Let’s know more about Salmonella!
Salmonellae are bacteria generally found in animal intestines and transmitted to humans through contaminated food. In most cases, they are responsible for gastroenteritis, which can lead to fatal typhoid fever.
Every year, millions of people around the world suffer from infections caused by these micro-organisms, which is one of the leading causes of food-borne infection worldwide. Their ubiquity in the food chain and their resistance raises major challenges for the study and detection of Salmonellae.
Let’s know more about this germ!
Salmonellae: General and Microbiological Characteristics
Species
Salmonellae are Gram-negative bacteria belonging to the genus Salmonella, in the Enterobacteriaceae family.
There are two species in the Salmonella genus: S. enterica and S. bongori. S. enterica is divided into 6 different subspecies (enterica, salamae, arizonae, diarizonae, houtenae and indica). Species and subspecies identification is based on biochemical characteristics. Then classified according to their serotypes meaning their reaction in the presence of a serum containing antibodies. Over 2,600 serotypes have been identified. The main serotypes in human infection are Enteridis (mainly present in the poultry industry) and Typhimurium (in pig industry).
Structure
Salmonellae have a peptidoglycan cell wall, as well as an outer membrane playing a key role in bacterial virulence, and mobile or immobile flagellum, the former enabling movement towards nutrients or away from hostile environments.
They also have a pili for adhesion to host cells for colonization and infection.
Salmonella metabolism
Salmonellae are facultative aero-anaerobic organisms, which means they can survive in the presence or absence of oxygen. In the presence of oxygen, they use aerobic respiration, but in its absence, they can ferment certain substrates to produce energy.
They use a variety of nutrient sources including carbon, sugars, amino acids and organic acids, enabling them to survive in a variety of environments, including animal intestines and outdoor environments.
They are also highly resistant to low-humidity production environments. They are able to survive in stressful conditions (notably heat) and dry environments thanks to biofilm formation on different types of surfaces, both biotic and abiotic.
Methods for Laboratory Detection and Identification of Salmonella
Culture and Isolation Techniques
Compliance with standards is essential to guarantee food safety, particularly for ready-to-eat food products. Methods of detection, enumeration and serotyping of Salmonella complies with NF EN ISO 6579-1.
Salmonella isolation and culture are crucial in the diagnosis of infections caused by these bacteria, and in food safety control.
Salmonella can be isolated from a variety of samples, including feces, food, water, clinical samples and environment.
Enrichment of the sample is necessary, and there are two types of enrichment. Non-selective enrichment promotes the growth of all bacteria, while selective enrichment inhibits the growth of unwanted bacteria to favor Salmonella.
After enrichment, cultures are plated on solid selective culture media. There are different types of culture media, such as:
Isolated colonies obtained on these conventional media must be confirmed by biochemical testing on TSI Agar, Urea Agar and L-Lysine decarboxylation medium.
Apart from reference method, there are also alternative methods that provide numerous advantages, such as IRIS Salmonella®.
This method is certified NF VALIDATION, for the fast detection of Salmonella spp. in all food and feed products, as well as environmental samples. It gives positive and negative screening result in just 37 hours.
As an alternative method, IRIS Salmonella® offers multiple advantages. This easy-to-use method, with just one enrichment broth and one plate , detects Salmonella spp. including serotypes such as Typhi and Paratyphi, lactose-positive and sucrose-positive Salmonella strains, immobile serovars and monophasic strains. It offers an excellent contrast between the white agar and the pink-magenta colonies of Salmonella and the blue-green colonies of associated flora and can also detect strains with little or no esterase activity that give uncharacteristic colonies on classical media.
Other alternative Salmonella detection techniques include PCR, used to detect Salmonella-specific genes. Or enzyme-linked immunosorbent assays, which highlight the immunological characteristics of Salmonella.
Implications for public health and food safety
Some numbers
Salmonella is a major pathogen in low-moisture foods. Even in very small quantities, the bacteria can cause illness and is a frequent cause of food poisoning in worldwide.
For example, in France, it is estimated that 198,000 cases of salmonellosis occur every year, among which 183,000 through food transmission. However, this figure may be underestimated, as not all infections of lesser severity are reported or diagnosed.
In the case of non-typhoid fevers, around 22% of reported cases require hospitalization, and 0.8% of cases result in death due to this infection, mainly among vulnerable people (elderly, immunocompromised, etc.). Typhoid (and paratyphoid) fevers are generalized infections, and the mortality rate is 10% without effective antibiotic treatment.
Due to the danger to public health and the foodborne nature of salmonellosis, globally speaking the regulatory requirement of Salmonella testing in food industries evolves towards stricter control and more complete monitoring. Today, Salmonella testing in food industry represents a market of more than $ 1,6 billion per year, with the US leading the market, followed by Europe and then Asia-Pacific.
What are the symptoms and complications?
Symptoms of salmonellosis generally appear between 6 hours and 6 days after exposure and manifest as gastroenteritis, or between 8 and 14 days for typhoid fevers, with symptoms such as:
– Diarrhea
– Abdominal pain
– Fever
– Nausea and vomiting
– Intense headaches
– Muscle pain
– Fatigue
In more severe forms, complications such as bacteremia, hemorrhage, digestive perforation, osteitis, cholecystitis and heart failure can occur.
In rare cases, Salmonella infection persists, and patient may develop chronic inflammation.
What are the sources of infection?
They are mainly found in animal intestines and are spread by infected animals. As a result, Salmonella can be found almost everywhere, notably:
– Animal products: Eggs, poultry, pork and unpasteurized dairy products are the main sources of infection. Reptiles are also a common vector of Salmonella, especially those in captivity.
– Vegetables and fruit: Although less common, raw vegetables and certain fruits can also be contaminated, especially if they have been in contact with manure or contaminated water.
– Processed foods: Some processed foods, such as deli meats or ready-made meals, can also be contaminated if hygiene measures are not observed during preparation.
– Environment: due to the presence of infected animals and their excrement, they can be easily found in all types of environments: natural, agricultural and industrial.
How to prevent it?
For the food industry, prevention is key to the successful control of Salmonella. Each manufacturer must think ahead and make preventive plan that is adapted to the installation, manufacturing process, materials, and intermediate and final product, while taking into consideration all risk factors that are specific to this industrial environment. This plan should include rigorous cleaning and disinfection and timely monitoring covering equipment, process, material, and products. It is also important to increase the awareness of factory staff regarding microbiological safety.
Due to its resistance, Salmonella can survive for years in an industrial environment. External contamination can also be brought into the production environment. When a contamination happens, its source and pathway must be identified and eliminated at all costs immediately.
Implementation of Good Manufacturing Practice and Good Hygiene Practice, as well as establishing HACCP guideline can greatly help to prevent Salmonella contamination and eliminate the hazard if the latter happens.
Conclusion
Salmonella represents a major public health issue, due to their ability to cause serious foodborne infections. Although significant progress has been made in the control and prevention of these bacteria, they continue to pose a risk, particularly in environments where hygiene measures are inadequate. Ongoing research is crucial to developing new methods for the detection, prevention and treatment of Salmonella infections.
For industries, a good monitoring of the production environment in terms of raw materials, equipment, plant design and staff hygiene practices are crucial. Therefore, establishing a complete and efficient control plan specific to each factory remains a key factor in the prevention of Salmonella contamination and infection.