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Salmonella is the most frequently reported foodborne illness in the United States, and is the second most common foodborne illness worldwide. The incidence of human salmonellosis varies depending on the geographic, demographic, socioeconomic, meterological and environmental factors. The case rate can be approximately one to three hundred per 100,000 population. In 1996, the CDC documented 39,027 salmonellosis cases in the U.S.
The Salmonella family includes over 2,300 serotypes of bacteria, but only two types, Salmonella enteritidis and Salmonella typhimurium, account for about half of all human infections. Strains that do not cause symptoms in animals can affect humans and vice versa.
Salmonella bacteria have been known to cause illnesses for more than 100 years when it was discovered by Dr. Daniel Salmon. Initially, the Salmonella serotype associated with foodborne illness was S. typhimurium. Then in the 1980's, a public health concern emerged with S. enteritidis since it was capable of systemic colonization of poultry, leading to widespread contamination of raw eggs or lightly cooked food containing eggs.
Since the 1990's, a specific type of S. typhimurium known as a definitive type (DT)104, has become a problem in the United Kingdom, Western Europe, and recently in the United States. Strains of S. typhimurium DT 104 are invasive and may contain large plasmids conferring resistance to antibiotics such as ampicillin, chloramphenicol, streptomycin, sulphonamides,tetracycline, trimethoprim and cirpofloxacin.
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Classification | | Members of the International Subcommittee on Taxonomy of Enterobacteriaceae have designated Salmonella choleraesuis as the single species within the genus Salmonella. S. choleraesuis consists of five subspecies: S. enterica (subsp.I); S. salamae (subsp. II); S. arizonae (subsp IIIa) S. diarizonae (subsp.IIIb); S. houtenae (subsp.IV); and S. bongori (subsp. V).
Species and subspecies of a genus are analyzed and classified more specifically by their antigenic formula. Strains of species and subspecies are classified into serogroups and serovars. Strains are divided into serogroups based on the differences in epitopes of lipopolysaccharide (LPS), a major component of the outer membrane of Gram-negative bacteria. Serogroups are further divided into serovars based on variation in flagellins or H antigens, the subunit proteins of flagella. Strains of individual serovars can be subdivided using a range of phenotypic and genotypic methods such as biotyping, phage typing, antibiotic susceptibility testing or resistotyping, restriction endonuclease analysis, ribotyping, IS200 typing, pulsed-field gel electrophoresis, plasmid profiling, and fingerprinting. The current method of choice for strain analysis is phage typing.
The genus Salmonella is comprised mostly of facultatively anaerobic, oxidase-negative, catalase-positive, Gram-negative rod-shaped bacteria. Most strains are motile and ferment glucose with production of both acid and gas. There are several biochemical tests used to characterize Salmonella from other genera within the family Enterobacteriaceae.
Epidemiologically, Salmonella strains can be classified according to their adaptation to human and animal hosts:
- Group 1 - S. typhi and S. paratyphi; causes enteric fever only in humans and in higher primates.
- Group 2 - Causes diseases in animals such as S. dublin in cattle, S. choleraesuis in pigs, but only infrequently in humans; when these strains do cause disease in humans, it is often invasive and can be life-threatening.
- Group 3 - S. enteriditis and S. typhimurium, the two most important strains for salmonellosis (transmitted from animals to humans); includes the remaining strains; typically, such strains cause gastroenteritis which is often mild and self-limiting but can be severe in the young, the elderly, and patients with weakened resistance against infectious diseases.
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Salmonella enteritidis | |
Salmonella enteritidis (SE) is a serovar in the subspecies S. enterica. The full name of SE is S. choleraesuis subsp. enterica serovar Enteritidis (most literature reports it as S. enteritidis).
S. enteritidis is a Gram-negative, motile, non-spore-forming rod and is facultatively anaerobic, catalase-positive and oxidase-negative. As with most Salmonella serovars, it ferments carbohydrates with production of acid and gas. SE is found mainly in avian species and is easily transmissible to mammals. Chickens are the common vehicle of SE transmission to humans through eggs and chicken.
SE contains several virulence factors that enhance pathogenicity. SE produces a heat-labile enterotoxin and a heat-labile cytotoxin. The cytotoxin inhibits protein synthesis in intestinal epithelial cells and damages intestinal mucosa which is characteristic of salmonellosis.
LPS or lipopolysaccharide of the bacterial cell wall is an additional virulence factor. The O-side chain oligosaccharide of the LPS appears to be a key to the virulence of SE. Changes to the structure of this side chain can affect SE ability to resist macrophage phagocytosis and the strain can become avirulent.
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Salmonella typhimurium | | S. typhimurium can be found in a broad range of species as well as the environment. Together with S. enteritidis, it causes the overwhelming majority of cases of zoonotic salmonellosis in many countries. Epidemiological investigations must discriminate among serovars, and a technique called phage typing is used for this purpose in many countries.
Antimicrobial therapy is used extensively to combat S. typhimurium infection in animals. The evolution of a strain resistant to the commonly-used antibiotics has made infections with S. typhimurium in food animals difficult to control. Multi-drug resistant S. typhimurium DT 104 initially emerged in cattle in 1988 in England and Wales. Subsequently, the strain has been isolated from poultry, sheep, pigs, and horses. S. typhimurium, rather awkwardly called R-type ACSSuT, has developed multiple drug resistance as an integral part of the genetic material of the organism. Unlike other Salmonella strains, multiresistant S. typhimurium DT 104 is likely to retain its drug resistance genes even when antimicrobial drugs are no longer used.
The primary route by which humans acquire S. typhimurium infection is by consumption of contaminated food of animal origin. Unlike S. enteritidis which is mainly associated with poultry and eggs, multi-drug resistant S. typhimurium DT 104 can be found in a broad range of food. Outbreaks in the United Kingdom and Northern Ireland have been linked to poultry, a variety of meats and meat products, and unpasteurized milk. In addition to acquiring infection from contaminated food, human cases have also occurred where individuals have had contact with infected cattle. A small proportion of cases may have contracted infection from pets such as cats and dogs, which can also be infected with this strain of Salmonella. Pets probably acquire the infection like humans, in other words through consumption of contaminated raw meat, poultry or poultry-derived products.
Apart from its multiple resistance to antibiotics, Salmonella typhimurium DT 104 has similar characteristics to other Salmonella and is no more resistant to food processing conditions such as acids, drying, preservatives or disinfectants than other serotypes.
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Salmonellosis | | Salmonellosis in humans is contracted mainly through the consumption of raw or undercooked contaminated food of animal origin such as meat, poultry, eggs and milk, although many other foods have been implicated in its transmission. In developed countries human to human transmission is uncommon but can occur. Little is known about the epidemiology in developing countries but spread within hospitals and health centres has been reported.
Clinical symptoms of human salmonellosis include acute onset of fever, abdominal pain, diarrhea, nausea and vomiting. Dehydration can become severe and life threatening, especially in elderly or young children. Antibiotic treatment is necessary in less than 2 percent of the clinical cases. The incidence is particularly high in children and the elderly accounting for up to 60 percent of all reported laboratory confirmed cases. Studies in developed countries indicate that more than 80 percent of all salmonellosis cases occur individually rather than as outbreaks.
Salmonella typhimurium DT 104 is becoming a public health concern in the United Kingdom because of the increase in cases of salmonellosis in man, with the main source of foodborne infection being meat products such as burgers and sausages. Contaminated meat products, such as sausages and burgers, are the main sources of foodborne infection. The organism is resistant to a wide range of anti-infectious agents and, as a result, the illness is more difficult to treat. A 3 percent mortality rate is associated with infection by this organism, especially in the elderly.
Salmonellosis is also an important animal disease affecting many species of domestic and wild animals. Salmonella research has focused on animal health and developing a poultry vaccine in order to control Salmonella transmission between animals, and from animals to humans.
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Pathogenesis of Salmonella | Infection begins with ingestion of a dose for the bacterium sufficient to broach the first-line host defenses and colonize the gastrointestinal tract. Whether of not Salmonella has a high or low infectious dose depends upon:
- Virulence factors
- Physiological state of the pathogen
- Life stage and immunty of the host
- Food matrix
Inherently, different strains of Salmonella possess a diversity of virulence factors and the number and type of these factors affect pathogenesis.
Pathogen survival is impacted both in the food matrix and entry into the host by the physiological state of the pathogen, stationary phase or active log phase.
The life stage of the host also is a factor. For example, children have a poorly developed immune system and low gastric acidity. The elderly and immuno- compromised also demonstrate a weak immune response to infection.
The food matrix high in fat or protein affects the infectious dose because fat and protein offer protection to the organism within it, both within the host and in the external environment. Salmonella is transported quickly to the lower gastrointestinal tract in foods composed of high fat serves as a barrier to gastric acidity.
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USDA Research | |
Microbial pathogen reduction from farm to fork is a focus of food safety research in the USDA Agricultural Research Service. In its National Program Food Safety 108, Salmonella is among one of the bacterial pathogens investigated.
Research Objectives:
- Understand the molecular ecology of Salmonella and develop effective intervention strategies to reduce their transmission in livestock.
- Investigate the transmission dynamics of antimicrobial resistance in integrated animal and human populations using molecular epidemiology.
- Use a novel genetic system to identify Salmonella proteins that are essential for growth, virulence or antibiotic resistance.
- Understand antibiotic resistance and prevent its spread and acquisition by other bacteria and other hosts.
- Monitor animal Salmonella isolates to determine the frequency, characteristics and trends of resistance determinants present in the bacterial population studies.
- Determine why Salmonella contamination persists in produce such as canteloupe.
- Develop intervention strategies to reduce Salmonella contamination in poultry products.
Develop molecular-based methods to detect, quantify, and characterize Salmonella.
- Understand Salmonella infection and contamination in the holding of market swine, and evaluate cost-effective perimarketing interventions that reduce the number of Salmonella infected swine entering the slaughter process.
- Determine the effect of stress on the migration and numbers of Salmonella in swine.
- Determine the recovery incidence of Salmonella in commercial turkeys.
- Develop a poultry vaccine against Salmonella enterica serovar Enteritidis.
- Develop improved methods for detecting Salmonella enteritidis (SE) infections in laying flocks and SE contamination in eggs.
- Determine whether Salmonella enteritidis infections are exacerbated in commercial hens that are undergoing molt via feed withdrawal.
- Investigate why Salmonella enteritidis strains vary in their ability to contaminate eggs and why it is the only serotype to routinely contaminate eggs.
For an update on Salmonella research conducted by the USDA/ARS view the
National Program 108: Food Safety 2005 Annual Reports.
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Resources |
- Salmonella typhimurium DT 104
Institute of Food Science and Technology, UK.
- Salmonellosis
NIH/National Institute of Allergy and Infectious Diseases
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Salmonella
FDA/CFSAN
- Progress Report on Testing of Raw Meat and Poultry Products
USDA/FSIS. 1998 - 2002.
- Salmonella Enteritidis Risk Assessment
USDA/FSIS/Salmonella Enteritidis Risk Assessment Team. August 1998.
- Surveillance for Outbreaks of Salmonella Serotype Enteritidis
CDC/MMWR. 1998 - 2002.
- Salmonella
WHO
- Genome Sequencing Center - The Enteric Bacteria
Washington University Medical School in St. Louis
- Second Report on Salmonella and Eggs
(PDF Format)
Advisory Committee on the Microbiological Safety of Food. May 2001.
- A Possible New Vaccine to KO Salmonella in Chicken Eggs
(PDF Format)
USDA/ARS. May 2003.
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