Intracellular activity of antibiotics in a model of human THP-1 macrophages infected by a Staphylococcus aureus Small Colony Variant isolated from a cystic fibrosis patient : Pharmacodynamic evaluation and comparison with isogenic normal phenotype and revertant strains

Nguyen, Hoang Anh [UCL] Denis, Olivier Vergison, Anne Theunis, Anne Tulkens, Paul M. [UCL] Struelens, Marc J. Van Bambeke, Françoise [UCL] et al.[show all]

Small Colony Variants of Staphylococcus aureus show reduced antibiotic susceptibility and intracellular persistence, potentially explaining therapeutic failures. The activities of oxacillin, fusidic acid, clindamycin, gentamicin, rifampin, vancomycin, linezolid, quinupristin-dalfopristin, daptomycin, tigecycline, moxifloxacin, telavancin, and oritavancin have been examined in THP-1 macrophages infected by a stable thymidine-dependent SCV in comparison with normal phenotype and revertant isogenic strains isolated from the same cystic fibrosis patient. SCVs grew slowly extracellularly and intracellularly (1 and 0.2 log CFU increase in 24 h, respectively). In confocal and electron microscopy, SCV and the normal phenotype bacteria remain confined in acid vacuoles. All antibiotics tested, except tigecycline, caused a net reduction in bacterial counts that was both time- and concentration-dependent. At an extracellular concentration corresponding to human Cmax (total drug), oritavancin caused a 2 log CFU reduction at 24 h, rifampin, moxifloxacin, and quinupristin-dalfopristin a similar reduction at 72 h, and all other antibiotics only a static effect at 24 h and 1 log CFU reduction at 72 h. In concentration-dependence experiments, response to oritavancin was bimodal (two successive plateaus -0.4 and -3.1 log CFU); tigecycline, moxifloxacin and rifampin showed maximal effects of -1.1 to -1.7 log CFU, and the other antibiotics -0.6 log CFU or less. Addition of thymidine restored SCV intracellular growth, but did not modify the activity of antibiotics (except quinupristin-dalfopristin). All drugs (except tigecycline and oritavancin) showed higher intracellular activity against normal or revertant phenotypes than against SCVs. The data may help rationalizing the design of further studies with intracellular SCVs.