Intrinsic fluoroquinolone resistance in Orientia tsutsugamushi

Abstract

Scrub typhus is a public health concern for a population of over a billion humans, with an estimated incidence of one million cases/year in endemic areas. Although doxycycline remains the standard therapy, fluoroquinolones have been used successfully in a few patients. However, there is also clinical evidence that fluoroquinolones are ineffective in the treatment of scrub typhus. To clarify this matter, we determined the in vitro susceptibility of Orientia tsutsugamushi strain Kato to ciprofloxacin and ofloxacin and sequenced the quinolone resistance-determining region (QRDR) of the gyrA gene, the target of fluoroquinolones, of 18 fresh isolates from the Lao PDR. Orientia tsutsugamushi strain Kato was resistant to ciprofloxacin and ofloxacin in vitro (minimum inhibitory concentration=8 microg/mL). All sequences obtained, including those from the two available genomes of O. tsutsugamushi (strains Boryong and Ikeda), had a Ser83Leu mutation in their QRDR domain that is known to be associated with fluoroquinolone resistance. These findings re-emphasise the usefulness of in silico analysis for the prediction of antibiotic resistance and suggest that fluoroquinolones should not be used in the treatment of scrub typhus.

Fig. 1

Growth curves of Orientia tsutsugamushi in L929 cells with different concentrations of (A) ciprofloxacin and (B) ofloxacin. The number of bacteria was expressed as log DNA copies. Experiments were repeated twice and each experiment was performed in duplicate. Data in the figure represent mean ± standard deviation.

Fig. 2

(A) DNA sequence alignment of the quinolone resistance-determining region (QRDR) domain of the gyrA genes of Orientia tsutsugamushi strains from Laos and from genomes of strains Boryong and Ikeda showing the Ser83Leu mutation (Escherichia coli numbering); and (B) three-dimensional representation of DNA gyrase showing position 83 in the QRDR.