Rapid testing methods for Salmonella monitoring in food production

Leveraging rapid testing methods to screen for Salmonella in food-producing and processing plants  

Salmonella is a type of bacteria that can cause severe foodborne illness. It is vital to detect its presence in food production environments before it can contaminate end products. While Listeria monocytogenes is commonly consumed by most people in small numbers without causing harm, one Salmonella bacteria can be enough to cause severe reactions.  

When detecting Salmonella, the most established method is ISO 6579:2017. This protocol provides a standardized approach for detecting bacteria in various food types and matrices. The ISO testing process itself includes multiple steps: 

• sample enrichment, 
• selective agar plating to detect the presence of Salmonella.  

ISO 6579:2017 protocol is recognized internationally and considered highly accurate. However, it is time-consuming and requires more resources than rapid test methods. 

With rapid testing methods, we can correspond to ISO 16140, where results can be obtained within hours. Rapid tests often use immunological or molecular techniques, such as lateral flow devices or polymerase chain reaction (PCR). Such advantages compared to ISO 6579, as simplicity, ease of use, and ability to be performed anywhere, at any time, make rapid tests highly appealing to the industry.  

Comparing the different rapid methods available for Salmonella monitoring based on the time to result 

Historically, swab-based rapid tests have been in the market for the past two decades and utilize colourimetric or chromogenic endpoints. These tests involve incubating a swab and interpreting the results based on changes in colour, pH or other indicators. Unfortunately, these tests have yet to perform optimally in the market, with high false negatives and false positives rates. This caused the appearance of new rapid tests that claim to detect Salmonella effectively.  

When it comes to more traditional methods, such as using Petrifilm or enrichment with differential media, the industry is well-versed in detecting Salmonella in the environment, albeit with a longer incubation time of two to three days. Even though these tests are considered rapid, a presumptive positive result can still take up to 48 to 72 hours. Nonetheless, these methodologies have been used for 20 years, and the industry is familiar with them.  

Read also: The Reliable Alternative To Chromogenic Rapid Tests 

While the industry would like to see faster, simpler, and less expensive methods, they acknowledge that these tests can occasionally produce confirmed positive results. Below we gathered a list of rapid Salmonella testing methods based on the time to result, including the time required for sample collection, transportation to the laboratory, sample preparation, inoculation onto selective media, incubation, and confirmation of Salmonella growth. Results are graded from the slowest to the fastest. 

Polymerase chain reaction (PCR) 

Polymerase chain reaction (PCR) is a molecular biology technique for amplifying specific DNA sequences. In rapid Salmonella testing, PCR detects Salmonella in a food sample by amplifying a particular gene or sequence unique to the bacterium. The amplified DNA is then detected and quantified, producing a positive or negative result. PCR is a highly sensitive and specific method for Salmonella detection. It is widely used in food safety testing because it detects low bacterium levels in a sample. However, PCR also requires specialized equipment and trained personnel and is generally performed in a laboratory setting, further increasing the cost of testing.  

Time to result: PCR tests can be performed on-site or in a laboratory setting, and results can be obtained within 2-6 hours after sample enrichment and preparation (together with sampling and incubating, the total time to get results is 3-4 days). 

Enzyme-linked immunosorbent assay (ELISA) 

ELISA is a rapid diagnostic test that uses antibodies and enzymes to detect the presence of specific target molecules, such as Salmonella. It operates on the immunoassay principle and is commonly used for point-of-care testing. The test works by coating a plate with antibodies specific to Salmonella and adding the food sample. If Salmonella is present in the sample, it will bind to the antibodies on the plate, forming an antigen-antibody complex. The presence of the complex is then detected by adding a secondary antibody conjugated to an enzyme, which generates a visible signal.  

Time to result: ELISA method requires sample collection and transportation to the laboratory. Results can be obtained within 3-6 hours after sample preparation and testing (together with sampling and incubating, the total time to get results is 3-4 days). 

Loop-mediated isothermal amplification (LAMP) 

LAMP is a highly sensitive and specific test that can detect Salmonella in a short amount of time. It is a molecular biology technique used for amplifying DNA and is performed under constant temperature conditions without needing temperature cycling. LAMP has the advantage of being simple, rapid, and suitable for field use. In addition, it can be performed onsite, eliminating the need for laboratory analysis and reducing the time it takes to obtain results. However, LAMP requires specialized equipment and training to perform accurately.  

Time to result: LAMP testing can be performed on-site or in a laboratory setting, and results can be obtained in about 2-4 hours after sample collection and preparation. 

Lateral flow immunoassay (LFIA) 

Lateral flow immunoassay (LFIA) is a rapid diagnostic test that utilizes antibodies and visual indicators to detect the presence of a specific target molecule, such as Salmonella. It operates on the principle of immunochromatography and is commonly used for point-of-care testing. The test works by flowing a sample, such as a food sample, over a strip coated with antibodies. If Salmonella is present, it will bind to the antibodies on the strip and generate a visible signal, indicating a positive result. This test is simple, quick, and does not require specialized equipment, making it suitable for food production facilities. However, it is less sensitive than PCR and may produce false negatives.  

Time to result: LFIA can be performed on-site or in a laboratory setting, and results can be obtained within 20-30 minutes after sample collection and preparation. 

N-Light^TM Salmonella Risk rapid test 

NEMIS Technologies AG developed the N-Light^TM Salmonella Risk detection method for detecting potential Salmonella contamination in a food producers’ environment. The N-Light^TM method is suitable for use in food processing areas and equipment as part of an environmental monitoring program. Using hand tools: Bench Top Luminometer, Dry Block Heater, N-Light^TM Test Salmonella Risk and Sterile Dry Swab with separate PBS Buffer, it takes only 5 easy steps to perform any pathogen screening directly in the factory. NEMIS developed a unique proprietary enrichment broth containing a mix of bacteriophages to eliminate competing microflora and grow the targeted pathogens selectively. Compared to LAMP and LFIA, N-Light^TM Salmonella Risk is more sensitive than any chromogenic method. 

Time to result: N-Light^TM Salmonella Risk is one of the fastest methods available in the market as the results can be obtained within 24 hours (if, to be more exact: 24 hours + 3 minutes + 10s.). 

Conclusion 

In conclusion, the cost-effectiveness of rapid testing methods will depend on the specific needs of the food production facility. While rapid testing methods have the potential to be more cost-effective than traditional methods, this will depend on factors such as the level of accuracy required, the cost of the test, and the ease of use. Therefore, food production facilities should carefully consider these factors when choosing a testing method to ensure they get the best value for their money.