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 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). He is also correct in noting that this particular florfenicol 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 florfenicol 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 florfenicol resistance and tetR/tetG genes (5–7). For example, the region of the florfenicol 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 (5–7).

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). These R plasmid DNA sequences in SGI1 also confirm direct or indirect horizontal gene transfer between bacteria in the aquaculture environment and S. enterica serovar Typhimurium DT104 (5–7).