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Clonal Expansion and Microevolution of Quinolone-Resistant Salmonella enterica Serotype Typhi in Vietnam from 1996 to 2004

Centre National de Référence des Salmonella, Centre Collaborateur de Référence et de Recherche pour les Salmonella, Unité de Biodiversité des Bactéries Pathogènes Émergentes, Institut Pasteur, Paris, France,¹ Laboratoire d'Épidémiologie de la Résistance Bactérienne, Institut National d'Hygiène et d'Épidémiologie, Hanoi, Vietnam,² Max-Planck Institut für Infektionsbiologie, Berlin, Germany,³ Faculté de Santé, Médecine et Biologie Humaine de Bobigny, Université Paris Nord, Paris, France⁴

Received 7 May 2007/ Returned for modification 9 July 2007/ Accepted 31 July 2007

Salmonella enterica serotype Typhi clinical isolates (n = 91) resistant to nalidixic acid (Nal^r) were collected from sporadic cases and minor outbreaks throughout Vietnam between 1996 and 2004. These isolates were typed and compared by four methods: Vi phage typing, PstI ribotyping, XbaI and SpeI pulsed-field gel electrophoresis (PFGE), and single-nucleotide polymorphism (SNP) analysis. The results indicated that 65% of the isolates were not typeable by Vi phage typing. In contrast, the ribotyping and, with more accuracy, the SNP analysis methods indicated that all Nal^r isolates belonged to a single clone (ribotype 3a, haplotype H58) that was found previously and that largely consisted of plasmid-encoded multidrug-resistant serotype Typhi isolates. PFGE demonstrated the occurrence of microevolution within this clone. We identified two major combined PFGE profiles: X1-S1 and X3-S6. X3-S6 predominated between 1996 and 2002 but was replaced by X1-S1 after 2002. Nevertheless, PFGE, with a Simpson's index of 0.78, was not considered an optimal discriminatory method for investigating typhoid fever outbreaks in Vietnam. The rate of quinolone resistance increased and the rate of multidrug resistance decreased during the study period. From 2002 to 2004, 80.6% of the isolates from South Vietnam were resistant only to Nal. The mechanism of Nal resistance in most of the isolates (94%) was a mutation in the quinolone resistance-determining chromosomal region of gyrA that led to the amino acid substitution Ser83Phe. No plasmid-located qnrA, qnrB, or qnrS was detected.

Published ahead of print on 29 August 2007.

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