Small Gram-negative bacillus
Antigens:
Usually motile (H antigen)
Possess polysaccharide capsule (K antigen on most Salmonella spp. or Vi antigen of Salmonella typhi and Salmonella paratyphi)
Specific O antigens
H2S produced from inorganic sulfur
Acid and gas produced from glucose
Note: S. typhi is a non-gas producer and produces minimal H2S
Salmonella spp. are relatively resistant to bile acids and this trait is utilized for selective isolation media
Family Enterobacteriaceae:
All Enterobacteriaciae:
ferment glucose
reduce
nitrates (NO3 to NO2 or all the way to N2)
are oxidase negative
The genus Salmonella contains over 2,000 sero-species and is one of the most important pathogens in the family Enterobacteriaceae
Taxonomically, all strains of Salmonella fall within one species, S. enterica, but this nomenclature has not caught on and the genus continues to be recognized by the popular species names, many named on the basis of serotyping and outbreaks
Salmonella enteritidis (enteritis)
- Salmonella typhimurium (enteritis)
- Salmonella cholerasuis (septicemia)
Salmonella typhi (enteric fever; asymptomatic carriage)
Salmonella paratyphi (enteric fever; asymptomatic carriage)
The majority of medically important isolates are found in only a few sero-species
Enteritis (acute gastroenteritis): nausea, vomiting, nonbloody diarrhea, fever (see clinical progression described below in Pathogenesis & Immunity section)
Enteric fever: prototype is typhoid fever and less severe paratyphoid fever (see clinical progression described below in Pathogenesis & Immunity section)
Septicemia (particularly S. choleraesuis)
Prolonged state with fever, chills, anorexia, and anemia
Lesions in other tissues
Asymptomatic carriage: gall bladder is the reservoir for S. typhi
Enteritis
Members of this genus are ubiquitous in the environment and are found in association with both warm and cold blooded animals
High infectious dose (108 CFU)
Foodborne disease: poultry, eggs, dairy products, cross-contaminated foods
- 6-48h incubation
Enteric fevers
Infectious dose (106 CFU)
Transmission via fecal-oral route = person-to-person spread by chronic carrier through fecally-contaminated food or water
- 10-14 day incubation to signs of sepsis; sustained fever (delirium) for one to several weeks before abdominal pain and gastrointestinal symptoms
Exposure to the O and H antigens stimulates the production of specific antibodies
Natural infection provides limited protection
Surface antigens: play a role in attachment and intracellular survival
Invasiveness
Penetrate mucus, adhere to and invade into epithelial layer (enterocytes) of terminal small intestine (ileum) and further into subepithelial tissue
Bacterial cells are internalized in endocytic vacuoles in which the organisms multiply
- PMN's confine infection to gastrointestinal (GI) tract, but organisms may spread hematogenously (through blood, i.e., septicemia) to other body sites
Inflammatory response mediates release of prostaglandins, stimulating cAMP and active fluid secretion with loose diarrheal stools
Epithelial destruction occurs during late stage of disease
Exotoxins
Surface antigens: play a role in attachment and intracellular survival
Vi antigen (capsular polysaccharide) of S. typhi and S. paratyphi
Smooth and rough variants
Invasiveness
Pass through intestinal epithelial cells in ileocecal region and infect the regional lymphatic system, invade the bloodstream, and infect other parts of the reticuloendothelial system
Organisms are phagocytosed by macrophages and monocytes, but survive, multiply and are transported to the liver, spleen, and bone marrow where they continue to replicate
- During the second week, the organisms reenter the bloodstream and cause prolonged bacteremia; biliary tree and other organs are infected; gradually increasing and sustained fever most likely from endotoxemia
During the second to third week, the bacteria colonize the gall bladder and reinfect the intestinal tract with diarrheal symptoms and possible necrosis of the Peyer's patches
Endotoxin: could be responsible for the fever or the enteric lesions seen in typhoid fever
Stool specimens and rectal swabs should be cultured soon after collection or placed in appropriate transport medium (e.g., Cary-Blair medium)
Readily isolated on selective/differential agar media (e.g., XLD, XLT-4, SS, or brilliant green agar supplemented with novobiocin)
Lactose nonfermenter
Suspect colonies are further analyzed using various biochemicals
Further identification should be pursued by serotyping and molecular diagnostic methodology
Enteritis: Supportive therapy, dehydration prevention, balancing electrolytes; Avoid antibiotics in gastroenteritis to prevent prolonged carrier state
Enteric fever or septicemia: ampicillin or chloramphenicol; resistant isolates occur
S. typhi carried in gall bladder; surgically remove and treat with ampicillin; 85% cure of carrier state
Observe water standards
Cook and store foods properly
Treat carriers, special precautions with food handlers
Vaccines for typhoid fever
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