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Chapter 1
detected by agglutination tests on glass slides with the corresponding
commercial anti-sera.
1.9. Methods used for epidemiological surveillance of
Salmonella
Bacteriophages (phages) are viruses that can only grow or replicate within
a bacterial cell. Phage typing can differentiate between strains of the same
serotype based on the principle that certain phages will only lyse particular
strains of a specific serotype. The lysis pattern can be compared to a
standard scheme for each serovar to determine the phage type of the
strain [18]. The typing schemes were devised around particular serovars of
Salmonella have proven very useful epidemiological tools for investigating
outbreaks of S. Typhimurium [19], S. Enteritidis [20] and S. Agona [21].
When used in conjunction with antimicrobial susceptibility testing, phage
typing has led to identification of a number of large international outbreaks
including multi-state outbreaks of S. Typhimurium DT104 [22].
Pulsed Field Gel Electrophoresis (PFGE) was adapted for Salmonella in the
1990’s and is still considered as the “Gold Standard” for supporting the
identification of epidemiological links between isolates [23]. Although
multilocus sequence typing (MLST) and whole genome sequencing (WGS)
are quickly becoming the more favoured options [24]. The procedure is
based on cutting the intact bacterial chromosome with site-specific
proteins (restriction endonucleases). The DNA fragments are then
separated using pulsed currents (electrophoresis separation) to generate a
banding pattern that forms the basis for assessing similarity between
isolates. However a major disadvantage with using PFGE to distinguish
between Salmonella strains is it has low discriminatory power for certain
serovars [25]. Other disadvantages include inter-laboratory variation and
reliance on reader interpretation of banding pattern.
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