What is Salmonella? What are its characteristics?
Salmonella (S.) is the genus name for a large number (over 2,500) of types of bacteria. Each type is distinctly identifiable by its specific protein coating. The types are otherwise closely related. Salmonella bacteria are rod-shaped, flagellated, Gram stain-negative and are known to cause disease in humans, animals, and birds (especially poultry) worldwide.The terminology that identifies the particular protein coats, or serovars, is not well settled, and what previously were thought to be various species of the genus Salmonella are now thought to be serovars of only two species, S. enterica and S. bongori. However, these designations are not always accepted in the scientific literature and so common serovars that have been named in the past are still used (for example, S. typhi, S. typhimurium, S. enteritidis, S. cholerasuis). The serovars are identified by the Kauffman-White classification that uses two major types of antigens (somatic O and flagellar H) to distinguish the over 2,500 types of Salmonella bacteria. Sometimes laboratories or other reporting agencies identify isolates simply as Salmonella spp (species) and do not identify the serovars.
Who discovered Salmonella?
The bacteria were first isolated by Theobald Smith in 1885 from pigs. The genus name "Salmonella" was derived from the last name of D. E. Salmon, who was Smith's director.
What kinds of diseases does Salmonella cause? What are symptoms of Salmonella infection?
Salmonellosis (gastroenteritis characterized by nausea, vomiting, and diarrhea) is the most common disease caused by the organisms. Over 1.4 million cases per year occur in the U.S., and the rest of industrialized countries have similar rates. Countries with poor sanitation have a much higher incidence of Salmonellosis.Typhoid fever occurs when some of the Salmonella organisms are not killed by the normal human immune defenses (macrophage cells). Salmonella then survive and grow in the human spleen, liver, and other organs and may reach the blood (bacteremia). Salmonella can be shed from the liver to the gallbladder, where they can continue to survive and be secreted into the patient's feces for up to a year. Symptoms include high fevers up to 104 F, sweating, inflammation of the stomach and intestines, and diarrhea. Symptoms usually resolve but many patients become Salmonella carriers. Approximately half of patients develop slow heartbeat (bradycardia), and about 30% of patients get flat, slightly raised red or rose-colored spots on the chest and abdomen. Typhoid fever is also referred to as enteric fever.Paratyphoid fever, also termed enteric fever, has symptoms like typhoid, but it's usually not as severe. Subtypes are A, B, and C and vary by having small changes in symptoms, such as more rose spots (A), in conjunction with Herpes labialis and gastroenteritis (B), rarely, with septicemia and abscesses (C).
How is Salmonella transmitted to humans?
Salmonellosis is spread to people by ingestion of Salmonella bacteria that contaminate food. Salmonella is worldwide and can contaminate almost any food type, but recent outbreaks of the disease involve raw eggs, egg products, fresh vegetables, cereal, and contaminated water. Contamination can come from animal or human feces that contact the food during its processing or harvesting. New data about types of food contamination (food poisoning by Salmonella spp) is available from the U.S. Centers for Disease Control and Prevention (CDC). Potential direct sources of Salmonella are pets such as turtles, dogs, cats, most farm animals, and humans that are infected or are carriers of the organisms.Although typhoid and paratyphoid fevers can be transmitted by the same methods as Salmonellosis listed above, the most frequent manner of transmission is by the feces of infected people contaminating the water or food source of uninfected people.
How do Salmonella spp cause disease in people?
Usually, Salmonella spp are ingested. Researchers suggest that at between 1 million to 1 billion bacteria are needed to cause infection. This means food, water, or other sources of contamination contain large amounts of bacteria. Although human stomach acid can reduce and sometimes eliminate Salmonella spp, occasionally some get through to the intestine and then attach and penetrate the cells. Toxins produced by the bacteria can damage and kill the cells that line the intestines, which results in intestinal fluid loss (diarrhea).Some Salmonella can survive in cells of the immune system and can reach the bloodstream causing blood infection (bacteremia). Other Salmonella spp can enter the gall bladder leaving the affected patient a chronic carrier of the organisms. Salmonella can then be shed with the bile from the gall bladder into the feces and then may infect other people. Perhaps the most famous such carrier, the first to be identified in 1907, was a cook named Mary Mallon, also known as "Typhoid Mary." She was suspected of infecting hundreds of individuals.
How do you diagnose Salmonella infections?
Feces of patients that have symptoms of food poisoning or typhoid fever are cultured for Salmonella. Often, the diagnosis is first suggested by recognition of an outbreak (multiple patients having symptoms of a disease all at about the same time and often, from the same food or water source). Usually, the laboratory needs to be notified that S. spp is suspected so they will choose the correct testing media (selective agar media). Serovars are identified by serotyping (detecting bacterial proteins by using specific immunological tests).
How are Salmonella infections treated?
Treatment for enteritis or food poisoning is controversial. Some doctors recommend no antibiotics since the disease is self-limited, while others suggest using antibiotics such as ciprofloxacin for 10-14 days. Patients identified as immunosuppressed (for example, patients with AIDS or undergoing cancer chemotherapy) should receive antibiotics. Some investigators believe antibiotics prolong the carrier state.Treatment for typhoid or enteric fevers with septicemia is not controversial. Antibiotics, often given intravenously, are needed. These S. spp also should be tested for antibiotic resistance as some Salmonella species have been reported to be resistant to multiple antibiotics.Supportive therapy for both enteritis and enteric fevers consists mainly of preventing dehydration and electrolyte abnormalities (for example, abnormal levels of potassium and sodium ions) with fluids containing electrolytes (for example, IV fluids or oral fluids like sports drinks).Carriers of Salmonella are considered to be infected even though they may show no symptoms. Carriers can infect other people and need to be cured of the carrier state. About 85% of carriers can be cured by a combination of surgery to remove their gallbladder and antibiotic treatments.
How can Salmonella infections be prevented?
Cleanliness is a key to prevention. Hand washing with soap and water, especially after handling eggs, poultry, and raw meat is likely to reduce the chance for infections. Chlorine-treated drinking water, washed produce, and not ingesting undercooked foods such as eggs can also reduce the chance of exposure to Salmonella. Avoiding direct contact with animal carriers of Salmonella (for example, turtles, snakes, pigs) also prevents the disease. Public-health authorities that enforce restaurant cleanliness and employee hand washing have helped in general prevention. Human carriers of Salmonella should never work in the food-handling service industry and ideally should undergo gallbladder removal and antibiotic therapy for an attempt for a cure of the carrier state. Although some Salmonella vaccines are available for poultry and animals, human vaccines are still under development.
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