A number of culture media have been developed for the isolation of Salmonella. Some media are differential and non-selective, i.e., they contain lactose and a pH indicator but do not contain an inhibitor for non-Salmonella species (e.g., bromocresol purple lactose agar). Other media are differential and somewhat selective; that is, in addition to lactose and a pH indicator, they contain an inhibitor for non-Enterobacteriaceae (e.g., MacConkey agar and eosin-methylene blue agar).
The most commonly used selective media for Salmonella SS agar, bismuth sulfite agar, Hektoen enteric (HE) medium, brilliant green agar, and xylose-lysine-deoxycholate (XLD) agar. All of these media contain both selective and differential components and are commercially available.
The media used for the identification of salmonella the same as those used for the identification of all Enterobacteriaceae. Most Salmonella are motile with peritrichous flagella, but non-motile variants may occasionally occur. Most strains grow on nutrient agar as smooth colonies, 2–4 mm in diameter. Most strains are prototrophs, requiring no growth factors. However, auxotrophic strains do occur, particularly in host-adapted serovars such as Typhi and Paratyphi A.
Table 1. Characteristics shared by most Salmonella belonging to subspecies I
- Motila, Gram-negative bacteria
- Lactose-negative; acid and gas from glucose, mannitol, maltose, and sorbitol; no acid from adonitol, sucrose, salicin, or lactose
- ONPG test negative (lactose-negative)
- Indole test negative
- Methyl red test positive
- Voges-Proskauer test negative
- Citrate-positive (growth on Simmons citrate agar)
- Lysine decarboxylase positive
- Urea negative
- Ornithine decarboxylase positive
- H₂S produced from thiosulfate
- Does not grow with KCN
- Phenylalanine and tryptophan deaminase negative
- Gelatin hydrolysis negative
Genetics of Salmonella
The genetic map of Salmonella LT2 does not differ significantly from that of Escherichia coli K-12. The F plasmid can be transferred to Typhimurium, and an Hfr strain of Typhimurium can then be selected. Conjugative chromosome transfer can occur from Typhimurium Hfr to E. coli or from E. coli Hfr to Typhimurium. Chromosomal genes responsible for O-, Vi-, and H-antigens can be transferred from Salmonella Escherichia.
Salmonella also harbor temperate phages and plasmids. Plasmids in Salmonella encode antibiotic resistance (resistance plasmids are common due to the selective pressure from widespread antibiotic treatment), bacteriocins, metabolic traits such as lactose or sucrose fermentation, or antigenic changes in the O-antigen.