Salmonellae are facultative, gram-negative, motile bacilli that ferment glucose but not lactose or sucrose. Salmonellae are able to utilize citrate as a sole carbon source where other genera require a more complex source of this nutrient. All salmonellae, with the exception of Salmonella typhi, produce gas during the fermentation process. Salmonellae are oxidase negative, reduce nitrates to nitrites, and do not require NaCl for growth. They grow at temperatures between 8 and 45°C in a pH range of 4-9 and require water activities (aw) above 0.94 (7). Salmonellae are sensitive to heat and are generally killed at temperatures of 70°C or above. They are also susceptible to pasteurization of milk at 72.1°C for 15 seconds (7). As a result, the bacteria is killed if food is cooked long enough to reach this temperature throughout. Salmonellae are resistant to drying and may survive for years in dust and dirt.
The genus Salmonella has been classically divided into three species: S. typhi, S. cholerae-suis, and S. enterica. These species are further cataloged by their antigenicity, as described by the Kauffmann-White scheme (8). Antigen formulas represent serotypes rather than species (S. enterica serotype Typhimurium), although designations such as S. typhimurium are still accepted. Serotypes are identified by highly specific O (somatic) and H (flagellar) antigens. A given serotype will contain a specific combination of multiple O and H antigens. Each serotype of salmonellae can be further subdivided into phage type depending upon their reactivity with a defined set of bacteriophages. Phage typing is generally used when origin and characteristics of an outbreak of infection must be determined. Phage typing is currently being used in the investigation of S. enterica serotype Enteriti-dis contamination of eggs and has played a key role in identifying certain phage types of Typhimurium that are highly antibiotic resistant. Both serotyping and phage typing are done only in major salmonellae typing centers, which have the collection of antisera and phages necessary for such work.
Salmonellae can also be broadly divided into groups on the basis of O antigen composition. The reagents for serogrouping are readily available commercially, and serogrouping can be done routinely in most standard microbiology laboratories. Most isolates from natural sources fall into five serogroups, A-E. S. typhi and S. cholerae-suis contain only one serotype each in groups D and C, respectively, while S. enterica contains over 2000 different serotypes in all groups including C and D. Considerable overlap in antigenic composition is responsible for the cross-reactivity commonly seen in serological tests with salmonellae.
Recently multilocus enzyme electrophoresis and comparative sequence analysis of housekeeping and rRNA genes have revealed that the genus Salmonella may actually contain only two lineages that have diverged considerably from each other during evolution (9,10). By using genetic distance determined by multilocus enzyme electrophoresis and results of DNA-DNA hybridization studies as criteria, it has been proposed that these lineages represent two distinct species, designated Salmonella enterica (11) and Salmonella bongori (12). S. enterica can be further subdivided into subspecies designated by Roman numerals (10). S. enterica subspecies I contains 1367 serotypes, is mainly isolated from warm-blooded animals (mammals and birds), and accounts for >99% of all clinical isolates. The remaining subspecies (II, Ilia, Illb, IV, VI, and VII) are mainly isolated from coldblooded animals and account for <1% of clinical isolates.
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