Abstract
In his letter (1), Cabello makes 2 observations regarding the debate concerning the origin of the floR gene in Salmonella enterica serovar Typhimurium DT104. The first observation is that the plasmid PASPPFLO contained cloned segments of an indigenous Vibrio damsela plasmid. However, PASPPFLO is not the name of a plasmid but is the GenBank locus identifier associated with the sequence (GenBank accession no. D37826) of a 3,745-bp region of the V. damsela plasmid pSP92088 that contained pp-flo (2,3). The second observation is that sequences flanking the floR gene in S. enterica serovar Typhimurium DT104 (GenBank accession no. AF071555) are homologous to those flanking the pp-flo gene sequenced from the V. damsela plasmid pSP92088 (4). On the basis of this homology, he seems to assume that these flanking sequences must have originated in V. damsela and, therefore, that they constitute a molecular signature that firmly establishes the aquaculture origin of this florfenicol resistance. What Cabello does not mention is that sequences flanking a wide range of floR genes, including those in plasmid R55 (GenBank accession no. AF332662), are also homologous to those found in S. enterica serovar Typhimurium DT104 (5,6). These data suggest that during horizontal transfer between species and genera, the association of floR with its flanking regions has been conserved (5,6). However, the data provide no evidence for postulating a unique association of these flanking sequences with V. damsela, and, therefore, do not provide evidence for an aquaculture origin of floR. If Cabello believes that sequences flanking floR in S. enterica serovar Typhimurium DT04 constitute a molecular signature that firmly establishes the aquaculture origin of floR in S. enterica serovar Typhimurium DT104, he should provide some explanation as to how this signature was also present in the R55 plasmid detected in a Klebsiella pneumoniae strain isolated in 1969 (5,7).
Highlights
To the Editor: In a letter recently published in Emerging Infectious Diseases, Smith [1] discussed evidence that he mistakenly believes to undermine the hypothesis that the florfenicol resistance gene present in some isolates of the epidemic Salmonella enterica serovar Typhimurium DT104 strain originated from a florfenicol resistance plasmid present in Vibrio damsela (Pasteurella piscicida) that infected fish farms in Japan in the 1990s [2]
Smith correctly states that the florfenicol resistance gene was present in S. enterica serovar Typhimurium DT104 strains isolated in the United States in 1985, before the gene was documented in V. damsela in Japan [1,3]
The region of the florfenicol resistance gene in Salmonella genomic island 1 (SGI1) contains 763 nt of the non–antimicrobial-drug resistance portion of the original V. damsela plasmid; the region of tetR/tetG contains 468 nt of the non–antimicrobial-drug resistance DNA segment of the P. piscicida plasmid [5,6,7]. The presence of these non–antimicrobial-drug resistance R plasmid DNA sequences in SGI1 constitutes a molecular signature that firmly establishes the aquaculture origin of the florfenicol resistance and the tetR/ tetG genes in the S. enterica serovar Typhimurium DT104 strain studied by Briggs and Fratamico and in the SGI1 of other bacteria [5]
Summary
To the Editor: In a letter recently published in Emerging Infectious Diseases, Smith [1] discussed evidence that he mistakenly believes to undermine the hypothesis that the florfenicol resistance gene present in some isolates of the epidemic Salmonella enterica serovar Typhimurium DT104 strain originated from a florfenicol resistance plasmid present in Vibrio damsela (Pasteurella piscicida) that infected fish farms in Japan in the 1990s [2]. Smith correctly states that the florfenicol resistance gene was present in S. enterica serovar Typhimurium DT104 strains isolated in the United States in 1985, before the gene was documented in V. damsela in Japan [1,3].
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