Intracellular Activity of Antibiotics in a Model of Human THP-1 Macrophages Infected by a Staphylococcus aureus Small-Colony Variant Strain Isolated from a Cystic Fibrosis Patient: Pharmacodynamic Evaluation and Comparison with Isogenic Normal-Phenotype and Revertant Strains

Abstract

Small-colony variant (SCV) strains of Staphylococcus aureusshow 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 strain in comparison with normal-phenotype and revertant isogenic strains isolated from the same cystic fibrosis patient. The SCV strain grew slowly extracellularly and intracellularly (1-and 0.2-log CFU increase in 24 h, respectively). In confocal and electron microscopy, SCV and the normalphenotype 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 the maximum concentration in human serum (total drug), oritavancin caused a 2-log CFU reduction at 24 h; rifampin, moxifloxacin, and quinupristin-dalfopristin caused a similar reduction at 72 h; and all other antibiotics had only a static effect at 24 h and a 1-log CFU reduction at 72 h. In concentration dependence experiments, response to oritavancin was bimodal (two successive plateaus of ؊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 produced results of ؊0.6 log CFU or less. Addition of thymidine restored intracellular growth of the SCV strain 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 SCV strains. The data may help rationalizing the design of further studies with intracellular SCV strains.Small-colony variant (SCV) strains are increasingly recognized as a major cause of the persistence and recurrence of Staphylococcus aureus infections, and specific therapeutic options remain so far ill explored (58). Microbiologically, SCV strains constitute a naturally occurring subpopulation of bacteria with a series of metabolic alterations that confer a particular phenotype characterized by slow growth, reduced hemolytic activity, and auxotrophy for hemin, menadione, or thymidine. SCVs easily revert to the normal phenotype and therefore often escape detection. Moreover, even if stable, they remain difficult to recognize in routine testing unless specifically looked for (42). Genetically, the expression of virulence regulators (agr and sarA) or the alternative stress regulator sigB and its dependent virulence genes (hla and spa) is downregulated in SCV strains compared with isogenic normal strains (26, 38). Clinically, SCV strains are preferentially isolated in chronic infections of skin, soft tissues, joint, and bone (41,59,62), as well as in the respiratory tract of cystic fibrosis (CF) pati...

Contrasting Effects of Acidic pH on the Extracellular and Intracellular Activities of the Anti-Gram-Positive Fluoroquinolones Moxifloxacin and Delafloxacin againstStaphylococcus aureus

“…Maximal efficacies of the two drugs are difficult to compare, since the plateau of activity could not be reached for delafloxacin at the concentrations tested in these experiments. Of interest, the dose-effect relationship best fits a bimodal sigmoid, as previously observed with the lipoglycopeptide oritavancin (32). This was interpreted in the latter case as suggesting a dual mode of action or a dual target for the drug.…”

Outcome and Prevention of Pseudomonas aeruginosa-Staphylococcus aureus Interactions During Pulmonary Infections in Cystic Fibrosis

“…In addition to the increased ability of these variants to form biofilms (Al Laham et al, 2007;Häußler et al, 2003;Mitchell et al, 2010aMitchell et al, , 2010bSingh et al, 2009Singh et al, , 2010von Götz et al, 2004), SCVs are well-known for their ability to infect and persist within non-professional phagocytes (Sendi & Proctor, 2009) and there is a limited choice of antibiotics able to act against intracellular bacteria. Nguyen et al (Nguyen et al, 2009a) reported a considerable decrease in the efficacy of most antibiotics against intracellular SCVs in comparison to that seen against extracellular bacteria, but, most importantly, in comparison to their efficacy against the normal-phenotype bacteria. Nevertheless, the authors noted that four antibiotics (quinupristin-dalfopristin, moxifloxacin, oritavancin and rifampicin) were more effective in killing intracellular SCVs.…”

Lipoglycopeptide Antibacterial Agents in Gram-Positive Infections: A Comparative Review

“…Against intracellular bacteria infecting THP-1 cells (native monocytes or differentiated in macrophages), oritavancin appears to be the most effective of all the antibacterials tested in this model, reaching a true intracellular bactericidal effect (-3 log 10 decrease in bacterial counts), as demonstrated against intracellular MSSA [68,69], MRSA, VISA (including strains collected from a patient with bacteremia) [70], or even small colony variants [71][72][73]. It also proved synergistic with other bactericidal antibacterials such as fluoroquinolones or rifampicin against small colony variants [74].…”