Antimicrobial susceptibility of travel-related Salmonella enterica serovar Typhi isolates detected in Switzerland (2002-2013) and molecular characterization of quinolone resistant isolates

Background: Typhoid fever is an acute, invasive, and potentially fatal systemic infection caused by Salmonella enterica subspecies enterica serotype Typhi (S. Typhi). Drug resistance to antimicrobials such as ciprofloxacin is emerging in developing countries, threatening the efficacy of treatment of patients in endemic regions as well as of travellers returning from these countries. Methods: We compared the antimicrobial resistance profiles of 192 S. Typhi isolated from patients over a time span of twelve years. Susceptibility testing was done by the disk diffusion method. A representative selection of isolates (n = 41) was screened by PCR for mutations in the quinolone resistance-determining regions (QRDRs) of the gyrA and parC genes and all 192 isolates were screened for plasmid-mediated quinolone resistance (PMQR) genes. Multilocus sequence typing (MLST) was used to investigate the sequence type of isolates from patients with a known history of international travel. Results: Resistance rates for nalidixic acid increased from 20 % to 66.7 % between 2002 and 2013. Resistance to ciprofloxacin was detected in 55.6 % of the isolates by 2013. Ciprofloxacin resistance was predominantly associated with the triple substitutions Ser83 -> Phe and Asp87 -> Asn in GyrA and Ser80 -> Ile in ParC. The plasmid-mediated resistance gene qnrS1 was detected in two isolates. Sequence type ST1 was associated with the Indian subcontinent, while ST2 was distributed internationally. Multidrug resistance was noted for 11.5 % of the isolates. Conclusions: Fluoroquinolone resistant S. Typhi constitute a serious public health concern in endemic areas as well as in industrialized countries. Increased surveillance of global patterns of antimicrobial resistance is necessary and the control of resistant strains is of the utmost importance to maintain treatment options.