Journal
PLOS ONE
Volume 8, Issue 4, Pages -Publisher
PUBLIC LIBRARY SCIENCE
DOI: 10.1371/journal.pone.0062774
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Funding
- National Science Council [NSC 98-2314-B-010-010-MY3]
- SFT [NSC 99-2320-B-400 -013-MY3]
- YTC [NSC 100-2311-B-005-001]
- National Health Research Institutes
- Immunology Research Center at Taipei Veterans General Hospital [V100E4-005]
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The genetic features of the antimicrobial resistance of a multidrug resistant Klebsiella pneumoniae strain harboring bla(NDM-1) were investigated to increase our understanding of the evolution of NDM-1. The strain, KPX, came from a Taiwanese patient with a hospitalization history in New Delhi. Complete DNA sequencing was performed; and the genes responsible for antimicrobial resistance were systematically examined and isolated by library screening. KPX harbored two resistance plasmids, pKPX-1 and pKPX-2, which are 250-kb and 141-kb in size, respectively, with bla(NDM-1) present on pKPX-1. The plasmid pKPX-1 contained genes associated with the IncR and IncF groups, while pKPX-2 belonged to the IncF family. Each plasmid carried multiple antimicrobial resistance genetic determinants. The gene responsible for resistance to carbapenems was found on pKPX-1 and that for resistance to aztreonam was found on pKPX-2. To our surprise, we discovered that bla(NDM-1) exists on pKPX-1 as multiple copies in the form of tandem repeats. Amplification of bla(NDM-1) was found to occur by duplication of an 8.6-kb unit, with the copy number of the repeat varying from colony to colony. This repeat sequence is identical to that of the pNDM-MAR except for two base substitutions. The copy number of bla(NDM-1) of colonies under different conditions was assessed by Southern blotting and quantitative PCR. The bla(NDM-1) sequence was maintained in the presence of the antimicrobial selection; however, removal of antimicrobial selection led to the emergence of susceptible bacterial populations with a reduced copy number or even the complete loss of the bla(NDM-1) sequence. The dynamic nature of the NDM-1 sequence provides a strong argument for judicious use of the broad-spectrum antimicrobials in order to reduce the development and spread of antimicrobial resistance among pathogens.
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