Salmonella Contamination and Testing

Salmonella Bacteria

Foodborne illness affects public health and development worldwide. Salmonella spp. are members of the Enterobacteriaceae family and there are more than 2600 known serovars (or variations) within the Salmonella species. The serovars of Salmonella are broadly classified into typhoidal and nontyphoidal Salmonella. The nontyphoidal Salmonella serovars Typhimurium and Enteritidis are two of the most common causes of gastroenteritis, but many others can also lead to illness.

Infection with Salmonella

Salmonellosis can be extremely severe, particularly for young children, the elderly and the immuno-compromised. It is generally contracted through the consumption of contaminated food of animal origin (mainly eggs, meat, poultry, and milk), although other foods have also been implicated in its transmission. Symptoms include diarrhoea, abdominal pain, nausea and vomiting and can last from 1 to 7 days.

Salmonella Contamination in Food

Salmonella bacteria are widely distributed in domestic and wild animals. They are prevalent in food animals such as poultry, pigs, and cattle. Salmonella can pass through the entire food chain from animal feed, primary production, and all the way to households or food-service establishments and institutions.¹

Salmonella can be quite resilient and are not killed by freezing, and can also survive in acidic foods. The high resistance to drying, combined with a very high heat resistance once dried, makes Salmonella a potential problem in most foods, particularly in dry and semi-dry products (e.g. milk powder, spices etc.).

This means that if contamination is present, it can be difficult to eradicate. For example, fatty foods can protect the cells from quite severe heat treatments, making pasteurisation ineffective.

Preventing Salmonella Contamination

It is therefore exceptionally important for all food manufacturers to rigorously test for Salmonella to ensure no contaminated products are released to the public.

ISO Method for Salmonella Detection

The International Organization for Standardization (ISO) recently revised the scope of the horizontal method for the detection, enumeration, and serotyping of Salmonella in the revised EN ISO 6579-1:2017 standard which has been expanded to incorporate ISO 6785 I IDF 93 and now includes milk and milk products, animal feces and environmental samples from the primary production stage.

For further information, read our article ISO 6579-1 for Salmonella Detection in the Food Chain or watch our 10 minutes presentation on compliant Salmonella spp. testing.

Media and ISO Compliance

When following ISO 6579-1:2017, regulatory compliant media should be used as the media formulation and preparation needs to be as described in the method. For piece of mind during audits or lab inspections, use media that has been released from an ISO/IEC 17025 accredited laboratory, such as our Granucult^® dehydrated media, or ready-to-use ReadyTube^® media, which also verifies compliance with DIN EN ISO 11133:2014.

Sample Preparation and Enrichment

Sample preparation and pre-enrichment are carried out with buffered peptone water, followed by selective enrichment using Muller-Kauffmann Tetrathionate-Novobiocin (MKTTn) Broth, Rappaport Vasiliadis Soy broth (RVS) broth, or Modified Semi-solid Rappaport Vasiliadis (MSRV) agar. Check out our gravimetric dilutors and sample homogenizers to aid with sample preparation.

Selective Media for Salmonella Detection

EN ISO 6579-1:2017 provides greater flexibility in the plating-out step of Salmonella detection so the procedures for the inoculation of the isolation medium have been made less prescriptive. More detailed directions for the choice of the second isolation medium, which must be used in addition to XLD (Xylose Lysine Deoxycholate) agar, are now given. This medium must be based on different diagnostic characteristics to balance the disadvantages of XLD e.g. the lack of detection of H₂S-negative Salmonella. As second diagnostic agar, we recommend using Rambach^® agar, which is based on acid formation with propylene glycol, and in combination with an included pH indicator, the colonies will have a characteristic red color. This enables species of Salmonella to be differentiated unambiguously, and allows identification of H₂S-negative Salmonella strains, e.g. S. Agona, which may otherwise be missed using XLD agar and a second agar based on the same diagnostic principle, such as XLT4 or Hektoen Enteric Agar.

Rapid Methods for Salmonella Detection

Immunological Methods

Lateral Flow Tests are immunoassays for detecting food pathogens. Acting as mini-laboratories, they include a built-in control reaction and definitive results are delivered in as little as 20 minutes, following that standard ISO 6579-1 enrichment method. Our Singlepath^® Salmonella test is AOAC-RI approved.

One of the simplest rapid methods available for the detection of Salmonella uses immunodiffusion technology to provide accurate and rapid results following a simple 1 or 2 step enrichment protocol. This 1–2 Test is also an AOAC official method of analysis.

For a high-throughput method, Enzyme Immunoassay (EIA) technology using complex antibody formulations can overcome the challenges of differential bacterial detection. Extensive AOAC validation along with built-in positive and negative controls is combined to provide users with the necessary confidence in the results. Assays are also validated by AFNOR according to ISO 16140.

Molecular Methods

Try a rapid accurate detection method featuring the latest innovations in genetic detection technology and food microbiology. The Assurance^® GDS system uses multiple layers of specificity, including immunomagnetic separation (IMS), highly specific primers, and a patented probe to ensure highly accurate results. GDS assays have been validated by AOAC and other international standards (AFNOR/MicroVal) according to ISO 16140.

Reference Materials for QC and Validation

In order to assure the quality of your test results, it is advisable to use certified reference microorganisms for verification. Our Vitroids™ and LENTICULE^® discs contain viable microorganisms in a certified quantity (generally accredited according to ISO/IEC 17025) are produced under reproducible conditions compliant with ISO Guide 34:2009 using authenticated strains from NCTC^®, NCPF^® and CECT^®. Consisting of pure cultures of bacteria in a solid water-soluble matrix, they are stable for at least one year and are in a viable state with a shelf life of 1-3 years. Each batch is provided with a comprehensive certificate of analysis.

References

1. WHO Salmonella (non-typhoidal) Fact Sheet.. [Internet].[cited 22 Jan 2018]. Available from: http://www.who.int/mediacentre/factsheets/fs139/en/