Minocycline enhances antimicrobial activity of unpolarized macrophages against Acinetobacter baumannii while reducing the inflammatory response

Minocycline activity against Acinetobacter baumannii ( AB ) in vivo is underestimated by standard methods of susceptibility testing. We examined pharmacologic effects of minocycline on primary immunity that may be contributing to the in vivo vs. in vitro discrepancy of minocycline activity against AB. Minocycline MICs against 10 AB strains were compared in standard bacteriologic media (Mueller-Hinton broth, MHB) and physiologic (RPMI) media. Macrophages were pretreated with minocycline or comparator antibiotics before AB co-culture. Macrophage cytokine production and phagocytosis of AB were measured without and with pre-treatment with minocycline. Two to eight-fold reduction in minocycline MIC against 10 AB strains occurred in RPMI compared to MHB, which was more pronounced than other antibiotic classes. Macrophages pretreated with 1, 5, 10, 30, 50, and 100 μg/mL minocycline before bacterial co-cultures significantly decreased AB inoculum at 6 hours of co-culture in a dose-dependent manner, with no bacterial colonies observed from co-cultures with macrophages pretreated with 30 μg/mL or more of minocycline. Macrophages pretreated with minocycline for 24 hours before zymosan stimulation led to significantly higher levels of phagocytosis. Macrophages treated with minocycline for 24 hours significantly decreased production of IL-6, TNF-α, and MCP-1 in a dose dependent manner. The minocycline in vivo efficacy may be attributed to enhanced activity in nutrient-limited, physiologic medium combined with increased macrophage phagocyte efficiency. Incorporating novel assays that recapitulate the in vivo environment will be important for understanding the host-pathogen-antibiotic relationship toward a goal of improved future drug discovery and overall treatment strategies against AB and other drug-resistant pathogens.