Molecular characterization of endemic salmonella infections ... - Evira

In 1977, 207 "definitive types" (DTs) of S. Typhimurium were defined by using 34 phages
(Anderson et al 1977). A different typing system, in which typing with another set of
bacteriophages is supplemented by biotyping, is used in the Netherlands (Parker 1983, EFSA
2005). Hungary also uses yet a different set of phages (EFSA 2005). Phage typing schemes have
also been described for serovars Adelaide, Anatum, Bareilly, Blockley, Braenderup,
Bovismorbificans, Enteritidis, Gallinarum, Hadar, Infantis, Montevideo, Newport, Panama,
Paratyphi B, Virchow and Weltevreden (Grimont et al 2000). Recently a phage typing scheme for
S. Agona was developed (Rabsch et al 2005).
Phage typing is routinely used for subtyping isolates of S. Typhimurium as it is one of the basic
methods employed in studying the epidemiology of this serovar. Even nowadays, when molecular
methods are used for the subtyping of isolates, phage typing is still used. Phage-typing does not
require any expensive equipment and is therefore cheap (Grimont et al 2000). However, stocks of
biologically active phages and control strains need to be maintained. Therefore phage typing is
only available at reference laboratories. It is very demanding even for experienced workers, and
subject to considerable biological and experimental variability (Maslow et al 1993). Moreover,
when only a few phage types tend to dominate over a period of time, the discriminatory ability of
this typing method becomes diminished. In the 1990s, a multiresistant phage type of S.
Typhimurium, DT104, emerged and spread in the UK, Denmark, USA and Canada (Kariuki et al
1999). In 2004, the dominating S. Typhimurium phage types overall among isolates obtained from
human infections in the EU were DT104 and DT120 (EFSA 2005).
2.5.1.2 Antimicrobial susceptibility testing
Salmonellae used to be susceptible to a wide range of antimicrobials in vitro. However, in 1958
resistance to tetracycline was first noted. In 1962, resistance to ampicillin was reported in Britain
and rapidly became common. Transferable resistances to streptomycin, tetracycline and
sulphonamides were observed in a S. Typhimurium strain of phage type 29 from 1958, which by
1968 had acquired a resistance spectrum with resistance to ampicillin, streptomycin,
sulphonamide, tetracycline and furazolidone. Resistance to chloramphenicol and kanamycin was
also reported (Parker 1983). Plasmids containing genes that confer resistance to antibiotics may
readily be acquired by Salmonella strains (Brenner 1984). After 1976, resistant strains of S.
Typhimurium phage types 204 and 193 caused epidemics in calves, with subsequent spread to
man. Plasmids coding for resistance to four to six antimicrobials, including chloramphenicol, were
carried by these strains (Parker 1983).
23