Description
Salmonella belong to the family Enterobacteriaceae. They are facultatively anaerobic, motile, gram-negative bacilli. The genus Salmonella is divided into two species and six subspecies. The total number of serovars (serotypes) now exceeds 2700.
However, the distribution of Salmonella species, subspecies and serovars according to their pathogenicity is not uniform.
The World Organisation for Animal Health (OIE) distinguishes three basic groups:
- The first, relatively small, group consists of serovars with narrow host specificity. These serovars are the causative agents of serious systemic diseases in animals and humans. Examples include the serovars Salmonella gallinarum and pullorum, which cause lung disease and typhoid fever in poultry, and the serovars Salmonella typhi and paratyphi A, B and C, which cause typhoid fever and paratyphoid fever in humans.
- Another group are the salmonellae of the subspecies salamae, arizonae, diarizonae, houtenae, indica, which are common in cold-blooded animals. These salmonellae only sporadically cause fatal disease in animals or humans.
- The third and largest group consists of Salmonella serovars that are primarily adapted to the digestive tract of warm-blooded animals. Infected animals are largely asymptomatic carriers and show only sporadic clinical signs of disease (e.g. juveniles). However, they are often a source of infection when they are shed into the environment or into the food chain as contaminated raw materials and food of animal origin. These serovars tend to cause the typical human faecal-oral disease known as salmonellosis.
The diagnosis of Salmonella is a sophisticated cascade procedure based on the non-selective multiplication of all the microflora present in the sample in a liquid culture medium. This step is followed by multiplication of the sample in different types of selective media that inhibit the growth of other bacteria. From these media, the sample is further inoculated onto selective chromogenic media, which allow initial colour differentiation of Salmonella while repeatedly suppressing the contaminating microflora. If an increase in suspect (colour-characteristic) colonies is observed on the latter media, further steps are taken to identify, subculture and, where appropriate, biochemically type the genus, species and subspecies of Salmonella. The confirmed Salmonella are then subjected to a serological test (serotyping) to detect the presence of specific antigens. The unique combination of antigens identified is then used to determine the corresponding serovar (serotype) according to the Kauffmann-White-Le Minor scheme. In addition, selected isolates may be subjected to further molecular genetic analysis.
