Emergence and Evolution of Multiply Antibiotic-ResistantSalmonella entericaSerovar Paratyphi Bd-Tartrate-Utilizing Strains Containing SGI1

Abstract

The first Australian isolate of Salmonella enterica serovar Paratyphi B D-tartrate-utilizing (dT ؉ ) that is resistant to ampicillin, chloramphenicol, florfenicol, streptomycin, spectinomycin, sulfonamides, and tetracycline (ApCmFlSmSpSuTc) and contains SGI1 was isolated from a patient with gastroenteritis in early 1995. This is the earliest reported isolation globally. The incidence of infections caused by these SGI1-containing multiply antibiotic-resistant S. enterica serovar Paratyphi B dT ؉ strains increased during the next few years and occurred sporadically in all states of Australia. Several molecular criteria were used to show that the early isolates are very closely related to one another and to strains isolated during the following few years and in 2000 and 2003 from home aquariums and their owners. Early isolates from travelers returning from Indonesia shared the same features. Thus, they appear to represent a true clone arising from a single cell that acquired SGI1. Some minor differences in the resistance profiles and molecular profiles also were observed, indicating the ongoing evolution of the clone, and phage type differences were common, indicating that this is not a useful epidemiological marker over time. Three isolates from 1995, 1998, and 1999 contained a complete sul1 gene but were susceptible to sulfamethoxazole due to a point mutation that creates a premature termination codon. This SGI1 type was designated SGI1-R. The loss of resistance genes also was examined. When strains were grown for many generations in the absence of antibiotic selection, the loss of SGI1 was not detected. However, variants SGI1-C (resistance profile SmSpSu) and SGI1-B (resistant to ApSu), which had lost part of the integron, arose spontaneously, presumably via homologous recombination between duplications in the In104 complex integron.SGI1 was first identified in Salmonella enterica serovar Typhimurium DT104 strains with the ampicillin, chloramphenicol, florfenicol, streptomycin, spectinomycin, sulfonamides, and tetracycline (ApCmFlSmSpSuTc) resistance phenotype (4), and SGI1, or variants of it with different resistance phenotypes and resistance genes, have since been found in many Salmonella serotypes (10,18,19,24,33) and also in Proteus mirabilis (2, 5). SGI1 is an integrating element that can move into new hosts by transduction (31) or via mobilization by an IncA/C plasmid (12), and it establishes itself by integrating into the host chromosome at the end of the thdF gene. However, these events likely are quite rare, and it is possible that all of the SGI1-containing strains of any particular serovar arose from a single cell that acquired the SGI1 genomic island and then expanded in an antibiotic-selective environment and subsequently spread around the globe. A single-cell origin for the widespread multiply antibiotic-resistant S. enterica serovar Typhimurium DT104 clone that contains SGI1 is supported by the finding that the earliest DT104 strains of this type isolated in the United States are identical to thos...

Stability, Entrapment and Variant Formation of Salmonella Genomic Island 1

“…However, our attempts to isolate SGI1-free segregants of the 17 SGI1 + S. T. strains and to detect the accumulation of SGI1-free attB site by PCR have failed even after 43 passages suggesting that the loss of SGI1 must be very rare and is not significantly beneficial for the cells under the given experimental conditions. The stability of SGI1 has also been reported for two S. enterica serovar Paratyphi B dT + isolates, however, the segregation assay was carried out in a smaller scale [23]. The complete loss of pathogenicity islands (PAIs) of the uropathogenic E. coli 536 was also investigated [24] and the deletion rate of four of the five PAIs ranged between 10 −5 –10 −6 .…”

Two Novel Salmonella Genomic Island 1 Variants in Proteus mirabilis Isolates from Swine Farms in China

“…The mobility and stability of the SGI1 variants have previously been confirmed in S. enterica (6,19,20). Many SGI1 variants can be excised from the chromosome, and the resulting free circular form may be transferable with the helper plasmid (6,20).…”

“…Nevertheless, homologous recombination within SGI1 integrons could occur in a few clones. Four SGI1-I variants changed to SGI1-C, containing only the aadA2 cassette in the MDR region already described (19,20). The exchange of gene cassettes between the two integrons in SGI1-B2 was detected in nine clones, generating a new SGI1 MDR region containing the dfrA17 and aadA5 cassettes (see Fig.…”

Genetic Context and Structural Diversity of Class 1 Integrons from Human Commensal Bacteria in a Hospital Intensive Care Unit

“…Approaches that reveal class 1 integron contexts include studies that investigate whole genomes (1) and whole mobile DNA regions, such as plasmids (30) and pathogenicity islands (6,7). Other studies may be more focused and target class 1 integrons and their context directly in subsets of isolates selected based on criteria such as location (18), presence of specific resistance genes (8), and source species.…”