Aquaculture and Florfenicol Resistance in Salmonella enterica Serovar Typhimurium DT104

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).


Aquaculture and Florfenicol Resistance in Salmonella enterica
Serovar Typhimurium DT104 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 fl orfenicol resistance gene present in some isolates of the epidemic Salmonella enterica serovar Typhimurium DT104 strain originated from a fl orfenicol resistance plasmid present in Vibrio damsela (Pasteurella piscicida) that infected fi sh farms in Japan in the 1990s (2).Smith correctly states that the fl orfenicol 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).He is also correct in noting that this particular fl orfenicol resistance gene was detected in a plasmid in Klebsiella pneumoniae in France in 1969 (1,4).
However, an earlier report by Briggs and Fratamico (5) clearly established that the fl orfenicol resistance genes and the tetracycline resistance genes tetG and tetR in the Salmonella genomic island 1 (SGI1) were surrounded by non-antimicrobial-drug resistance DNA.This DNA is homologous to DNA sequences in plasmids PASPPFLO and pJA8122 (see Figure 1 and Table 2 in reference 5) (5-7).In addition to antimicrobial drug resistance genes, PASPPFLO and pJA8122 contain cloned DNA segments of indigenous R plasmids found in V. damsela and V. anguillarum, respectively; these cloned DNA segments span sequences that extend beyond their fl orfenicol resistance and tetR/tetG genes (5-7).For example, the region of the fl orfenicol resistance gene in 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 fi rmly establishes the aquaculture origin of the fl orfenicol 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).These R plasmid DNA sequences in SGI1 also confi rm direct or indirect horizontal gene transfer between bacteria in the aquaculture environment and S. enterica serovar Typhimurium DT104 (5-7).

Felipe C. Cabello
In Response: In his letter (1), Cabello makes 2 observations regarding the debate concerning the origin of the fl oR gene in Salmonella enterica serovar Typhimurium DT104.The fi rst observation is that the plasmid PASPP-FLO contained cloned segments of an indigenous Vibrio damsela plasmid.However, PASPPFLO is not the name of a plasmid but is the GenBank locus identifi er associated with the sequence (GenBank accession no.D37826) of a 3,745-bp region of the V. damsela plasmid pSP92088 that contained ppfl o (2,3).
The second observation is that sequences fl anking the fl oR gene in S. enterica serovar Typhimurium DT104 (GenBank accession no.AF071555) are homologous to those fl anking the pp-fl o gene sequenced from the V. damsela plasmid pSP92088 (4).On the basis of this homology, he seems to assume that these fl anking sequences must have originated in V. damsela and, therefore, that they constitute a molecular signature that fi rmly establishes the aquaculture origin of this fl orfenicol resistance.What Cabello does not mention is that sequences fl anking a wide range of fl oR genes, including those in plasmid R55 (Gen-Bank 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 fl oR with its fl anking regions has been conserved (5,6).However, the data provide no evidence for postulating a unique association of these fl anking sequences with V. damsela, and, therefore, do not provide evidence for an aquaculture origin of fl oR.If Cabello believes that sequences fl anking fl oR in S. enterica serovar Typhimurium DT04 constitute a molecular signature that fi rmly establishes the aquaculture origin of fl oR 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).