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Letter
Diversification in Salmonella
Typhimurium DT104
John Threlfall,*
Katie L. Hopkins,* and Linda R. Ward*
*Health Protection Agency, Centre for Infections, London, United Kingdom
Suggested
citation for this article
To the Editor: Multidrug-resistant (MDR) Salmonella Typhimurium
definitive phage type (DT) 104 with chromosomally encoded resistance to
ampicillin, chloramphenicol, streptomycin/spectinomycin, sulfonamides,
and tetracyclines (ACSSpSuT) was first identified and characterized in
the United Kingdom in the early 1990s (1). MDR DT104
has subsequently caused numerous outbreaks throughout the world (1).
The organism is characterized by a distinctive Xba I-generated
pulsed-field profile (PFP), designated Xtm 1 (2), carriage
of the 90-kb S. Typhimurium serovar-specific plasmid (SSP), and
presence of the 43-kb Salmonella genomic island 1 (SGI 1), which
is composed of integrons containing, respectively, the ASu (blaCARB-2
and sul1) and SSp (aadA2) genes, with intervening plasmid-derived
genes coding for chloramphenicol/florfenicol (cmlA) and tetracycline
resistance (tetG) (1,3,4). The same genetic characteristics
have been observed in MDR strains of the closely related DTs 12 and 104b
(5) and in some strains of phage type U302 (4).
All isolates of MDR DT104 ACSSpSuT contain the same gene cassettes irrespective
of source or country of origin. Although MDR DT104 has declined during
the last 5 years, the organism remains the most common MDR Salmonella
in the United Kingdom and many other European countries (6).
Since 1998, MDR DT104 has undergone changes in both resistance spectrum
and genetic structure. In the United Kingdom, outbreaks of MDR DT104 have
been caused by new subclones with additional resistance to trimethoprim
(Tm) (R-type ACSSpSuTTm) (7), by clones with decreased
susceptibility to ciprofloxacin (CpL) (R-type ACSSpSuTCpL)
(1), and by strains of R-type SSpSu. In 2002, an outbreak
of MDR DT104 ACSSpSuTTm with >200 cases was recognized (7).
The outbreak strain was characterized by 3 plasmids of 6.8, 3.0, and 1.5
kb. The 6.8-kb plasmid coded for resistance to sulfonamides and trimethoprim,
with trimethoprim resistance being mediated by dhfr1b. The outbreak
strain lacked the S. Typhimurium SSP and was negative by polymerase
chain reaction (PCR) for a 437-bp internal fragment of the Salmonella
plasmid virulence (spv)C gene. The absence of the SSP was
reflected in the PFP, which was identical to Xtm 1 but lacked a fragment
of ≈90 kb that corresponds to the presence of the SSP (4).
A strain of R-type ACSSpSuT that also lacked the SSP and with a PFP indistinguishable
from that of the ACSSpSuTTm strain caused a simultaneous outbreak with
>40 cases (7). This strain was also characterized
by 2 plasmids of 3.0 and 1.5 kb but did not possess the 6.8-kb sulfonamide-trimethoprim
resistance plasmid.
Decreased susceptibility to ciprofloxacin coupled with resistance to
nalidixic acid (Nx) was first reported in MDR DT104 in 1996 in the United
Kingdom (1). Four mutations in gyrA, each giving
rise to resistance to Nx/CpL, were subsequently identified
in MDR DT104. The most common mutation was aspartate (Asp)-87 to asparagine
(AAC) and involved a change from aspartate (GAC) to AAC. An identical
mutation was identified in a MDR DT104 strain responsible for an outbreak
in Denmark in 1998 (8). In 1999, an Asp-87 to glycine
mutation was identified in a CpL MDR DT104 strain responsible
for a major outbreak in northwestern England (1). Subsequently,
>500 sporadic MDR DT104 infections in the United Kingdom have involved
strains with CpL.
In 2004, 2 simultaneous outbreaks of DT104 were recorded in the United
Kingdom in which the strains were of R-type SSpSu, 1 involving >100
cases and 1 involving >50 cases. Isolates were characterized by pulsed-field
gel electrophoresis coupled with plasmid profile analysis. Resistance
genes were identified by using PCR with primers specific for blaTEM
(A), blaCARB-2, cmlA, catI (C), catIII
(C), aadA2, sul1, and tetG (4).
Testing for SGI1 was conducted by using primers U7-L12 and LJ-R1 for the
left junction and primers 104-RJ and 104-D for the right junction (9).
All isolates exhibited the Xtm 1 PFP, but strains from the 2 outbreaks
could be differentiated by the presence or absence of an additional plasmid
of ≈3 kb. By using PCR, all isolates possessed aadA2 and
sul1 but were negative for blaTEM,blaCARB-2,
cmlA, catI, catIII, and tetG. Similarly, all
strains were positive for the left junction of SGI1. These results indicate
that the SGI1 was in the same chromosomal location as for DT104 ACSSpSuT
but was lacking the right junction.
These findings demonstrate that evolutionary changes have occurred involving
both loss and acquisition of drug resistance genes, the development of
chromosomal mutations conferring resistance to Nx/CpL, and
in certain subclones, the loss of the SSP. None of these changes has altered
the virulence of the organism, with bloodstream invasion in only 1.6%
of 408 cases in the United Kingdom. These findings contrast with those
in the United States, where MDR nontyphoidal Salmonella have been
associated with excessive numbers of bloodstream infections (10).
All facets of the organism should be explored for epidemiologic investigations.
These should include phage type, antibiogram, plasmid profile, and PFP,
coupled with the identification of resistance genes and integrons, and,
when appropriate, the characterization of mutations conferring resistance
to quinolone antimicrobial agents.
These studies were
in part supported by a grant from the Department for Environment, Foods
and Rural Affairs, reference V02136.
References
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Suggested citation
for this article:
Threlfall J, Hopkins
KL, Ward LR. Diversification in Salmonella Typhimurium DT104 [letter].
Emerg Infect Dis [serial on the Internet]. 2005 Jun [date cited].
Available from http://www.cdc.gov/ncidod/EID/vol11no06/05-0100.htm
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