JAC Advance Access originally published online on August 30, 2006
Journal of Antimicrobial Chemotherapy 2006 58(4):904-905; doi:10.1093/jac/dkl340
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Correspondence |
Quinolone resistance determinant qnrA3 in clinical isolates of Salmonella in 2000–2005 in Hong Kong
1 Public Health Laboratory Services Branch, Centre for Health Protection Department of Health, Hong Kong SAR China 2 Princess Margaret Hospital, Hospital Authority Hong Kong SAR China 3 Queen Elizabeth Hospital, Hospital Authority Hong Kong SAR China 4 Yan Chai Hospital, Hospital Authority Hong Kong SAR China
*Corresponding author. Tel: +852-2319-8685; Fax: +852-2776-1446; E-mail: alf{at}dh.gov.hk
Keywords: QnrA , ciprofloxacin , fluoroquinolones , Salmonella Enteritidis
Sir,
The plasmid-mediated quinolone resistance determinant qnrA3 encoding the gyrase protecting pentapeptide has recently been described in four unrelated isolates of Salmonella enterica serotype Enteritidis in Hong Kong.1 In order to assess the prevalence of qnr-like elements in clinical isolates of Salmonella in our locality, we reviewed the antimicrobial susceptibility results of strains collected between 2000 and 2005 by the Salmonella Typing Centre, Microbiology Division, Public Health Laboratory Services Branch (PHLSB). Strains with reduced ciprofloxacin susceptibility were selected for further studies by molecular methods.
Altogether, 1496 strains had ciprofloxacin susceptibility results available for examination. Apart from 23 isolates that were non-serotypeable, 69 different serotypes were represented by the strains, with the top 5 serotypes in descending order being Enteritidis (n = 481), Typhimurium (n = 220), Derby (n = 104), Muenster (n = 102) and Typhi (n = 70).
The vast majority of the strains were susceptible to ciprofloxacin by disc diffusion assay according to NCCLS (now CLSI) guidelines and only 37 strains (2.5%) were either intermediate or resistant to ciprofloxacin (Table 1). The susceptibility classification of these 37 isolates was verified by MIC determination using Etest (AB BIODISK, Solna, Sweden) and all ciprofloxacin-intermediate and ciprofloxacin-resistant isolates had ciprofloxacin MICs of 2 and 
4 mg/L, respectively. Noteworthy was that while the 21 ciprofloxacin-resistant strains belonged to five different serotypes, all 16 ciprofloxacin-intermediate strains were serotype Enteritidis except for 1 Paratyphi A. These 37 ciprofloxacin-intermediate and ciprofloxacin-resistant strains were subjected to PCR amplification for qnr-like sequence using the primer pair Qnr-F and Qnr-R as described previously.1 While none of the 21 ciprofloxacin-resistant strains was positive, 8 (all Enteritidis) of the 16 ciprofloxacin-intermediate strains produced amplicons of the expected size, 657 bp. DNA sequencing revealed that all eight ORFs were 100% identical to the qnrA3 gene previously found in Shewanella algae2 and four other isolates of Salmonella Enteritidis from Hong Kong.1
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Our results have shown that qnrA3 was the only qnr allele found present among the Salmonella clinical isolates examined since as early as 2002 in Hong Kong. Also, the qnrA3 gene appeared to be associated with the serotype Enteritidis and intermediate ciprofloxacin resistance.
The qnrA3 gene was thought to originate from S. algae2 and it was tempting to postulate that its presence in the eight Salmonella Enteritidis isolates identified in this work was the result of clonal dissemination following a single horizontal gene transfer event. Indeed, PFGE analyses by two restriction enzymes XbaI and BlnI both showed that the eight qnrA3 positive isolates did have indistinguishable patterns. On the other hand, the same pair of patterns was also shared by 51.2% of 966 other Salmonella Enteritidis isolates according to a PFGE fingerprint database developed at PHLSB. This increased genetic similarity among Salmonella Enteritidis compared with other serotypes in Hong Kong has also been reported previously.3 Moreover, six of the qnrA3-bearing plasmids conjugated from the eight Salmonella isolates to Escherichia coli recipients had identical sizes and RFLP patterns (data not shown), while only two were different (isolated in 2002 and 2003). Therefore, it is possible that these six isolates with RFLP indistinguishable plasmids are from the same lineage. Nonetheless, the chronology of discovery may well be different to that of the actual evolution, such that the evidence presented here may be considered as circumstantial rather than conclusive.
The association of qnrA3 with the ciprofloxacin-intermediate isolates and its absence in ciprofloxacin-resistant isolates is interesting. One of the possible explanations is that after the host has developed high-level resistance such as mutations in the quinolone resistance-determining region (QRDR) in the topoisomerase genes, the reliance on the QnrA determinant for survival would be significantly reduced. The QnrA determinant would then be lost by the host via either deletion of the gene sequence or loss of the gene bearing plasmid. In fact, Martínez-Martínez et al.4 have previously proposed that qnrA confers low-level resistance and allows the bacterial population to reach a concentration at which secondary chromosomal mutations for higher quinolone resistance may occur. Furthermore, Ellington and Woodford5 have recently hypothesized that Qnr may be the antitoxin of a toxin-antitoxin system that is involved in plasmid addiction and/or compatibility. In brief, if the host has successfully acquired high-level resistance to fluoroquinolone, which confers cross-resistance to the toxin (yet to be discovered), the plasmid mediating the QnrA determinant can be ditched as ‘excess baggage’. However, many pieces of the puzzle are still missing including identification of the hypothetical toxin and the exact mechanism of transfer of the QnrA determinant between vectors and across genera. A previous study in vitro has also demonstrated that the QnrA determinant only confers protection from the bactericidal effect by quinolones but not fluoroquinolones, raising the possibility that QnrA without the relevant chromosomal mutations actually provides little or no protection against fluoroquinolones or other similar agents.6
In summary, although the precise role of the QnrA determinants in the development of fluoroquinolone resistance is still not known, its increasing occurrence in important pathogens should still warrant close attention. Vigilance and research effort into determining its presence and function must be continued if fluoroquinolones are to be preserved for treatment of infections such as severe salmonellosis.
Transparency declarations
None to declare.
Acknowledgements
We would like to thank the staff of the Microbiology Division, Public Health Laboratory Services Branch, Centre for Health Protection, Department of Health, for their excellent technical support, and the Director of Health for permission to publish this work.
References
1
Cheung TK, Chu YW, Chu MY, et al. (2005) Plasmid-mediated resistance to ciprofloxacin and cefotaxime in clinical isolates of Salmonella enterica serotype Enteritidis in Hong Kong. J Antimicrob Chemother 56:586–9.
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Nordmann P and Poirel L. (2005) Emergence of plasmid-mediated resistance to quinolones in Enterobacteriaceae. J Antimicrob Chemother 56:463–9.
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Ling JM, Koo IC, Kam KM, et al. (1998) Antimicrobial susceptibilities and molecular epidemiology of Salmonella enterica serotype Enteritidis strains isolated in Hong Kong from 1986 to 1996. J Clin Microbiol 36:1693–9.
4 Martínez-Martínez L, Pascual A, Jacoby GA. (1998) Quinolone resistance from a transferable plasmid. Lancet 351:797–9.[CrossRef][Web of Science][Medline]
5
Ellington M and Woodford N. (2006) Fluoroquinolone resistance and plasmid addiction systems: self-imposed selection pressure? J Antimicrob Chemother 57:1026–9.
6 Rodríguez-Martínez JM, Conejo MC, Martínez-Martínez L, et al. (2005) Bactericidal activity of fluoroquinolones against plasmid-mediated QnrA-producing Escherichia coli. Clin Microbiol Infect 11:402–4.[CrossRef][Web of Science][Medline]
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  M. J. Ellington and N. Woodford Comment on: Quinolone resistance determinant qnrA3 in clinical isolates of Salmonella in 2000-2005 in Hong Kong J. Antimicrob. Chemother., January 1, 2007; 59(1): 157 - 157. [Full Text] [PDF]
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