The Salmonella

The Salmonella-Arizona Group
Salmonellae are often pathogenic for humans or animals when acquired by the oral route. They are transmitted from animals and animal products to humans, where they cause enteritis, systemic infection, and enteric fever.
Morphology & Identification
Salmonellae vary in length. Most isolates are motile with peritrichous flagella. Salmonellae grow readily on simple media, but they almost never ferment lactose or sucrose. They form acid and sometimes gas from glucose and mannose. They usually produce H2S. They survive freezing in water for long periods. Salmonellae are resistant to certain chemicals (eg, brilliant green, sodium tetrathionate, sodium deoxycholate) that inhibit other enteric bacteria; such compounds are therefore useful for inclusion in media to isolate salmonellae from feces.
Classification
The members of the genus Salmonella were originally classified on the basis of epidemiology, host range, biochemical reactions, and structures of the O, H, and Vi (when present) antigens. The names (eg, Salmonellatyphi, Salmonella typhimurium) were written as if they were genus and species. There are more than 2500 serotypes of salmonellae, including more than 1400 in DNA hybridization group I that can infect humans. Four serotypes of salmonellae that cause enteric fever can be identified in the clinical laboratory by biochemical and serologic tests. These serotypes should be routinely identified because of their clinical significance. They are as follows: Salmonella Paratyphi A (serogroup A), Salmonella Paratyphi B (serogroup B), Salmonella Choleraesuis (serogroup C1), and Salmonella Typhi (serogroup D). The more than 1400 other salmonellae that are isolated in clinical laboratories are serogrouped by their O antigens as A, B, C1, C2, D, and E; some are nontypeable with this set of antisera.
Variation
Organisms may lose H antigens and become nonmotile. Loss of O antigen is associated with a change from smooth to rough colony form. Vi antigen may be lost partially or completely. Antigens may be acquired (or lost) in the process of transduction.
Pathogenesis & Clinical Findings
Salmonella Typhi, Salmonella Choleraesuis, and perhaps Salmonella Paratyphi A and Salmonella Paratyphi B are primarily infective for humans, and infection with these organisms implies acquisition from a human source. These organisms almost always enter via the oral route, usually with contaminated food or drink. The mean infective dose to produce clinical or subclinical infection in humans is 105–108 salmonellae Among the host factors that contribute to resistance to salmonella infection are gastric acidity, normal intestinal microbial flora, and local intestinal immunity .
Salmonellae produce three main types of disease in humans, but mixed forms are frequent .


Table 3 Clinical Diseases Induced by Salmonellae.



  Enteric Fevers Septicemias Enterocolitis
Incubation period 7–20 days Variable 8–48 hours
Onset Insidious Abrupt Abrupt
Fever Gradual, then high plateau, with "typhoidal" state Rapid rise, then spiking "septic" temperature Usually low
Duration of disease Several weeks Variable 2–5 days
Gastrointestinal symptoms Often early constipation; later, bloody diarrhea Often none Nausea, vomiting, diarrhea at onset
Blood cultures Positive in first to second weeks of disease Positive during high fever Negative
Stool cultures Positive from 2nd week on; negative earlier in disease Infrequently positive Positive soon after onset