Combination antibiotic therapy is required to eliminate Bartonella henselae in multiple microenvironments

Bartonella are gram-negative, facultative intracellular bacteria. Infection by Bartonella manifests as different clinical syndromes collectively known as bartonellosis. The well-known diseases caused by these bacteria are cat scratch disease (Bartonella henselae), trench fever (Bartonella quintana) and Carrion's disease (Bartonella bacilliformis). Excluding B. bacilliformis, which is evolutionarily more distinct than the other species, Bartonella infections result in self-limiting disease that is often undiagnosed and untreated. However, individuals with compromised immune systems or other undefined conditions may experience clinical manifestations, which can become life-threatening and need to be treated with effective antibiotics. To date, there is no standard treatment course for these infections, and many doctors prescribe antibiotics based on limited case studies. Within the host, Bartonella can grow extracellularly, intracellularly, and in biofilms. To determine an effective antibiotic strategy, it is important to understand Bartonella susceptibility in each of these growth conditions. We hypothesized that combination antibiotic treatments are required to effectively eliminate Bartonella henselae growth, particularly in biofilm and intracellular environments. Our previous work has shown that B. henselae treatment with single antibiotics in different media, as well as in DH82 canine macrophages, was ineffective in eliminating bacteria. We expanded this work with different antibiotics supported by case reports, as well as double and triple combinations. The following antibiotics were tested: doxycycline, gentamicin, azithromycin, azlocillin, rifampin, and clarithromycin. We found that while monotherapy may inhibit growth extracellularly, it is ineffective when used against intracellular bacteria or pre-existing biofilms. Gentamicin in combination with either rifampin or azlocillin significantly reduced bacterial growth in multiple microenvironments. The effectiveness of combination therapy supports the notion that Bartonella species utilize host cells and biofilms as antibiotic evasion strategies.