The antimicrobial susceptibility of Borrelia burgdorferi isolated from human spinal fluid was determined in vitro and in vivo. A broth dilution technique was used to determine the MBCs of four antimicrobial agents. The Lyme disease spirochete was most susceptible to ceftriaxone (MBC, 0.04 ,ug/ml) and erythromycin (MBC, 0.05 ,ug/ml), then tetracycline (MBC, 0.8 ,ug/ml), and finally penicillin G (MBC, 6.4 ,g/ml). Syrian hamsters were used to determine the 50% curative doses (CD_0s) of the four antimicrobial agents. Ceftriaxone and tetracycline had the highest activities, with CD50s of 240 and 287 mg/kg, respectively. Both erythromycin and penicillin G possessed low activities. The CD50 of erythromycin was 2,353 mg/kg, and the CD50 of penicillin G was >1,975 mg/kg.

Hamsters passively immunized with as little as 0.0125 ml of immune rabbit serum (indirect fluorescentantibody titer, 1:8,192) were protected from challenge with 1,000 50% infective doses of Borrelia burgdorferi. Cross-protection studies with Minnesota and Connecticut isolates of B. burgdorferi indicated no major differences in their capacity to elicit mutually protective antibodies in rabbits.

The immunogenicity of a single dose of a whole-cell vaccine of inactivated Borrelia burgdorferi1HSF (human spinal fluid isolate) was evaluated. Syrian hamsters were vaccinated subcutaneously and challenged by the intraperitoneal injection of 1,000 50% infectious doses of B. burgdorferi HSF 30 or 90 days postvaccination.Animals were sacrificed 14 days after challenge, and the kidneys and spleens were examined for spirochetes by cultural procedures. At 30 days postvaccination, 86 to 100% protection against infection was achieved in hamsters receiving 50 and 100 ,ug (dry weight) of vaccine. Protection was decreased to 60% with 25 ,ug of vaccine and was absent with 10 ,ug of vaccine. Resistance to infection decreased to 25, 40, and 5% for the 100-, 50-, and 25-,Lg vaccine doses, respectively, at 90 days postvaccination.

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The in-vitro and in-vivo susceptibility of Borrelia burgdorferito tetracycline, erythromycin and azithromycin was investigated. A macrodilution broth technique was used to determine MBCs. B. burgdorferi was most susceptible to azithromycin (MBC 0.04 mg/l) followed by erythromycin (MBC 0.16 mg/l) and tetracycline (MBC 1.6 mg/l). Syrian hamsters were used to determine ED50S for the three antimicrobials. Azithromycin was most effective in the elimination of spirochaetes from experimentally infected hamsters with an ED50 of 3.71 (+/- 1.9) mg/kg followed by tetracycline (ED50 15.6 (+/- 4.58) mg/kg). Erythromycin possessed low activity, having an ED50 of 122.2 (+/- 51.9) mg/kg. Tissue concentrations of azithromycin exceeding the MBC were present 24 h after the final treatment.