In vitro antimicrobial susceptibility of Borrelia burgdorferi sensu strictu. Reduction of viable spirochetes in the presence of different concentrations of antibiotics (ceftriaxone, doxycycline). Cotrimoxazole, which is considered ineffective, is used as a control. Only in high doses does cotrimoxazole show a reduction in the amount of spirochaetes after a few days. Tests were performed on microtitre plates by broth dilution; spirochete count determined by dark-field microscopy.