[Real-time PCR system]
It will be more effective if there is more sensitive optical instrument and staining dye which can detect very small amount of product than naked eyes and EtBr. Micro PCR, which was developed for this purpose, uses real-time ABI 7500 PCR machine as a detector and SYBR Green I reagent as a staining dye. First, selection of specific primers; primer size (17~25 mer), hybridization ability, secondary structure within primer, GC contents (40~60%), melting temperature (Tm) (55~65°C). Second, factors affecting Tm; product size, GC content of product. Third, effect of commercial SYBR green reagent; Takara, A&B, Qiagen and in house reagent. Fourth, products size; approximately 60, 100, 200, 300, 400, 500, 600 and 700 bp. Fifth, running conditions of real-time PCR. to enable simultaneous detection, each PCR products were designed to have different melting temperature, at least 2°C apart from each other. As shown in Fig. 3 primer of 60 bp showed two peaks of positive and negative control at the same position. Primer of 284 bp did not showed non-specific products at all but it was also unavailable because the peak of positive control was too weak. The Tm value was 86.8°C. Only the primer of 678 bp was proved to be available. Although a weak undesired peak was appeared beneath 75°C, it is ignorable because its temperature is sufficiently low. The Tm value was measured as 86.7°C. Therefore, the primer of 678 bp is finally selected for SYBR Green I system. A. 60 bp To apply real-time SYBR Green I system to food, various food samples were artificially contaminated by the Salmonella and incubated for enrichment. After overnight enrichment, Salmonella spp. gave positive PCR reaction. No signals were observed in negative (un-inoculated) controls. As shown in this result, no other strains but the inoculated Salmonella enteritidis was detected (Fig. 4). In conclusion, the SYBR Green I PCR assay combined with DNA extraction using boiling method offers rapid and nonsequence-specific detection of amplicons. The minimum detection limit was 10 cells / ml with pure culture, which is far more sensitive than conventional PCR which has detection limit of 10,000 cells / ml. In conclusion, we developed a highly sensitive and specific real-time PCR assay for detection of the five food-borne pathogenic bacteria in food samples. This newly developed assay was successfully used to monitor the dynamics of this novel bacterium in food (Abu et al, 2005).

Rapid and simple detection of invA gene in Salmonella spp. by isothermal target probe amplification (iTPA)
Nucleic acid amplification methods are widely used for detection of food-borne pathogens and the PCR is the most popular and useful method, requires a high precision thermal cycling instrument, which often prevents PCR from being used in routine food pathogen detection by the food industry. Recently, Jung et al. (Jung et al. 2010) developed a new highly sensitive and specific isothermal amplification and detection system called isothermal target and probe amplification (iTPA) by employing DNA-RNA-DNA chimeric primers and a FRET probe. The iTPA reaction is done under isothermal conditions between 55 and 65°C using a simple incubator such as a water bath or block heater which is sufficient for amplification. The detection of the fluorescence signal is acquired directly from the amplification reaction tube without any post-amplification handling that reduces the risk of any amplicon-carryover cross-contamination. The iTPA assay is highly specific for the target sequence because the primers and probe recognize five distinct regions on the targeted DNA. The iTPA method is based on a combination of novel ICA (isothermal chain amplification) and FRET cycling probe technology (CPT). In the ICA method, which relies on the strand displacement activity of DNA polymerase and the RNA degrading activity of RNase H, two displacement events occur in the presence of four specially designed primers that lead to powerful amplification of target DNA. Since the amplification is initiated only after hybridization of the four primers, the ICA method leads to high specificity for the target sequence. In the CPT method, a DNA-RNA-DNA chimeric probe is hybridized with the target DNA, and the RNA region of the duplex is specifically cleaved by RNase H. The cleaved probe fragments are disassociated from the target DNA and another intact probe is again hybridized and then cleaved. In the cycling events, a single target DNA molecule results in a large number of cleaved probe fragments, which can be designed to generate fluorescent signals (Fig. 1). In the present study, a sensitive and specific iTPA assay for detecting Salmonella spp. in experimentally inoculated food samples was developed.

[iTPA primers, FRET probe, and reaction conditions]
The Salmonella invA gene (GenBank: EU348369) was used as the target for iTPA primer and probe design. Four primers, two outer and two inner, and one FRET probe which recognized five distinct regions of the target sequence were designed using the DNASTAR software (Maison, WI). Oligonucleotide sequences and locations of the primers and the probe are shown in Table 1 , and 2 l l of DNA template (2 l of sterilized water was used for a negative control). The iTPA reaction mix was incubated at 58°C for 60 min in a water-bath and then cooled to room temperature. After a quick spin-down, the reaction tube was inserted into a RF-1000 fluorescent reader (Raplegene, Inc., Sungnam, South Korea) to read the relative RFU (fluorescence relative unit) signal. The RF-1000 fluorescent reader calculates the F-score and it is display on the LCD window. The result was interpreted as a Salmonella spp. positive if the F-score was ≥35 or a Salmonella spp. negative if the F-score was < 35. This cut-off value was determined using uninoculated food samples that had also undergone cultural pre-enrichment. F-scores of uninoculated egg yolk samples and chicken meat samples were 20±9.5 and 20±7.2 respectively (p≤0.001). The equation used to calculate the F-score is the following: F-score = [(fluorescence of the sample -fluorescence of the negative control) / fluorescence of the negative control] x 100. Sterilized water was used for the negative control reaction instead of the extracted nucleic acid from inoculated food samples. Since uninoculated food samples may be contaminated, we used sterile water as the negative control.

Name
Sequence ( Table 4. iTPA primers and FRET probe used in this study to detect Salmonella spp.

[iTPA specificity and detection limits]
Fifty bacterial strains were used to determine the iTPA specificity. DNA templates were prepared from bacterial cultures and aliquots were subjected to the iTPA assay. For Salmonella strains, the genomic DNA was isolated from the overnight culture grown in LB www.intechopen.com medium and then quantified. One picogram of genomic DNA was used as the template. For non-Salmonella strains, the genomic DNA was isolated from the overnight culture grown in LB medium and then 2 L of the DNA extraction TE buffer solution was used. Specificity tests were repeated 10 times. To determine iTPA detection limits, serial 10-fold dilutions of a mid-log phase S. typhimurium KCTC2515 culture (ca. 10 8 CFU) grown in LB broth were prepared in PBS and quantified using the standard plating method. The detection limit tests were repeated 10 times and the lower limits of detection (CFU per assay) were reported.

[iTPA testing in experimentally inoculated food samples]
Three kinds of foods were used for the study: peanut butters, egg yolk and chicken breasts. Food samples were processed as described in a previous study  with slight modifications. Briefly, A 500 ml Erlenmeyer flask (LB broth 225 ml) containing 25 g of chicken breast was incubated at 37°C overnight and then 9 ml of this solution was transferred into a 10 ml conical tube followed by adding 1 ml of inoculated buffered peptone water of Salmonella spp.(1.0 x 10 9 CFU / ml) to prepare a stock solution. Plastic food bags containing 25 g of chicken breast were inoculated with 1 ml of serial dilutions (1:10 to 1:10 8 ) of the stock solution and vigorously mixed using a homogenizer (Pro-media SH-001, ELMEX Ltd., Tokyo, Japan) for about 30 sec to distribute the bacteria followed by adding 225 ml of freshly made LB broth to prepare pre-enriched solutions. The sample preparations for peanut butters and other food samples were the same except that for peanut butters which required an additional washing with a washing solution (0.05% NaOH, 0.5% Tween 20 in PBS buffer solution) due to the high viscosity. 100 L of the preenriched solution was mixed with the washing solution and centrifuged at 10,770 x g for 5 min followed by discarding the supernatant. The pellet was washed with 100% ethanol and then with TE buffer solution twice. The washed pellet was suspended in 200 L of TE buffer solution and heated at 100°C for 10 min in a dry heating block. The crude cell lysate was centrifuged at 10,770 x g for 5 min and an aliquot (2 L) of the supernatant was used for the iTPA assay. For negative samples, the same amount of aliquot (2 L) of uninoculated food samples that had also undergone cultural pre-enrichment was used. The inoculated food sample tests were repeated 10 times and the lower limits of detection (CFU per assay) were reported.

[Inclusivity and exclusivity of the iTPA assay]
The Salmonella spp. invA-based iTPA assay, which required only a water bath and the RF-1000 fluorescent reader successfully detected 10 Salmonella spp. strains while showing negative results for 40 non-Salmonella spp. strains (Table 1), indicating that the invA-based iTPA assay was specific for Salmonella spp.. The PCR assay using iTPA outer primers yielded amplicons of the expected size (117 bp) for all 10 Salmonella spp. strains. (data not shown) Two sample t-tests were performed for negatives and positives in pure culture. The mean F-score of the negatives was 3.97±0.44 and the mean F-score of the positives was 82.9±6.1 (p≤0.001, data not shown). For a rigorous exclusivity comparison, the positive strains were used at a low concentration of the genomic DNA (1pg, ca. 10 2 CFU) as the template while the negative strains were used at very high concentration of the genomic DNA (ca. 10 5 CFU). Neither false positive nor false negative results for the 50 bacterial strains were observed by the iTPA assay using two primer sets and a FRET probe, indicating good specificity. (Table 1)

[Detection limits of the iTPA assay]
The detection limits of the iTPA assay using serial in S. Typhimurium strain were determined. KCTC2515 were determined and the lowest number of cells detected was 4 x 10 1 CFU per iTPA reaction (

cells in inoculated food samples]
The detection limits of Salmonella spp. inoculated in three food samples are shown in Table  3. In inoculation experiments, the invA-based iTPA assay using the serial dilution platforms consistently detected at an initial inoculums level of less than 10 CFU in the pre-enriched food samples (egg yolk, chicken breast, and peanut butter). In Table 4 the results of the Fscore measurement for the iTPA reaction for 60 min at 58°C in the artificially contaminated samples are shown. For statistics, one-way analysis of variance (ANOVA) test was performed (Clarke and Cooke 1998). The mean F-scores for uninoculated peanut butter, egg yolk and chicken breast were 14±4.7, 20±10, and 20±7.2, respectively. The mean F-scores for inoculated peanut butter, egg yolk and chicken breast were 87.34±30.24, 59.09±36.16 and 68.24±26.33(p≤0.001), respectively. The lowest detection limit achieved in this study was a less than 10 CFU per 25 g of food samples.
In this study, we designed a set of DNA-RNA-DNA chimeric primers and a FRET probe to specifically target the Salmonella spp. invA gene. So, a novel and rapid DNA detection system has been developed which we have termed isothermal target and probe amplification (iTPA). By simultaneously utilizing the dual amplification powers of the target DNA and FRET probe, we have demonstrated that iTPA can be used to rapidly detect less than 10 CFU of Salmonella spp. in food samples after pre-enrichment. The four chimeric primers and one FRET probe were designed from five regions of the Salmonella spp. invA gene coding sequence that are highly specific for Salmonella spp. (Table 1). In conclusion, we have developed a DNA detection system which is conveniently performed by requiring only a water bath and a fluorometer and has great potential in applications for hand-held or point-of-care-testing (POCT) diagnostics. The invA-based iTPA assay developed in this study is a specific, sensitive, and rapid method for the detection of Salmonella spp. in food samples. This simple method is expected to enable a rapid risk assessment of pathogen contamination of foods at a low cost.  Food samples Dilution No. of Bacteria # (CFU) F-score Peanut butter Egg yolk Chicken meat 10 -5 10 -6 10 -7 10 -8 10 -9 10 -5 10 -6 10 -7 10 -8 10 -9 10 -5 10 -6 10 -7 10 -8 10 -9 1.7±0.4 x 10 3 1.7±0.4 x 10 2 1.7±0.4 x 10 1 1.7±0.4 x 10 0 1.7±0.4 x 10 -1 un-inoculated 1.7±0.4 x 10 3 1.7±0.4 x 10 2 1.7±0.4 x 10 1 1.7±0.4 x 10 0 1.7±0.4 x 10 -1 un-inoculated 1.7±0.4 x 10 3 1.7±0.4 x 10 2 1.7±0.4 x 10 1 1.7±0.4 x 10 0 1.7±0.4 x 10 -1 un-inoculated  Table 6. Detection limits for inoculated food samples by iTPA assay