Salmonella is a genus of bacteria widely recognized for causing foodborne illness in humans and animals. This bacterium is a significant public health concern globally, leading to numerous infections, hospitalizations, and deaths each year. It is one of the leading causes of diarrhoeal diseases worldwide, impacting both developed and developing countries.
Microbiological Profile of Salmonella
Salmonella bacteria are rod-shaped and Gram-negative. This means they do not retain the crystal violet stain used in Gram staining, appearing red or pink under a microscope, due to their specific cell wall structure. They are also motile, moving independently through liquid environments using flagella, which are whip-like appendages extending from the cell body.
Salmonella is categorized into over 2,600 different serotypes, such as Salmonella enterica serotype Enteritidis and Salmonella enterica serotype Typhimurium. These serotypes are differentiated by variations in their surface antigens, which can influence the type and severity of illness they cause.
Environmental Survival and Growth
Salmonella is a hardy bacterium that can survive in various environments outside a host. It is a mesophile, growing best at moderate temperatures, typically 35-43°C. Refrigeration can slow or stop its growth, but freezing does not destroy the bacteria, allowing them to persist in frozen foods.
The bacterium tolerates a wide range of pH levels, growing between 3.8 and 9.5, with an optimal range of 7.0-7.5. This helps it survive in acidic conditions, such as those found in some foods. Salmonella is a facultative anaerobe, meaning it can generate energy both with and without oxygen. These characteristics enable Salmonella to persist in diverse settings, including farm equipment, water sources, and food processing plants, making its control challenging.
Mechanisms of Infection
Infection with Salmonella typically begins when a sufficient number of bacteria are ingested, often through contaminated food or water. Upon entering the body, the bacteria must survive the highly acidic environment of the stomach. While many bacteria are destroyed by stomach acid, Salmonella possesses mechanisms that allow a portion of the ingested population to pass into the intestines.
Once in the small and large intestines, Salmonella actively invades the cells lining the intestinal mucosa. This invasion involves the bacterium inducing its own uptake into these host cells, where it can then multiply. Some bacteria may also be taken up by immune cells, such as macrophages, which can transport the bacteria to other parts of the body, leading to systemic infection.
The body’s immune system responds to the presence of Salmonella by initiating an inflammatory response in the intestines. This inflammation results in the characteristic symptoms of salmonellosis, such as fever, abdominal pain, and diarrhea.
Common Sources and Transmission
Salmonella bacteria are widely distributed in nature, with a large and varied animal reservoir. Domestic and wild animals, including poultry, pigs, cattle, and reptiles like turtles and lizards, commonly harbor Salmonella in their intestinal tracts, often without showing signs of illness. These animals then shed the bacteria through their feces, which can contaminate the environment.
Transmission to humans primarily occurs through the consumption of contaminated food of animal origin, such as undercooked meat, poultry, and eggs. Raw milk and unpasteurized dairy products also serve as vehicles for transmission. Produce like fruits and vegetables can become contaminated if washed with contaminated water or come into contact with animal manure. Cross-contamination in kitchens, where Salmonella from raw foods spreads to ready-to-eat items or surfaces, is another significant route of infection.