Class 1 integrons in Dutch Salmonella enterica serovar Dublin isolates from clinical cases of bovine salmonellosis

Fifty-nine Salmonella enterica serovar Dublin (Salmonella Dublin) isolates from clinical cases of bovine salmonellosis between 1993 and 2004 were tested for their susceptibility to 15 antimicrobial agents and the presence of class 1 integrons. Integrons were further analyzed by conserved segment PCR-RFLP. DNA sequencing was used to identify the inserted gene cassette. Twelve (20.3%) isolates were multidrug-resistant. A combination of resistance against chloramphenicol, streptomycin and sulphonamides was the most common phenotype observed. Multidrug-resistance (MDR) was found to be strongly associated with the presence of integrons, since a class 1 integron with the aadA1 gene cassette encoding resistance to streptomycin and spectinomycin was found in all 12 multidrug-resistant isolates. The presence of the aadA1 gene in Salmonella Dublin has not been reported before. None of the integron carrying Salmonella Dublin isolates could transfer its antimicrobial resistance to E. coli K12 by conjugation. Analysis of plasmid profiles and pulsed field gel electrophoresis (PFGE) patterns showed at least some clonality among the Salmonella Dublin isolates, but 11 different types could be distinguished based on both XbaI and BlnI-PFGE patterns. Thus, the Dutch Salmonella Dublin strains were closely related but not clonal.