Characterization of
Salmonella enterica
Serovar Typhimurium DT104 Isolated from Denmark and Comparison with Isolates from Europe and the United States
Authors: Baggesen D. L. , Sandvang D. , Aarestrup F. M. 
Published: December 2019

A total of 136 isolates of
Salmonella enterica
serovar Typhimurium DT104 from Denmark (
n
= 93), Germany (
n
= 10), Italy (
n
= 4), Spain (
n
= 5), and the United Kingdom (
n
= 9) were characterized by antimicrobial resistance analysis, plasmid profiling, pulsed-field gel electrophoresis (PFGE) with the restriction enzymes
Xba
I and
Bln
I, and analysis for the presence of integrons and antibiotic resistance genes. The isolates from Denmark were from nine pig herds, while the isolates from other countries were both of animal and of human origin. All but 10 isolates were resistant to ampicillin, chloramphenicol, spectinomycin, streptomycin, sulfonamides, and tetracycline. Five isolates from the United Kingdom and Spain were sensitive to all antibiotics examined, whereas four isolates from the United Kingdom and the United States were also resistant to one or more of the antibiotics, namely, gentamicin, neomycin, and trimethoprim. All but two strains had the same PFGE profiles when the
Xba
I restriction enzyme was used, while seven different profiles were observed when the
Bln
I restriction enzyme was used. Different dominating
Bln
I types were observed among European isolates compared with the types observed among those from the United States. All the isolates harbored common 95-kb plasmids either alone or in combination with smaller plasmids, and a total of 11 different plasmid profiles were observed. Furthermore, all but one of the multidrug-resistant isolates contained two integrons,
ant (3")-Ia
and
pse-1
. Sensitive isolates contained no integrons, and isolates that were resistant to spectinomycin, streptomycin, and sulfonamides had only one integron containing
ant (3")-Ia
. When restriction enzyme
Bln
I was used, the 14 isolates from one of the nine herds in Denmark showed unique profiles, whereas isolates from the remaining herds were homogeneous. Among isolates from seven of nine herds, the same plasmid profile (95 kb) was observed, but isolates from two herds had different profiles. Thus, either PFGE (with
Bln
I) or plasmid profiling could distinguish isolates from three of nine pig herds in Denmark. The epidemiological markers (antimicrobial susceptibility testing, plasmid profiling, and PFGE) applied demonstrated high in vivo stability in the Danish herds. This may indicate that some different strains of multidrug-resistant
S. enterica
serovar Typhimurium DT104 have been introduced into Danish food animal herds. The presence of isolates from six different countries with similar profiles by PFGE with
Xba
I and highly homogeneous profiles by PFGE with
Bln
I indicate that multidrug-resistant
S. enterica
serovar Typhimurium DT104 has probably been spread clonally in these countries. However, some minor variation could be observed by using plasmid profiling and profiling by PFGE with
Bln
I. Thus, a more sensitive technique for subtyping of strains of DT104 and a broader investigation may help in elucidating the epidemiological spread of DT104 in different parts of the world.