Etiology
Nontyphoidal salmonellosis is caused by infection with Salmonella enterica serotypes other than S Typhi or S Paratyphi.
The most common serotypes identified in the US associated with human disease are S Enteritidis, S Typhimurium, S Newport, S Javiana, and S I 4,[5],12:i:- in descending order.[9]
Pathophysiology
Salmonella infections occur mainly via ingestion of contaminated foodstuffs, although they can also be waterborne. Occasionally food handlers transmit infection, and contamination of drugs/solutions has been described.
Organisms must survive the low pH found in the hostile gastric environment. Conditions of achlorhydria, which may occur among infants, in pernicious anemia, or with the use of antacid medications, increase the risk of infection. After passing through the stomach, the organisms invade the mucosa lining the distal ileum and proximal colon. Initial host responses involve neutrophil infiltration, followed by lymphocytes and macrophages.[34] It is at this stage that clinical manifestations such as abdominal cramps and diarrhea occur.
The infective dose required for clinical illness has been estimated as approximately 10⁶ organisms.[35] These estimates are based on challenge studies, but the infective dose in foodborne outbreak is sometimes considerably lower, suggesting that certain food vehicles may protect Salmonella from being destroyed in the stomach.[35][36] The higher the ingested dose, the shorter the incubation period may be. Larger ingested dosages may also be associated with more severe disease.[37] Fewer organisms, in the range of 10² to 10³, may also cause disease in highly susceptible hosts. For instance, patients with low gastric pH (given that acidity is an important barrier to Salmonella infections) may develop disease after ingesting a smaller quantity of bacteria.[35][38][39]
Classification
Modified Kauffman and White scheme for classifying Salmonella[1][2]
The classification of Salmonella is complex, and many familiar species designations such as Salmonella typhi and Salmonella choleraesuis, based on antigenic characteristics, are no longer regarded as taxonomically correct.[3] The genus Salmonella is now considered to consist of only 2 species, S enterica and S bongori. Nearly all human infections are due to serotypes within the species S enterica.
Within S enterica there are 6 subspecies (I, II, IIIa [formerly Arizona], IIIb, IV, and VI). Most human cases are due to subspecies type I infections. Of note, the former subspecies V is now the separate species S bongori.
Within S enterica, there are >2500 serotypes, which are classified based on a modified scheme of Kauffman and White. These serotypes are differentiated by their somatic O-antigen (lipopolysaccharide), capsular antigen when present (Vi), and flagellar H antigens. The names are often derived from the city in which the serotype was first described (e.g., S Heidelberg). Other serotype names are defined by a formula.
Of note, the names of Salmonella often omit the species name and use the serovar instead (S Newport), but the formal name would be Salmonella enterica serotype Newport.
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