Salmonellosis- Food Infection and Food Poisoning by Salmonella
Salmonella is a genus of bacteria that belongs to the family Enterobacteriaceae and causes various diseases in humans and animals, such as gastroenteritis, typhoid fever, and paratyphoid fever. Salmonella infections are usually transmitted by consuming contaminated food or water, or by contact with infected animals or people. Salmonella can survive in a wide range of environmental conditions, such as low pH, high temperature, and bile salts.
The name Salmonella comes from Daniel Elmer Salmon (1850-1914), a U.S. veterinary surgeon who was the head of the Bureau of Animal Industry, a division of the U.S. Department of Agriculture. In 1885, he and his assistant Theobald Smith isolated the first strain of Salmonella from the intestine of a pig that died of hog cholera. They named it Salmonella choleraesuis, after the disease and the host animal.
However, Salmonella was actually discovered earlier by other scientists who did not recognize its significance or did not name it properly. In 1879, Karl Joseph Eberth observed rod-shaped bacteria in the spleen and lymph nodes of typhoid patients, but he could not grow them in pure culture. In 1880 and 1881, he published his findings and suggested that these bacteria were the cause of typhoid fever. His discovery was confirmed by Robert Koch and other bacteriologists in Germany and England.
The genus Salmonella now comprises two species: Salmonella enterica and Salmonella bongori. S. enterica is further divided into six subspecies that include over 2,600 serotypes based on their antigenic variations. S. enterica subsp. enterica contains most of the human pathogens, such as S. Typhi, S. Paratyphi, S. Enteritidis, and S. Typhimurium. S. bongori is mainly found in cold-blooded animals and rarely causes disease in humans.
Salmonella has been responsible for many outbreaks and epidemics throughout history, such as typhoid fever in ancient Greece and Rome, the Great Plague of Athens in 430 BC, the Plague of Justinian in 541 AD, and the Irish Famine Fever in 1846-1849. In modern times, Salmonella has caused foodborne illnesses associated with poultry, eggs, meat, dairy products, fruits, vegetables, spices, chocolate, peanut butter, and pet food.
Salmonella remains a major public health problem worldwide, especially in developing countries where sanitation and hygiene are poor. According to the World Health Organization (WHO), there are an estimated 93.8 million cases of salmonellosis and 155,000 deaths annually. Typhoid fever alone affects about 21 million people and kills about 200,000 people every year.
Salmonellosis is a serious disease that has a clinical spectrum of gastroenteritis and typhoid fever. Gastroenteritis, also known as food poisoning, is a common form of Salmonella infection that affects the intestinal tract. It usually occurs after consuming contaminated food or water, or through contact with infected animals or people. Typhoid fever, also known as enteric fever, is a systemic infection that affects the bloodstream and other organs. It is caused by specific strains of Salmonella, such as S. Typhi and S. Paratyphi, and is usually common in developing countries with poor sanitation.
The effects of Salmonellosis vary depending on the type and dose of the bacteria, the health and immune status of the person, and the duration of the infection. Most people with Salmonellosis develop diarrhea, fever, and abdominal cramps within 6 hours to 6 days after exposure. Some people may also have nausea, vomiting, or a headache. The symptoms usually last for 4 to 7 days and are self-limiting in most cases. However, some people may experience more severe or prolonged effects, such as dehydration, bloody stools, septicemia (blood infection), reactive arthritis (joint inflammation), or intestinal perforation (hole in the bowel). In rare cases, Salmonellosis can be fatal if not treated promptly.
The effects of typhoid fever are more serious and can be life-threatening. The symptoms usually start within 10 to 14 days after exposure and include high fever, headache, constipation, abdominal pain, rash, and enlarged spleen and liver. If untreated, typhoid fever can cause complications such as toxemia (blood poisoning), myocarditis (heart inflammation), intestinal hemorrhage (bleeding), or meningitis (brain infection). The fatality rate of typhoid fever can be as high as 20% without treatment.
Salmonellosis can be prevented by practicing good hygiene, cooking food thoroughly, avoiding cross-contamination, drinking safe water, and getting vaccinated for typhoid fever if traveling to high-risk areas. Salmonellosis can be treated by drinking fluids to prevent dehydration, taking antibiotics for severe or systemic infections, and seeking medical attention if symptoms persist or worsen.
Salmonella bacteria are Gram-negative, meaning they have a thin cell wall that does not retain the purple dye used in the Gram staining technique. They are rod-shaped (bacilli) and non-spore-forming, which means they do not produce dormant forms that can resist harsh conditions. They are facultatively anaerobic, which means they can grow with or without oxygen, but prefer oxygen for a greater yield of energy. They are also motile, meaning they can move by using their flagella, which are whip-like structures that extend from their cell surface. Salmonella bacteria have different types of antigens on their surface, which are molecules that trigger an immune response. The most important antigens for Salmonella are:
- O antigen: This is a part of the lipopolysaccharide (LPS) layer that covers the cell wall. It consists of repeating sugar units that vary among different Salmonella strains. The O antigen determines the serogroup of Salmonella, which is a broad classification based on antigenic similarity.
- H antigen: This is a protein that forms the flagella. It also varies among different Salmonella strains and determines the serotype of Salmonella, which is a more specific classification based on antigenic similarity.
- Vi antigen: This is a capsule-like layer that surrounds some Salmonella strains, especially those that cause typhoid fever (S. Typhi and S. Paratyphi). It helps the bacteria evade the immune system and resist phagocytosis (engulfment by white blood cells).
Salmonella bacteria can grow in a wide range of temperatures (5 to 45°C) and pH levels (3.8 to 9.5). They can also resist bile salts, which are produced by the liver and secreted into the intestine to help digest fats. Salmonella bacteria produce hydrogen sulfide (H2S), which is a gas that gives a rotten egg smell. This can be detected by using media that contain iron salts, such as Hektoen enteric agar or Xylose lysine deoxycholate agar, which turn black when H2S reacts with iron.
Salmonella bacteria are diverse and adaptable microorganisms that can cause various types of infections in humans and animals. They have evolved mechanisms to survive in different environments and hosts, and to evade or overcome the host immune system. Understanding their characteristics can help in their identification, diagnosis, prevention, and treatment.
Salmonellosis is a foodborne infection that can be transmitted through various sources, such as:
- Contaminated food and water. Salmonella bacteria can be found in the gastrointestinal tract of many animals, birds, and reptiles, especially poultry. These animals can contaminate the food and water they come in contact with, such as meat, eggs, milk, fruits, and vegetables. People who consume raw or undercooked food or drink contaminated water can get infected with Salmonella.
- Poor hygiene and sanitation. Salmonella bacteria can also spread from person to person through the fecal-oral route. This means that people who do not wash their hands properly after using the toilet or handling animals can transfer the bacteria to other people or objects. People who touch contaminated surfaces or utensils or eat with unwashed hands can also get infected.
- Direct contact with animals. Some animals, such as poultry and reptiles, can carry Salmonella bacteria without showing any signs of illness. People who have direct contact with these animals or their feces can get infected if they do not wash their hands thoroughly afterward. Pets can also spread the bacteria within the home environment if they eat food contaminated with Salmonella.
To prevent Salmonellosis, it is important to follow these steps:
- Cook food thoroughly and avoid cross-contamination between raw and cooked food.
- Wash hands frequently with soap and water, especially before and after handling food, animals, or their feces.
- Drink only pasteurized milk and treated water.
- Refrigerate food promptly and store it at an adequate temperature.
- Avoid eating raw sprouts, tropical fish, peanut butter, and chocolate that may be contaminated with Salmonella.
Salmonellosis is a global public health problem that affects millions of people every year. According to the World Health Organization (WHO), Salmonella is one of the four key causes of diarrheal diseases, which account for 550 million cases and 220 million deaths annually. Salmonella infections can range from mild gastroenteritis to severe systemic infections such as typhoid fever and paratyphoid fever.
The epidemiology of salmonellosis varies by geographic region, serotype, host, and source of transmission. Some factors that influence the distribution and prevalence of Salmonella include:
- Animal reservoirs: Salmonella can be found in the gastrointestinal tract of many animals, such as poultry, pigs, cattle, reptiles, and rodents. These animals can shed the bacteria in their feces and contaminate the environment, water, and food. Some serotypes are host-specific and cause more severe disease in humans, such as S. Typhi and S. Paratyphi A, which are mainly transmitted by humans. Other serotypes, such as S. Enteritidis and S. Typhimurium, can infect a wide range of hosts and cause gastroenteritis.
- Food production and consumption: Salmonella can survive in various food products, such as eggs, meat, dairy products, fruits, vegetables, and processed foods. The industrialization and globalization of the food supply chain increase the risk of cross-contamination and widespread outbreaks. The demand for ready-to-eat and raw or lightly cooked foods also increases the exposure to Salmonella. Proper cooking, pasteurization, refrigeration, and hygiene practices can reduce the risk of salmonellosis.
- Human behavior and demographics: Salmonella can be transmitted from person to person through the fecal-oral route or by handling contaminated food or utensils. Poor hygiene practices, such as inadequate handwashing, can facilitate the spread of the bacteria. Travelers to endemic areas may acquire Salmonella infections and introduce new strains to their home countries. Children, elderly people, immunocompromised patients, and malnourished individuals are more susceptible to severe salmonellosis and its complications.
- Antimicrobial resistance: The use of antibiotics in food animal production and human medicine can select for resistant strains of Salmonella that are harder to treat and more likely to cause invasive infections. Some serotypes, such as S. Typhimurium DT104 and S. Newport MDR-AmpC, have developed resistance to multiple classes of antibiotics. Antimicrobial resistance is a major threat to public health and requires coordinated surveillance and control measures.
The epidemiology of salmonellosis is constantly changing due to the interaction of these factors and the emergence of new strains and sources of infection. Therefore, it is important to monitor the trends and patterns of Salmonella infections using molecular typing methods, such as pulsed-field gel electrophoresis (PFGE), multilocus sequence typing (MLST), or whole-genome sequencing (WGS). These methods can help identify the sources, routes, and risk factors of salmonellosis outbreaks and guide prevention and control strategies.
Salmonellosis is caused by the ingestion of Salmonella bacteria in contaminated food or water. The bacteria must survive the acidic environment of the stomach and reach the small intestine, where they attach to and invade the epithelial cells. Salmonella can invade two types of cells in the intestinal mucosa: enterocytes and M cells. Enterocytes are the cells that line the surface of the intestine and absorb nutrients, while M cells are specialized cells that transport antigens from the lumen to the underlying lymphoid tissue.
Salmonella uses two different mechanisms to invade enterocytes and M cells. To invade enterocytes, Salmonella uses a type III secretion system (T3SS) encoded by a genomic region called pathogenicity island 1 (SPI-1). This system allows Salmonella to inject proteins called effectors into the host cell cytoplasm. These effectors manipulate the host cell actin cytoskeleton and induce membrane ruffling, which leads to bacterial internalization. To invade M cells, Salmonella uses another type III secretion system encoded by pathogenicity island 2 (SPI-2). This system also injects effectors into the host cell, but these effectors interfere with the normal function of the phagosome, which is the membrane-bound compartment where bacteria are enclosed after internalization. By doing so, Salmonella prevents the fusion of the phagosome with lysosomes, which are organelles that contain enzymes that degrade bacteria. This allows Salmonella to survive and replicate within the phagosome.
After invading the intestinal epithelium, Salmonella can disseminate to other organs through the bloodstream or the lymphatic system. This can result in systemic infections such as enteric fever, bacteremia, or focal infections in different tissues. The severity and outcome of Salmonellosis depend on several factors, such as the serotype and virulence of the bacteria, the dose and route of infection, and the immune status and genetic susceptibility of the host. Some of the factors that contribute to Salmonella virulence are:
- The presence of flagella, which enable motility and chemotaxis
- The expression of fimbriae, which mediate adhesion to host cells
- The production of lipopolysaccharide (LPS), which is an endotoxin that triggers inflammation
- The synthesis of siderophores, which scavenge iron from the host
- The secretion of toxins, such as enterotoxin and cytotoxin, which damage host cells and induce fluid secretion
- The modulation of host immune responses, such as apoptosis, cytokine production, and antigen presentation
Salmonellosis is a complex disease that involves multiple interactions between the bacteria and the host at different levels. Understanding these interactions can help to develop better strategies for prevention, diagnosis, and treatment of this important public health problem.
The symptoms of salmonellosis vary depending on the type and severity of the infection. The most common form of salmonellosis is gastroenteritis, which is also known as food poisoning. Gastroenteritis usually occurs within 6 to 48 hours after ingesting contaminated food or water. The common symptoms of gastroenteritis are:
- Diarrhea (that can be bloody)
- Stomach (abdominal) cramps
- Blood in the stool
Gastroenteritis usually lasts for 2 to 7 days and is self-limiting, meaning that it resolves without specific treatment. However, some people may develop complications such as dehydration, electrolyte imbalance, reactive arthritis, or irritable bowel syndrome.
The other extreme form of salmonellosis is enteric fever, which is caused by Salmonella Typhi and Salmonella Paratyphi. Enteric fever is also known as typhoid fever or paratyphoid fever. Enteric fever usually occurs within 10 to 14 days after ingesting contaminated food or water. The symptoms of enteric fever are:
- Constipation or diarrhea
- High fever (up to 104°F or 40°C)
- Severe headache
- Abdominal distension
- Rose-colored spots on the chest or abdomen
- Enlarged spleen and liver
- Delirium or confusion
Enteric fever can be life-threatening if not treated promptly with antibiotics. Complications of enteric fever include intestinal perforation, hemorrhage, septic shock, myocarditis, pneumonia, meningitis, or osteomyelitis.
Some people may have a mild or asymptomatic infection with Salmonella, meaning that they do not show any signs or symptoms. However, they may still carry and shed the bacteria in their feces and transmit the infection to others. This is especially common among children under 5 years old and people with weakened immune systems.
The diagnosis of Salmonellosis depends on the isolation and identification of Salmonella bacteria from the patient`s stool, blood, tissue, or fluids. The following methods are commonly used for this purpose:
- Culture method: This is the standard method for detecting Salmonella in clinical specimens. The specimens are inoculated on selective and differential media such as Desoxycholate Citrate Agar, Xylose Lysine Deoxycholate Agar, Salmonella-Shigella Agar, or MacConkey Agar. If the sample is small or diluted, enrichment media such as Tetrathionate or Selenite F broth are used to increase the number of bacteria. The typical colonies of Salmonella are identified by their morphology, biochemical reactions, and serological tests.
- Serology test: This is a method that measures the antibodies against the antigens of Salmonella in the patient`s blood. The most common serology test is the Widal agglutination test, which detects antibodies against the somatic (O) and flagellar (H) antigens of Salmonella. A high somatic (O) antibody titer indicates acute infection, while a high flagellar (H) antibody titer indicates enteric fever. However, this test has low specificity and sensitivity, especially in endemic areas where cross-reactions with other pathogens may occur. Therefore, this test has been replaced by more accurate methods such as enzyme-linked immunosorbent assay (ELISA) or polymerase chain reaction (PCR).
- Blood culture for enteric fever: This is a method that isolates Salmonella from the blood clot of patients with enteric fever. The blood clot is digested with streptokinase enzyme and then cultured on selective media. This method can be done at any stage of illness and has a high sensitivity and specificity.
- Molecular typing methods: These are methods that identify specific strains of Salmonella based on their genetic characteristics. Some of these methods are PCR-based methods, pulsed-field gel electrophoresis (PFGE), and plasmid typing. These methods are useful for epidemiological investigations, outbreak detection, and antibiotic resistance monitoring.
Salmonellosis treatment depends on the severity and location of the infection. The main goal is to replace fluids and electrolytes lost to diarrhea and vomiting. Water, oral rehydration solutions, energy drinks, herbal tea, broth, and juice are some options. In some cases, anti-diarrheal medicines or antibiotics may be recommended. Life-threatening infections may require a combination of drugs and surgery.
Antibiotics are not recommended for Salmonella enterocolitis, but in the case of enteric fever and typhoid fever, antibiotics such as Ciprofloxacin, Azithromycin, and Ceftriaxone are used. The use of antibiotics in uncomplicated cases shortens the illness but may increase the antibiotic-resistant strains. Typhoid vaccines are recommended for travelers, but the vaccine is ineffective against S. Paratyphi A, B, and C.
The major reservoir is poultry and livestock; therefore, to prevent the infection, removing the Salmonella-infected animals, changing the slaughtering methods, maintaining good hygiene practices during processing, and protecting processed foods from cross-contamination are recommended. It is important to consume properly cooked foods, pasteurized milk and milk products, and refrigerate foods at an adequate temperature in terms of storage.
Chlorine solutions, iodine, quaternary ammoniums, and phenolic compounds are very good at killing Salmonella on surfaces. However, it is very important to get rid of organic matter and bedding first, followed by wet cleaning with high-pressure hot water/steam and then disinfection.
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