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).

References

(1.) Smith P. Aquaculture and florfenicol resistance in Salmonella enterica Typhimurium DT104. Emerg Infect Dis. 2008;14:1327-8. DOI: 10.3201/eid1412.080162

(2.) Angulo FJ, Griffin PM. Changes in antimicrobial resistance in Salmonella enterica serovar Typhimurium. Emerg Infect Dis. 2000;6:436-8.

(3.) Ribot EM, Wierzba RK, Angulo FJ, Barrett TJ. Salmonella enterica serotype Typhimurium DT104 isolated from humans, United States, 1985, 1990, and 1995. Emerg Infect Dis. 2002;8:387-91.

(4.) Cloeckaert A, Baucheron S, Chaslus-Dancla E. Nonenzymatic chloramphenicol resistance mediated by IncC plasmid R55 is encoded by a floR gene variant. Antimicrob Agents Chemother. 2001;45:2381-2. DOI: 10.1128/AAC.45.8.2381-2382.2001

(5.) Briggs CE, Fratamico PM. Molecular characterization of an antibiotic resistance gene cluster of Salmonella typhimurium DT104. Antimicrob Agents Chemother. 1999;43:846-9.

(6.) Kim E, Aoki T. Sequence analysis of the florfenicol resistance gene encoded in the transferable R-plasmid of a fish pathogen, Pasteurella piscicida. Microbiol Immunol. 1996;40:665-9.

(7.) Zhao J, Aoki T. Nucleotide sequence analysis of the class G tetracycline resistance determinant from Vibrio anguillarum. Microbiol Immunol. 1992;36:1051-60.

DOI: 10.3201/eid1504.081171

Felipe C. Cabello

Address for correspondence: Felipe C. Cabello, Department of Microbiology and Immunology, New York Medical College, Valhalla, NY 10595, USA; email: [e-mail]

Author affiliation: New York Medical College, Valhalla, New York, USA

In Response: 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 ppflo (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 (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 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).

Acknowledgment

I thank A. Cloeckaert and D. McIntosh for helpful suggestions.

References

(1.) Cabello FC. Aquaculture and florfenicol resistance in Salmonella enterica serovar Typhimurium DT104. Emerg Infect Dis. 2009;15:623.

(2.) Kim E, Aoki T. Sequence analysis of the florfenicol resistance gene encoded in the transferable R-plasmid of a fish pathogen Pasteurella piscicida. Microbiol Immunol. 1996;40:665-9.

(3.) Kim EH, Yoshida T, Aoki T. Detection of R plasmid encoded with resistance to florfenicol in Pasteurella piscicida. Fish Pathology. 1993;28:165-70.

(4.) Briggs CE, Fratamico PM. Molecular characterization of an antibiotic resistance gene cluster of Salmonella typhimurium DT104. Antimicrob Agents Chemother. 1999;43:846-9.

(5.) Cloeckaert A, Baucheron S, Chaslus-Dancla E. Nonenzymatic chloramphenicol resistance mediated by IncC plasmid R55 is encoded by a floR gene variant. Antimicrob Agents Chemother. 2001;45:2381-2. DOI: 10.1128/AAC.45.8.2381-2382.2001

(6.) Schwarz S, Kehrenberg C, Doublet B, Cloeckaert A. Molecular basis of bacterial resistance to chloramphenicol and florfenicol. FEMS Microbiol Rev. 2004;28:519-42. DOI: 10.1016/j.femsre.2004.04.001

(7.) Chabbert YA, Scavizzi MR, Witchitz JL, Gerbaud GR, Bouchaud DH. Incompatibility groups and the classification of fi-resistance factors. J Bacteriol. 1972;112:666-75.

DOI: 10.3201/eid1504.090023

Peter Smith

Address for correspondence: Peter Smith, Department of Microbiology, National University of Ireland, University Road, Galway, Ireland; email: [e-mail]

Author affiliation: National University of Ireland, Galway, Ireland