Salmonella genomic island 1-J variants associated with change in the antibiotic resistance gene cluster in multidrug-resistant Salmonella enterica serovar Virchow isolated from humans, Taiwan, 2004–2006

Salmonella genomic island 1 (variant SGI1-J3) has been previously identified in multi-drug resistant (MDR) Salmonella enterica serovar Virchow isolated from humans in 1994. In this study, antimicrobial resistance, genotypes and genetic relationship were investigated in 96 S. Virchow isolates collected from humans in 2004–2006. XbaI-PFGE analysis separated 96 isolates into two main related clusters, I and II, which consisted of four major pulsotypes differing in prevalence by year. The majority of isolates were MDR to chloramphenicol, sulfonamide, trimethoprim and tetracyclines associated with antimicrobial resistance genes dfrA1, floR2, sulI and tet(G) of variant SGI1-J3. Among nine variants, we determined two novel variants, SGI1-J4 and -J5, which have undergone different homologous recombinational events resulting in partial deletions of the MDR region. The first one contained an empty integron structure and the second presented a deletion extending from the IS6100 element to the adjacent SGI1 backbone. SGI1-J3 is largely encountered in clonally related MDR S. Virchow isolates collected from humans, which spread vertically. The genomic island SGI1 appears to be largely responsible for the diversity of MDR phenotypes among S. Virchow isolates in Taiwan.