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Salmonella Factsheet

Salmonella species (spp.) are a group of bacteria capable of causing disease which live naturally in the intestinal tracts of humans and other warm-blooded animals, especially birds. They are the second most common cause of bacterial foodborne illness in Ireland (Campylobacter spp. is the most frequent cause). They are facultative anaerobic Gram-negative rods and members of the Enterobacteriaceae group. 

The genus Salmonella contains two species: Salmonella enterica and Salmonella bongori. Salmonella enterica is an important agent of foodborne illness and is sub-classified into six subspecies, of which S. enterica subspecies enterica is the most important for human health. 

The genus S. enterica can be subdivided into more than 2,500 serotypes. S. Typhimurium and S. Enteritidis are the most frequently isolated serotypes in human clinical cases in Ireland. Serotypes are further subdivided by resistance to bacteriophages (phage types or lystotypes), antibiotics or heavy metals; biochemical characteristics (biovars or biotypes); or sensitivity to or production of bacteriocins.

  • Growth and survival characteristics

    Table 1 outlines some of the key growth characteristics of Salmonella spp. that can be used by food business operators to assess the potential risk of the food product(s) supporting the growth of Salmonella under various conditions.

    Table 1. Growth characteristics of Salmonella spp. under different conditions
    Conditions Minimum Optimum Maximum
    Temperature (°C) 5 35–43 47
    pH 3.8 7–7.5 9.5
    Water activity (aw) 0.94 0.99 >0.99

    Salmonella spp. are not particularly heat resistant and most serotypes are killed by normal cooking conditions such as cooking to a core temperature of 75°C instantaneously or an equivalent time–temperature combination (e.g., 70°C for 2 minutes). However, a few highly heat-resistant serotypes have been reported (e.g. S. Senftenberg 775W and S. Irumu). Heat resistance is influenced by water activity (aw), nature of the solutes, and pH of the suspending medium. Greater heat resistance is observed for cells in sucrose compared with NaCl at the same aw values. 

    The heat resistance of Salmonella spp. also increases with increasing concentration of milk solids and fat content in certain food products such as dairy powders and milk or white chocolate. This phenomenon is due to the low aw environment created by high-fat and high-milk solids mixtures, which provides a protective, desiccation-tolerant environment for the pathogen. Research has shown that increasing the milk solids level (such as in milk chocolate) increases Salmonella heat resistance (Table 2). Additionally, for certain low-moisture food products such as chocolate and peanut butter, the high fat content contributes to increased heat tolerance for Salmonella spp. if present in the product because it restricts water availability and protects the cells. This high heat resistance means that standard pasteurisation or typical heat treatments for high-moisture foods are not sufficient to control the potential hazard of Salmonella spp. in these types of products. Thus, specialised, validated heat treatments are required instead in the manufacturing of low-moisture food products.

    Table 2. Heat resistance of S. Typhimurium in milk chocolate at various temperatures (D-values)
    Temperature D-value (min)*
    90°C 75
    80°C 222
    70°C 816

    * D-value: time in minutes at a given temperature to achieve a 90% reduction in the number of viable cells.

  • The illness

    Human infection with Salmonella spp. is referred to as salmonellosis. Although it can arise from contact with infected animals, consumption of contaminated food is the most usual cause. People infected with Salmonella typically develop symptoms between 12 and 36 hours after infection, but this can range between 6 and 72 hours. The most common symptom is diarrhoea, which can sometimes be bloody. Other symptoms may include fever, headache, and abdominal cramps. The illness usually lasts 4 to 7 days. Diarrhoea can occasionally be severe enough to require hospital admission. Older adults (>65 years), infants and young children (<4 years) and those with impaired immune systems are more likely to have severe illness.

  • Sources of Salmonella spp. and routes of transmission

    Salmonella spp. naturally live in the intestinal tract of humans and warm-blooded animals. Animals become infected through direct contact with other infected animals, feed or water. Salmonella spp. can be shed in the faeces of infected animals. Feet, hair, and skin of animals can become contaminated as they walk, sit, or lie in faecally contaminated ground or litter. Poor hygiene practices on farms can result in Salmonella spp. spreading to human hands. Spread to carcasses may occur in the abattoir during slaughter. Agricultural commodities may be contaminated via faecally polluted streams, rivers, coastal waters, or improperly treated sewage. In food processing, cross-contamination from raw food or infected food handlers can occur. 

  • Salmonella spp. and food

    Salmonella spp. are usually passed to humans by eating foods contaminated with animal faeces. It is rarely possible to tell if food has been contaminated as it usually looks and smells normal. Many foods have been identified as vehicles for transmission to humans, particularly foods of animal origin but also foods of non-animal origin that may be subject to faecal contamination. Foods of particular importance are eggs, pig meat, poultry meat, milk, chocolate, fruit, and vegetables. Although Salmonella spp. do not form spores, they can survive for long periods in foods. Thorough cooking of food kills Salmonella spp. and reduces the risk of illness. Cooked food can become cross-contaminated with Salmonella spp. if it comes into contact with raw food via poor hygienic practices. Food processing environments can also become contaminated with Salmonella spp. thereby acting as a source of cross-contamination. Adequate cooking and temperature control, and prevention of cross-contamination are important control measures for Salmonella spp. For foods which will not be cooked before consumption (termed ready-to-eat foods), such as fresh fruit and vegetables, contamination at the growing and distribution stages should be prevented through good agricultural practice.

  • Control of Salmonella spp. in the food chain

    As low numbers of Salmonella spp. can cause illness, control measures are needed at all stages in the food chain to protect consumer health and business integrity. Examples include:

    • Implementation of Good Hygiene Practices (GHP) and Good Manufacturing Practices (GMP) at farm, slaughterhouse, manufacturing, processing, catering, and retail; with particular attention to preventing cross-contamination between raw and ready-to-eat foods.
    • Implementation of a Hazard Analysis and Critical Control Point (HACCP)-based food safety management system, including good process control (e.g., temperature control during cooking and storage).
    • Testing against microbiological criteria, as appropriate, when validating and verifying HACCP-based procedures and other hygiene control measures.

  • Incidence of the disease and foodborne outbreaks

    Salmonellosis is a notifiable disease in Ireland. This information is used to investigate cases thus preventing spread of infection and further cases. The information will also facilitate the early identification of outbreaks. The most up-to-date data on notifiable infectious diseases is available on the HPSC’s National Notifiable Disease Hub (https://notifiabledisease.hpsc.ie/). This hub uses aggregated data to display a dynamic series of dashboards which provide an overview of notifiable infectious diseases in Ireland. The dashboard is updated on a weekly basis and is managed by HPSC.

    The European Food Safety Authority (EFSA) and the European Centre for Disease Prevention and Control (ECDC) also produce an annual One Health Zoonoses reports on zoonotic trends in the European Union which provides information on clinical cases, foodborne outbreaks and implicated sources of salmonellosis. The One Health Zoonoses report is typically published at the end of each calendar year and reports zoonoses data at a European level for the previous year. The One Health Zoonoses reports are available to view at:

    Ireland’s national zoonoses report is also published annually by the FSAI, in collaboration with the Department of Agriculture, Food and the Marine (DAFM), the Health Service Executive (HSE), the Sea-Fisheries Protection Authority (SFPA) and the Health Protection Surveillance Centre (HPSC). The report brings together the results of thousands of tests carried out on samples of food and feed, as well as tests on material of animal or human origin, in an effort to determine the pattern and extent of infection by zoonotic pathogens transmitted to humans from animals. 

    View the Irish national zoonoses reports.

  • Legislation

    All food business operators have a legal responsibility to produce safe food (Regulation 178/2002, as amended). Safety is ensured through a preventative approach by implementing a food safety management system based on HACCP principles. All food business operators, except primary producers, are legally obliged to implement and maintain permanent procedures based on HACCP (Article 5 of Regulation 852/2004, as amended). 

    All food business operators, including primary producers, must also implement good hygiene practices (GHP). Regulation 852/2004, as amended, lays down hygiene requirements for all foodstuffs. Regulation 853/2004, as amended, lays down more specific hygiene requirements for foods of animal origin. 

    Commission Regulation (EC) No 2073/2005, as amended, lays down microbiological criteria for combinations of food commodities and microorganisms, their toxins, or metabolites. It requires operators to ensure compliance as part of GHP/HACCP, including testing when validating and verifying procedures. The Regulation differentiates between food safety criteria (defining acceptability of a foodstuff) and process hygiene criteria (indicating whether production operates hygienically).

    The Regulation sets legal microbiological food safety and process hygiene criteria for Salmonella spp. in a wide range of foods when placed on the market during their shelf-life, including: fresh poultry meat, minced meat, meat preparations and meat products, mechanically separated meat, gelatine and collagen, dairy products (including infant formula and follow-on formula), egg products, live bivalve molluscs and live echinoderms, tunicates and gastropods, sprouted seeds, ready-to-eat pre-cut fruit and vegetables, unpasteurised juices and reptile meat. 

    Refer to the Commission Regulation (EC) No 2073/2005 of 15 November 2005 on microbiological criteria for foodstuffs for food categories and associated sampling plans relevant to Salmonella spp. Note that any amendments to the legislation since it was initially published in 2005 must be taken into account when determining compliance with the requirements of the Regulation. 

  • Related

    Guidance on compliance with legislative requirements:

    For further information on Salmonella spp. see: