Pharmacodynamic Evaluation of the Activity of Antibiotics against Hemin- and Menadione-Dependent Small-Colony Variants of Staphylococcus aureus in Models of Extracellular (Broth) and Intracellular (THP-1 Monocytes) Infections

Abstract

Staphylococcus aureus small-colony variants (SCVs) persist intracellularly, which may contribute to persistence/recurrence of infections and antibiotic failure. We have studied the intracellular fate of menD and hemB mutants (corresponding to menadione-and hemin-dependent SCVs, respectively) of the COL methicillin-resistant S. aureus (MRSA) strain and the antibiotic pharmacodynamic profile against extracellular (broth) and intracellular (human THP-1 monocytes) bacteria. Compared to the parental strain, SCVs showed slower extracellular growth (restored upon medium supplementation with menadione or hemin), reduced phagocytosis, and, for the menD SCV, lower intracellular counts at 24 h postinfection. Against extracellular bacteria, daptomycin, gentamicin, rifampin, moxifloxacin, and oritavancin showed similar profiles of activity against all strains, with a static effect obtained at concentrations close to their MICs and complete eradication as maximal effect. In contrast, vancomycin was not bactericidal against SCVs. Against intracellular bacteria, concentration-effect curves fitted sigmoidal regressions for vancomycin, daptomycin, gentamicin, and rifampin (with maximal effects lower than a 2-log decrease in CFU) but biphasic regressions (with a maximal effect greater than a 3-log decrease in CFU) for moxifloxacin and oritavancin, suggesting a dual mode of action against intracellular bacteria. For all antibiotics, these curves were indistinguishable between the strains investigated, except for the menD mutant, which systematically showed a lower amplitude of the concentration-effect response, with markedly reduced minimal efficacy (due to slower growth) but no change in maximal efficacy. The data therefore show that the maximal efficacies of antibiotics are similar against normal-phenotype and menadione-and hemin-dependent strains despite their different intracellular fates, with oritavancin, and to some extent moxifloxacin, being the most effective.

Clinical Significance and Pathogenesis of Staphylococcal Small Colony Variants in Persistent Infections

“…The regular loss of activity of SCVs toward antifolate agents and aminoglycosides is explicable by the metabolic mechanisms leading to the auxotrophism of respective SCV types (189). Negative effects on the bactericidal activity of antibiotics due to SCV phenotype characteristics have been reported for sessile and planktonic as well as extra-and intracellularly occurring SCV cells (128,136,190).…”

“…Nevertheless, there was no general direct correlation shown between the growth rates of intracellular SCV mutants and their susceptibilities to antimicrobial agents (136).…”

“…The authors also used their infection model to test antibiotic combinations with rifampin or oritavancin against intracellular SCVs, and they achieved a further decrease of the intracellular bacterial growth (135). Finally, concentration-dependent testing revealed that maximal efficacies of antibiotics were similar against wild-type and SCV strains, despite their different modes of infection (136).…”

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].…”

Renaissance of antibiotics against difficult infections: Focus on oritavancin and new ketolides and quinolones

“…Among the other remarkable features of this molecule, one should mention its prolonged half-life (terminal half-life  360 h), which can be attributed to both a high protein binding (85%-90%) (4) and an exceptional capacity to accumulate within eukaryotic cells (23), reaching concentrations as high as 560 mg/L in alveolar macrophages of healthy adults having received a cumulative dose of 4 g (24) (Table III). Coupled to the bactericidal character of the drug, this high accumulation confers to oritavancin a high efficacy against intracellular bacteria, including small colony variants of S. aureus, which are generally particularly recalcitrant to antibiotic action (25,26).…”