Foodborne diseases / Salmonellas
SalmonellasSalmonellas have a worldwide distribution and numerous animal reservoirs have been recognised. Many foods, particularly those of animal origin, are common sources of infection for humans.
1. Bacteriological characteristics
Salmonellas are Gram-negative, rod shaped bacteria belonging to the family Enterobacteriaceae. The majority of strains are motile. There are over 2200 salmonella serotypes, which are differentiated on the basis of their somatic (O) and flagella (H) antigens, and these can be divided arbitrarily into three unequal groups, namely, the species-specific serotypes (e.g. S. dublin in cattle and S. gallinarum and S. pullorum in poultry), the “invasive” serotypes (e.g. S. enteritidis and S. typhimurium, which are specifically mentioned in the “Zoonoses Directive 92/117/EEC), and the “non-invasive” serotypes, which form the great majority of the serotypes currently recognised.
2. Disease in humans
Salmonella food poisoning in humans usually develops within 12-24 hours and presents as gastroenteritis. Other signs include abdominal pain, diarrhoea, vomiting and fever. Death occurs occasionally in the very young or the very old. Most incidents are associated with relatively few serotypes, probably about 200, with S. enteritidis and S. typhimurium being prevalent in European countries.
3. Sources of infection and epidemiology
Farm animals, particularly pigs and poultry, may be infected with salmonellas without exhibiting any clinical signs. These inapparent intestinal infections are not detected by traditional meat inspection procedures. The carcass becomes contaminated from the surface of the animal or the intestinal contents, or from contact with contaminated equipment or aerosols. The prevalence is influenced, inter alia, by husbandry methods and hygiene standards during slaughter and subsequent handling of the carcass meat and offals. Raw meat, the drip from raw meat, mechanically recovered meat, cooked meat, particularly if undercooked, pâtés and fermented meat products may all act as sources of infection. Cross-contamination from raw to prepared foods may also occur in retail shops and kitchens.
4. Growth properties in relation to meat and meat products
Salmonellas grow under both aerobic and anaerobic conditions and over a wide temperature range (5-46°C), although the optimum is 35-43°C, and growth is reduced substantially below 15°C. The pH range for growth is 4.5-9.0 with an optimum of 6.5-6.7. Growth is inhibited by a water activity below 0.94.
Raw meat is usually contaminated with only small numbers of salmonellas which in themselves may not be dangerous. However, any multiplication will increase the potential risk to consumers. Salmonellas can survive freezing and have been detected on frozen foods after years of frozen storage. The organisms are also capable of surviving for long periods in the environment.
The importance of salmonellas as pathogens has resulted in the development of a plethora of culture media and considerable interest in novel or rapid methods for detection (e.g. hydrophobic grid membrane filtration, serological techniques, impedance methods and magnetic immuno-polymerase chain reaction assay). Traditionally, the isolation of salmonellas from feed and other non-clinical and environmental sources involves a three stage process. This includes resuscitation or pre-enrichment in a non-selective broth (e.g. buffered peptone water), which allows cells debilitated by e.g. desiccation or freezing to recover, and selective enrichment in a broth which is inhibitory to organisms other than the salmonellas (e.g. selenite, tetrathionate and Rappaport-Vassiliades broths). A selective (e.g. bismuth sulphite agar, brilliant green phenol red agar, xylose lysine deoxycholate agar) or a differentiating (e.g. Rambach agar) plating medium is then used in the third stage. The identity of salmonella-like colonies is subsequently confirmed using biochemical and serological tests. Within serotypes, different strains can be differentiated by techniques such as phage-typing and plasmid profiling, but these techniques are not used in routine laboratories.
6. Control measures in slaughterhouses and cutting plants
Salmonellas can survive and multiply in the environment and are, therefore, independent of animal hosts. Most serotypes are potentially pathogenic but they can frequently colonise the animal’s intestine without causing clinical illness. This means that it is difficult to identify effective control points in (poultry) meat production and control measures can be expected to do little more than reduce the risk of spreading the organisms. Therefore these organisms are best controlled on the farm.
The complete decontamination of raw foods is impossible unless sufficient doses of irradiation are used; a process not favoured in the EU. Other methods may reduce salmonella numbers but will not eliminate the organisms, for example, hot water or lactic acid sprays or trisodium phosphate treatment. Currently the EU considers the approval of decontamination treatments.