Fig. 2. Scatter plot showing the relationship between nalidixic acid and ciprofloxacin MIC, and zone size diameter, for 52 S. Typhi isolates with (A) no mutation in topoisomerasegenes; (B) a mutation in gyrB only; and (C) a mutation in gyrA only.
3. Results and discussionAmong 891 S. Typhi isolates the numbers of each antibacterialphenotype isolated were as follows: of wild type (NALS-CIPS) 46 (5.2%), classical fluoroquinolone decreased susceptibility(NALR-CIPI) 675 (75.8%), non-classical fluoroquinolone decreasedsusceptibility (NALS-CIPI) 80 (9%) and fluoroquinolone resistance90 (10%).
The proportion of NALRincreased from 76.1% in 2004 to 97.0% in2011. Of interest is that among the NALS-isolates CIPIwas isolatedevery year although the number varied (Fig. 1). In 2011 only 1/104isolates was fully susceptible to ciprofloxacin and nalidixic acid.The NALS-CIPIisolates appear to be established within the popula-tion. In order to assess the reasons for this stability we decided toinvestigate the genetic variation present within the population ofisolates with the NALS-CIPIphenotype.
No plasmid-encoded determinants were detected. No muta-tions were observed in the QRDR of parC and parE. Mutations werefound in the QRDR of gyrA and gyrB. Results are shown in Table 2.From the mutations detected we split all isolates into three groups.Group A (n = 16): isolates with wild type gyrA and gyrB gene; GroupB (n = 34): gyrB mutation only; and Group C (n = 2) gyrA mutationonly. Scatter plots correlating the zone diameters and MIC of CIPvs NAL and CIP MIC vs CIP zone diameters of these isolates areshown in Fig. 2. Although there is some heterogeneity in this groupof 52 isolates it is clear that each group: A, B and C, could be dis-tinctly identified based on the MIC data for CIP and NAL (Table 2).It is also clear that the mutations on gyrB (Group B) are a commoncause of the NALS-CIPIphenotype in S. Typhi in India. Group B iso-lates (mutation in gyrB) were distinct, with NAL (3–12 mg/L) andCIP (0.064–0.5 mg/L) MIC being 3- and 15-fold higher respectivelythan the wild type group A isolates but where difficult to identify