Molecular characterization of endemic salmonella infections ... - Evira

2.5.1.4 Biotyping
In biotyping, the pattern of activity for 20 or fewer cellular metabolic enzymes is established. Such
data can be routinely obtained if automated systems for species identification are available in the
laboratory. Variation in gene expression is most commonly the reason for isolates of the same
strain to differ in one or more biochemical reactions. However, random mutations may also alter
the result (Maslow et al 1993). Strains of the same serovar might show different sugar fermentation
patterns, which are determined by the presence or absence of enzymes, and hence genetically
determined. For example, the xylose+ and xylose- character of S. Typhi may be of epidemiological
interest (Brenner 1984). The utilization of d-tartrate in serovar Paratyphi B is used to separate
biotype Java that is associated with diarrhoea, from biotype Paratyphi B that is associated with
paratyphoid. Biotype Java can utilize d-tartrate whereas biotype Paratyphi B cannot (Grimont et al
2000).
In many serovars, a subdivision according to the biochemical character yields only a small number
of identifiable biotypes. This is because a significant number of strains behave differently in only a
few biochemical tests (Parker 1983). Although biotyping is considered highly reliable, the
discriminatory power of biotyping is generally low. If multiple isolates of an unusual serovar are
detected an outbreak can be identified effectively, and occasionally epidemic strains manifest
unique biotypes (Maslow et al 1993). In conventional biotyping, the results are expressed after a
definite incubation time. When the rate of each reaction is read, and the kinetics of each evaluated,
a biochemical fingerprint of an isolate is obtained (Möllby et al 1993).
2.5.2 Molecular genetic typing methods
2.5.2.1 Plasmid profile
The first genotypic method used for strain separation within Enterobacteriaceae was plasmid
profiling (Riley and Cohen 1982). Bacteria of the same clonal line are expected to carry the same
plasmids since copies of the resident plasmid are distributed between the two daughter cells when
the bacterium divides (Olsen 2000). For strains that lack plasmids or have only one or two of them,
plasmid profiling has poor discriminatory power. Other potential problems arise from the fact that
plasmids are extra-chromosomal elements. Plasmids carrying resistance determinants may spread
rapidly from one strain to another and persist for prolonged periods (Maslow et al 1993). However,
in strains with plasmids coding for drug resistance, plasmids can be lost if antibiotic selective
pressure is withdrawn. Consequently, interpretation of typing results may be difficult (Threlfall et al
1994).
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