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: Study of Antibiotic Combinations

Abstract

In a companion paper (H. A. Nguyen et al., Antimicrob. Agents Chemother. 53:1434-1442, 2009), we showed that vancomycin, oxacillin, fusidic acid, clindamycin, linezolid, and daptomycin are poorly active against the intracellular form of a thymidine-dependent small-colony variant (SCV) strain isolated from a cystic fibrosis patient and that the activity of quinupristin-dalfopristin, moxifloxacin, rifampin, and oritavancin remains limited (2-to 3-log CFU reduction) compared to their extracellular activity. Antibiotic combination is a well-known strategy to improve antibacterial activity, which was examined here against an intracellular SCV strain using combinations with either rifampin or oritavancin. Time-kill curve analysis using either concentrations that caused a static effect for each antibiotic individually or concentrations corresponding to the maximum concentration in human serum showed largely divergent effects that were favorable when antibiotics were combined with rifampin at low concentrations only and with oritavancin at both low and high concentrations. The nature of the interaction between rifampin, oritavancin, and moxifloxacin was further examined using the fractional maximal effect method, which allows categorization of the effects of combinations when dose-effect relationships are not linear. Rifampin and oritavancin were synergistic at all concentration ratios investigated. Oritavancin and moxifloxacin were also synergistic but at high oritavancin concentrations only. Rifampin and moxifloxacin were additive. This approach may help in better assessing and improving the activity of antibiotics against intracellular SCV strains.Small-colony variant (SCV) strains of Staphylococcus aureus are notoriously difficult to eradicate with most commonly used antistaphylococcal agents (30). Failures favor both selection and acquisition of antibiotic resistance because SCV strains are hypermutators (8). In the companion paper (21), we compared the intracellular activity of a series of 13 antibiotics against a methicillin-susceptible thymidinedependent SCV strain isolated from a cystic fibrosis (CF) patient. We showed that most of these antibiotics are poorly active, with only moxifloxacin, rifampin, and oritavancin able to decrease the intracellular inoculum to less than 1% of the initial value.Antibiotic combination is a well-known strategy both to prevent the emergence of resistant organisms and to increase activity against extracellular organisms thanks to additive, and even sometimes synergistic, effects between coadministered drugs (12,23). Available data suggest that combining antibiotics can improve intracellular activity against both normal and SCV phenotypes of S. aureus strains (2-4). These studies, however, used only a limited number of antibiotics and fixed concentrations, making it difficult to appreciate the exact nature of interactions between combined drugs. We therefore undertook to reexamine this issue by including a series of commonly used antibiotics. These were combined with either rifamp...

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

“…The postphagocytosis inoculum was typically 1.0 to 2.5 ϫ 10 6 CFU per mg of cell protein. As discussed previously (33), the large dilution of samples before spreading on agar plates for CFU counting ensured the absence of a carryover effect. To further check for the absence of a carryover effect, we examined the capacity of cell lysates obtained from cells incubated with delafloxacin to impair the growth of S. aureus ATCC 25923.…”

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

“…(Baltch et al, 2008). Nguyen et al (2009b) also showed additive or synergistic effects between oritavancin, moxifloxacin and rifampicin against intracellular SCVs.…”

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

“…Most antibiotics tested caused a reduction of bacterial counts, whereas all drugs showed higher intracellular activity against normal than against SCV strains (133). 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).…”