Molecular characterization of endemic salmonella infections ... - Evira

PFGE. PFGE has been used both for subdivision within serotype such as S. Brandenburg (Baquar
et al 1994a), S. Berta (Ellis et al 1998), and S. Javiana (Lee et al 1998) and phage type such as S.
Typhi (Nair et al 1994), and S. Enteritidis (Lukinmaa et al 1999).
S. Agona does not carry IS200 (Gibert et al 1995) and is unlikely to be subdivided by ribotyping
(Threlfall and Hampton, unpublished). However, PFGE proved useful in the analysis of an
international outbreak caused by S. Agona contaminated snacks (Threlfall et al 1996). In
investigating an outbreak of human salmonellosis caused by S. Infantis, PFGE typing results
indicated a common source for the outbreak (Wegener and Baggesen 1996). In characterizing
human and environmental isolates of S. Infantis, 35 distinct PFGE profiles were detected. None of
the environmental isolates shared the profile common to all the human isolates, therefore the
source for the human outbreak could not be identified (Murakami et al 1999). PFGE results
indicated a spread of a single strain of S. Infantis among humans in Argentina (Merino et al 2003).
Moreover, PFGE proved to be very useful for subdivision of strains of S. Typhimurium definitive
type (DT) 12, 193 and 104 in an outbreak. In particular, the PFGE profiles of DT12 and DT193
were identical, whereas DT104 yielded a clearly different profile (Corbett-Feeney and Ni Riain
1998). In a study by Kariuki et al (1999), isolates of 11 different phage types of S. Typhimurium
were divided into eight PFGE clusters. No consistent pattern of association between the phage
types of the isolates and a particular PFGE cluster was seen.
A problem reported infrequently in some serovars such as S. Saintpaul, is the presence of
unspecific nuclease activity (Baggesen et al 1996). This phenomenon makes the preparation and
analysis of PFGE profiles impossible. Recently, Liesegang and Tschäpe (2002) determined that
the DNA is degraded during electrophoresis by Tris radicals in the running buffer. By adding
thiourea to the Tris-containing buffer, the degradation of the DNA could be prevented.
In a study of 110 S. enterica subspecies enterica isolates and 25 serotypes genotyped by
multilocus sequence typing (MLST), PFGE and amplified fragment length polymorphism (AFLP),
PFGE was found to be as discriminatory as AFLP. Moreover, PFGE fingerprints were easier to
interpret and reproduce, in addition to being less time-consuming to analyse. Similarly, it is easier
to compare PFGE profiles between laboratories, and therefore PGFE is the preferred molecular
typing method for surveillance and outbreak investigations (Torpdahl et al 2005). In typing 85
clinical isolates of S. Typhimurium, PFGE was superior to MLST, as MLST could reveal no
differences between the nucleotides within the four selected genes in the analysed isolates (Fakhr
et al 2005). However, for the typing of the highly homogenous S. Typhimurium DT104 in outbreak
investigations, even PFGE is not discriminative enough (Murphy et al 2001).
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