|Abstract||This National Standards Method (NSM) describes the identification of members of the family|
Enterobacteriaceae. There are a large number of species included in the family. In routine clinical
microbiology laboratories it is usual to attempt identification by use of biochemical tests. The level of identification depends on the site of infection, the immune status of the host and the need for
epidemiological surveillance. Issue 3
Because of the large number of species involved, this NSM will concentrate on the most common
genera and species isolated from clinical specimens. The identification of Enterobacteriaceae can be simplified by taking advantage of the fact that three species comprise 80 to 95% of all isolates in the clinical setting. These are Esherichia coli, Klebsiella pneumoniae and Proteus mirabilis. The other species can be easily identified using biochemical tests.
The nomenclature of the Enterobacteriaceae is complicated and has been based on biochemical and antigenic characteristics. Recently, the application of new technologies such as DNA hybridisation has resulted in numerous changes in classification of the Enterobacteriaceae. In 1972 there were 26 recognised species, now there are in excess of 170.
Characteristics of Enterobacteriaceae:
Members of the Enterobacteriaceae are Gram-negative, straight rods, some of which are motile. Most species grow well at 37°C, although some species grow better at 25 - 30°C. They are facultatively anaerobic, oxidase-negative and catalase-positive (except Shigella dysenteriae type 1). They are distributed worldwide and may be found in soil, water, plants and animals.
Common genera of the family Enterobacteriaceae:
The genus Klebsiella contains five species and four subspecies. Four species, previously named
Principles of identification:
Colonial morphology, Gram’s stain, oxidase and the use of several biochemical tests identify isolates from clinical material. Enteric pathogens such as Salmonella species should be identified
biochemically and typed serologically. Hafnia, Morganella and Proteus species can resemble nonmotile salmonella biochemically and can agglutinate in polyvalent salmonella antisera. Because of the diversity of biochemical activities, all the reactions of every species are not described in this NSM. Therefore only a few screening tests are included together with results for the more common genera and species.
If further identification or confirmation is required, isolates should be sent to the Reference Laboratory. Careful consideration should be given to isolates that give an unusual identification.