LETTER
Nontyphoidal salmonellae are among the principal bacterial pathogens implicated in cases of food-borne gastroenteritis worldwide (1). Multidrug resistance, including ampicillin, chloramphenicol, streptomycin, sulbactam, and tetracycline (ACSSuT) resistance and ciprofloxacin or ceftriaxone resistance, has occurred in strains of Salmonella enterica serovar Typhimurium and has had significant impact on the effectiveness of current strategies to control and manage diseases associated with food-borne infections (2–5). A comprehensive review of data from the literature indicated that S. Typhimurium produced up to a 3-fold-higher mortality rate than an average Salmonella infection. Worse still, the ACSSuT, nalidixic acid (Nal), and ACSSuT-Nal resistance types were, respectively, associated with 4.8-, 10.3-, and 13.1-fold-higher mortality rates (6).
In this study, 239 human clinical S. Typhimurium isolates from hospitals in Hong Kong during 2005 to 2011and 546 human clinical S. Typhimurium isolates from the State Key Laboratory for Infectious Disease Prevention and Control, National Institute for Communicable Disease Control and Prevention (ICDC), Chinese Center for Disease Control and Prevention, Beijing, China, in China during 200 to 2011 were characterized for their resistance to 15 antimicrobials, as shown in Table 1, according to CLSI guidelines (7). S. Typhimurium from both Hong Kong and China showed high levels of resistance to quinolone, fluoroquinolones, and other antibiotics, such as ACSSuT and trimethoprim. In particular, the rates of resistance to nalidixic acid of S. Typhimurium strains from Hong Kong and China were 73% and 63%, respectively. The corresponding rates of resistance to ciprofloxacin (MIC ≧ 2 mg/liter) were 34% and 36%, respectively (Table 1).
Table 1.
Rates of resistance to 15 antimicrobial agents of oqxAB-positive and -negative S. Typhimurium isolates from Hong Kong and China
| Antibiotics(s) | Breakpoint(s) (μg/ml) | % of resistant strains (% of strains exhibiting intermediate resistance) |
|||||
|---|---|---|---|---|---|---|---|
| Hong Kong isolates (n = 239) |
ICDC isolates (n = 546) |
||||||
| Overall | oqxAB positive (n = 67) | oqxAB negative (n = 172) | Overall | oqxAB positive (n = 159) | oqxAB negative (n = 387) | ||
| Ampicillin | ≧32 | 67 | 100 | 55 | 47 (3) | 96 (1) | 27 |
| Amoxicillin-clavulanic acid | ≧32, ≧16 | 6 (18) | 15 (40) | 3 (9) | 6 (20) | 17 (45) | 2 (10) |
| Cefotaxime | ≧4 | 3 | 3 | 3 | 4 | 3 | 5 |
| Ceftriaxone | ≧4 | 3 | 3 | 3 | 4 | 3 | 5 |
| Ceftazidime | ≧16 | 3 | 3 | 3 | 2 | 1 | 2 |
| Chloramphenicol | ≧32 | 50 | 100 | 30 | 43 | 100 | 20 |
| Gentamicin | ≧16 | 23 | 42 | 15 | 35 | 70 | 21 |
| Nalidixic acid | ≧32 | 73 | 100 | 63 | 63 | 100 | 48 |
| Ciprofloxacin | ≧4 | 11 (23) | 18 (76) | 9 (2) | 20 (16) | 48 (50) | 9 (2) |
| Streptomycin | 52 | 61 | 49 | 28 | 42 | 22 | |
| Sulfamethoxazole | ≧512 | 70 | 100 | 59 | 55 | 100 | 36 |
| Tetracycline | ≧16 | 68 | 88 | 60 | 49 | 96 | 30 |
| Kanamycin | ≧32 | 36 | 63 | 26 | 44 | 85 | 27 |
| Trimethoprim | ≧16 | 38 | 73 | 25 | 42 | 100 | 18 |
| Olaquindox | ≧32 | 28 | 100 | 0 | 29 | 100 | 0 |
The mechanisms mediating the high prevalence of fluoroquinolone resistance in S. Typhimurium was investigated. We previously found that oqxAB, a plasmid-mediated resistance-nodulation-cell division (RND) efflux pump conferring resistance to multiple antibiotics, existed in Salmonella isolates recovered from food samples (8). We then assessed the prevalence of oqxAB in human clinical S. Typhimurium isolates. It showed that the prevalence of oqxAB increased markedly in S. Typhimurium from 2005 to 2011. Among the Hong Kong isolates, the overall oqxAB-positive rate was about 28% (67 out of 239 isolates), yet this resistance determinant was not detectable in salmonellae in 2005 and an increasing trend was observed only from 2006 onwards, with positivity rates of 12%, 24%, 34%, 34%, 37%, and 43% detectable among the S. Typhimurium isolates recovered in 2006 through 2011, respectively. A similar trend was observable among the ICDC isolates. The overall oqxAB-positive rate of S. Typhimurium strains was 29% (159 out of 546 isolates), and the yearly rates were 0%, 13%, 26%, 32%, 36%, 39%, and 42% during the years 2005 to 2011, respectively.
Screening of other plasmid-mediated quinolone resistance (PMQR) genes [qnrA, qnrB, qnrC, qnrD, qnrS, qepA, oqxAB, and aac(6′)Ib-cr] in these S. Typhimurium isolates detected the high prevalence of aac(6′)Ib-cr in 85% and 83% of the oqxAB-positive S. Typhimurium isolates from Hong Kong and China, respectively, while none of the oqxAB-negative S. Typhimurium isolates were positive for aac(6′)Ib-cr. No other PMQR genes were detected in these S. Typhimurium isolates. oqxAB was also found to be associated with multidrug resistance in S. Typhimurium. Among the S. Typhimurium isolates of the oqxAB-positive group, 94% and 98% from Hong Kong and the ICDC, respectively, were resistant to ciprofloxacin (MIC ≧ 2 mg/liter), while the corresponding resistance rate in oqxAB-negative S. Typhimurium Hong Kong and ICDC isolates was only 11% (Table 1). In addition, the ACSSuT resistance type was detected in 58% and 53% of the oqxAB-positive S. Typhimurium isolates from Hong Kong and China, respectively, while the corresponding rate in oqxAB-negative S. Typhimurium was only around 10%.
The clonal relationships of oqxAB-positive salmonella isolates were examined by pulsed-field gel electrophoresis (PFGE) according to the PulseNet PFGE protocol for salmonella (9). For the 159 oqxAB-positive ICDC isolates, 61 PFGE patterns were observed among the oqxAB-positive strains. Interestingly, the majority (37%) of oqxAB-positive isolates belonged to one PFGE type, designated CN0006 (Fig. 1). For 67 oqxAB-positive Hong Kong isolates, 18 PFGE patterns were detectable among the oqxAB-positive isolates; again 57% of the oqxAB-positive strains belonged to the CN0006 type (Fig. 1). Using an 80%-cutoff Dice coefficient index for the PFGE profiles, about 83% and 66% of S. Typhimurium isolates from Hong Kong and China, respectively, were related to the CN0006 clone. This finding suggested that CN0006 and related clones are responsible for the expansion of the ACSSuT-ciprofloxacin-oqxAB-aac(6′)Ib-cr type of S. Typhimurium in clinical settings in China. Multilocus sequence typing (MLST) was performed using primer sets as suggested at www.mlst.net. Results of MLST showed that all of the 20 randomly selected oqxAB-positive strains from Hong Kong and 16 out of the 20 randomly selected S. Typhimurium strains from the ICDC belonged to a specific sequence type, ST34, a sequence type that was frequently associated with the ACSSuT resistance type of S. Typhimurium, which is also frequently reported in the European Union (10). The quick expansion of the multidrug-resistant S. Typhimurium ST34 clone will pose a notable threat to clinical Salmonella infection control. Urgent actions are required to halt its further transmission in both environmental and clinical settings.
Fig 1.
PFGE patterns of oqxAB-positive S. Typhimurium strains recovered in Hong Kong and China during the period of 2006 to 2011. All oqxAB-positive S. Typhimurium strains in this study from Hong Kong (HK) and China (ICDC) were subjected to XbaI digestion and PFGE analysis.
ACKNOWLEDGMENTS
We thank Julia Ling for her assistance in the collection of clinical salmonella isolates in Hong Kong and Georges M. Halpern for critical reading of the manuscript.
This work was supported by the Chinese National Key Basic Research and Development (973) Program (grant 2013CB127200) and the Research Fund for the Control of Infectious Diseases of the Food and Health Bureau, the Government of the Hong Kong SAR (grant ZJE4 to S.C.).
We declare that we have no conflicts of interest.
Footnotes
Published ahead of print 24 June 2013
REFERENCES
- 1.Gomez TM, Motarjemi Y, Miyagawa S, Kaferstein FK, Stohr K. 1997. Foodborne salmonellosis. World Health Stat. Q. 50:81–89 [PubMed] [Google Scholar]
- 2.Centers for Disease Control and Prevention 1997. Multidrug-resistant Salmonella serotype Typhimurium—United States, 1996. MMWR Morb. Mortal. Wkly. Rep. 46:308–310 [PubMed] [Google Scholar]
- 3.Antunes P, Coque TM, Peixe L. 2010. Emergence of an IncIgamma plasmid encoding CMY-2 beta-lactamase associated with the international ST19 OXA-30-producing β-lactamase Salmonella Typhimurium multidrug-resistant clone. J. Antimicrob. Chemother. 65:2097–2100 [DOI] [PubMed] [Google Scholar]
- 4.Chiu CH, Wu TL, Su LH, Chu C, Chia JH, Kuo AJ, Chien MS, Lin TY. 2002. The emergence in Taiwan of fluoroquinolone resistance in Salmonella enterica serotype choleraesuis. N. Engl. J. Med. 346:413–419 [DOI] [PubMed] [Google Scholar]
- 5.Glynn MK, Bopp C, Dewitt W, Dabney P, Mokhtar M, Angulo FJ. 1998. Emergence of multidrug-resistant Salmonella enterica serotype typhimurium DT104 infections in the United States. N. Engl. J. Med. 338:1333–1338 [DOI] [PubMed] [Google Scholar]
- 6.Molbak K. 2005. Human health consequences of antimicrobial drug-resistant Salmonella and other foodborne pathogens. Clin. Infect. Dis. 41:1613–1620 [DOI] [PubMed] [Google Scholar]
- 7.CLSI 2012. Performance standards for antimicrobial susceptibility testing; twenty-second informational supplement. CLSI document M100-S22 Clinical and Laboratory Standards Institute, Wayne, PA [Google Scholar]
- 8.Wong MH, Chen S. 2013. First detection of oqxAB in Salmonella spp. isolated from food. Antimicrob. Agents Chemother. 57:658–660 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 9.Ribot EM, Fair MA, Gautom R, Cameron DN, Hunter SB, Swaminathan B, Barrett TJ. 2006. Standardization of pulsed-field gel electrophoresis protocols for the subtyping of Escherichia coli O157:H7, Salmonella, and Shigella for PulseNet. Foodborne Pathog. Dis. 3:59–67 [DOI] [PubMed] [Google Scholar]
- 10.Antunes P, Mourao J, Pestana N, Peixe L. 2011. Leakage of emerging clinically relevant multidrug-resistant Salmonella clones from pig farms. J. Antimicrob. Chemother. 66:2028–2032 [DOI] [PubMed] [Google Scholar]