Assaying Chlamydia pneumoniae persistence in monocyte-derived macrophages identifies schisandrin lignans as phenotypic switchers

Antibiotic-tolerant persister bacteria involve frequent treatment failures, relapsing infections and the need for extended antibiotic treatment. Taking persisters into account in susceptibility assays is thus an essential success factor in antibacterial drug discovery. The virulence of the obligate intracellular bacterium Chlamydia pneumoniae is tightly linked to its propensity for persistence, but current susceptibility screening on this gram-negative respiratory pathogen relies on permissive epithelial cells. To establish an improved antichlamydial susceptibility assay allowing the analysis of both actively growing and persister bacteria, we studied C. pneumoniae clinical isolate CV-6 infection kinetics in THP-1 macrophages by qPCR and quantitative culture. Indicated by the steady increase of chlamydial genome copy numbers and infectious progeny as well as the failure of azithromycin to eradicate the intracellular forms of the bacterium, the macrophages were found to harbor a subpopulation of persister C. pneumoniae cells. The potential of the assay for the discovery of anti-persister molecules against intracellular bacteria was demonstrated by the identification of the differential effects of two dibenzocyclooctadiene lignans on C. pneumoniae infection. While schisandrin reverted C. pneumoniae persistence and promoted productive infection, schisandrin C was superior to azithromycin in eradicating the C. pneumoniae infection. The phenotypic switch was associated with the suppression of cellular glutathione pools, implying that targeting glutathione homeostasis may provide a novel means for intracellular bacteria resuscitation. In conclusion, these data highlight the value of macrophages over permissive cell lines in anti-persister agent discovery on intracellular bacteria and targeting host cell redox status to fight persistent infections.