title: In vitro Synergism between Chloroquine and Antibiotics against Orientia tsutsugamushi.
abstract: BACKGROUND
  To investigate whether chloroquine enhances the effect of antibiotics against Orientia tsutsugamushi, the causative organism of scrub typhus, we compared the effect of antibiotics in combination with chloroquine with the effect of antibiotics alone in vitro.
  
  
  METHODS
  The Boryong or AFSC-4 strain was inoculated into ECV304 cells, and incubated in medium containing doxycycline (4 µg/mL), rifampin (4 µg/mL), azithromycin (0.5 µg/mL), chloroquine (1 µg/mL), and each of these antibiotics in combination with chloroquine for 7 d. Immunofluorescence (IF) staining for O. tsutsugamushi was performed 4 hr and 7 d after inoculation of the bacteria, and IF-positive foci were enumerated.
  
  
  RESULTS
  Chloroquine inhibited the growth of O. tsutsugamushi by 15.5%. In combination with chloroquine, the antimicrobial effects increased by 4.4% for doxycycline (a 92.9% reduction of bacterial numbers for doxycycline versus a 97.3% reduction for doxycycline plus chloroquine), 4.6% for rifampin (90.0% versus 94.6%), and 8.3% for azithromycin (86.9% versus 95.2%). The antimicrobial effect of the antibiotics alone was significantly different compared to the combined effect of antibiotics and chloroquine (Wilcoxon signed-rank test, P = 0.001).
  
  
  CONCLUSIONS
  The combined use of chloroquine with an antibiotic for the treatment of O. tsutsugamushi infections may be useful for increasing the efficacy of the antibiotics.
authors.affiliation: Department of Internal Medicine, Inha University School of Medicine, Incheon, Korea.
authors.forename: Dongwook
authors.initials: D
authors.lastname: Son
authors.affiliation: Department of Internal Medicine, Inha University School of Medicine, Incheon, Korea.
authors.forename: Moon-Hyun
authors.initials: MH
authors.lastname: Chung
keywords: Azithromycin; Chloroquine; Combination drug therapy; Doxycycline; Drug synergism; Orientia tsutsugamushi; Rifampin; Scrub typhus