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Abstract
Background
Scrub typhus, an infectious disease caused by Orientia tsutsugamushi, is endemic in Korea. With the introduction of tetracycline and chloramphenicol in clinical practice, the mortality due to scrub typhus has markedly decreased. In 1995, scrub typhus poorly responsive to doxycycline was reported in Thailand; the need for safe antibiotics for the treatment of scrub typhus acquired during pregnancy or for children is emerging; also, broader spectrum antibiotics having anti-Orientia activity may be preferred for empirical therapy of enteric fever syndrome and for complicated scrub typhus. The anti-Orientia activities of various antibiotics, including recently licensed antibiotics, were investigated by flow cytometry.
Materials and Methods
O. tsutsugamushi strain Boryong was inoculated into the ECV304 cell line. The infected cells were stained with FS15, a monoclonal antibody reacting against a linear epitope on 56-kDa major outer membrane protein of O. tsutsugamushi. Then the antimicrobial susceptibilities were measured by flow cytometry and expressed as a growth index (total mass of Orientia). A concentration at which no further decrease in growth index occurred was defined as the minimal inhibitory concentration (MIC). Microbial susceptibilities to the following antibiotics were measured: quinupristin-dalfopristin (Synercid), levofloxacin, ciprofloxacin, moxifloxacin, metronidazole, linezolid, clindamycin, chloramphenicol, doxycycline, azithromycin, and rifampin.
Results
Considering the usual serum concentrations of rifampin (MIC=0.025-0.05 µg/mL), azithromycin (MIC=0.05-0.5 µg/mL) and doxycycline (MIC=0.05-0.1 µg/mL), these antibiotics exhibited very low MICs. Synercid (MIC=0.2-1.0 µg/mL), clindamycin (MIC=1.0 µg/mL) and chloramphenicol (MIC=1-2 µg/mL) exhibited moderately low MICs; moxifloxacin (MIC=8 µg/mL), ciprofloxacin (MIC=25.6 µg/mL or more) and levofloxacin (MIC=30 µg/mL) exhibited relatively high MICs; and cefotaxime (MIC>50 µg/mL), metronidazole (MIC>30 µg/mL) and linezolid (>30 µg/mL) exhibited high MICs.
Figures and Tables
Figure 1
Growth index of O. tsutsugamushi strain Boryong at increasing concentrations of doxycycline. Growth index was calculated by multiplying the number of infected cells with mean fluorescence intensity of these cells. 100% represents the percentage of infected cells without doxycycline. The minimal inhibitory concentration of doxycycline was 0.1 µg/mL. Bars represent means±SD (n=2).
Figure 2
Growth index of O. tsutsugamushi strain Boryong at increasing concentrations of synercid (quinupristin-dalfopristin). The minimal inhibitory concentration of synercid was 1 µg/mL. Bars represent means±SD (n=3).
Figure 3
Pooled data on growth indices of O. tsutsugamushi strain Boryong at increasing concentrations of azithromycin. The minimal inhibitory concentration of azithromycin, which is defined as the growth index less than 10%, is 0.05 or 0.1 µg/mL.
Figure 4
Growth indices of O. tsutsugamushi strain Boryong at increasing concentrations of levofloxacin. The minimal inhibitory concentration of levofloxacin was 30 µg/mL.
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