Sequential Evolution of Vancomycin-Intermediate Resistance Alters Virulence in Staphylococcus aureus: Pharmacokinetic/Pharmacodynamic Targets for Vancomycin Exposure

Abstract

e Staphylococcus aureus possesses exceptional virulence and a remarkable ability to adapt in the face of antibiotic therapy. We examined the in vitro evolution of S. aureus in response to escalating vancomycin exposure by evaluating bacterial killing and the progression of resistance. A hollow-fiber infection model was utilized to simulate human doses of vancomycin increasing from 0.5 to 4 g every 12 h (q12h) versus a high inoculum (10 8 CFU/ml) of methicillin-resistant S. aureus (MRSA) USA300 and USA400. Host-pathogen interactions using Galleria mellonella and accessory gene regulator (agr) expression were studied in serially obtained isolates. In both USA300 and USA400 MRSA isolates, vancomycin exposure up to 2 g q12h resulted in persistence and regrowth, whereas 4 g administered q12h achieved sustained killing against both strains. As vancomycin exposure increased from 0.5 to 2 g q12h, the bacterial population shifted toward vancomycin-intermediate resistance, and collateral increases in the MICs of daptomycin and televancin were observed over 10 days. Guideline-recommended exposure of a ratio of the area under the concentration-time curve for the free, unbound fraction of the drug to the MIC (fAUC/MIC ratio) of 200 displayed a 0.344-log bacterial reduction in area, whereas fAUC/MICs of 371 and 554 were needed to achieve 1.00-and 2.00-log reductions in area, respectively. The stepwise increase in resistance paralleled a decrease in G. mellonella mortality (P ‫؍‬ 0.021) and a gradual decline of RNAIII expression over 10 days. Currently recommended doses of vancomycin resulted in amplification of resistance and collateral damage to other antibiotics. Decreases in agr expression and virulence during therapy may be an adaptive mechanism of S. aureus persistence.

Differential Activity of the Combination of Vancomycin and Amikacin on Planktonic vs. Biofilm-Growing Staphylococcus aureus Bacteria in a Hollow Fiber Infection Model

“…Several in vitro studies in dynamic systems including the HF model ( Nicasio et al, 2012 ; Lenhard et al, 2016 ) have investigated the antibacterial activity of drugs combined with vancomycin against planktonic S. aureus . However, the use of dynamic in vitro systems, such as the CDC biofilm device or others, to study the effects of combinations on biofilm is rarely reported.…”

“…These results are in agreement with previous studies which demonstrated the low activity of vancomycin on large bacterial inocula ( LaPlante and Rybak, 2004 ; LaPlante and Woodmansee, 2009 ) and on biofilms ( Hogan et al, 2016 ). One study involving a HF model showed that a peak concentration as high as 80 mg/L was needed to achieve bactericidal activity against a large inoculum of a MRSA strain with a MIC of 1 μg/mL for vancomycin ( Lenhard et al, 2016 ). One proposed explanation for the inoculum effect and reduced efficacy of vancomycin is that bacteria at high density are in a stationary growth phase with low dividing rate and low cell wall synthesis ( Brown et al, 1988 ; Lamp et al, 1992 ).…”

Compounding Effects of Climate Warming and Antibiotic Resistance

“…In this section we explore the implications when they are exposed sequentially. Previous studies have explored the effects of sequential exposure of both different antibiotics (Lenhard et al, 2015;MacGowan and Bowker, 1998;Miller et al, 1996) and antibiotics and temperature (Andrade-Linares et al, 2016;Hilker et al, 2016;Manrique et al, 2016;Rangel, 2011). Even transient exposure to a stressor can modify how an organism responds to subsequent stress.…”

Impact of Staphylococcus aureus accessory gene regulator (agr) system on linezolid efficacy by profiling pharmacodynamics and RNAIII expression

“…Similarly, relative RNAIII expression also transiently increased 410x baseline when USA 300 was exposed to 2 g of vancomyin q12h in a previous HFIM experiment. 17 In the present study, we sought to characterize the impact of agr-dysfunction on linezolid pharmacodynamics. Time-killing studies evaluating S. aureus with a group I, II or IV agr system suggest agr dysfunction did not significantly alter the activity of linezolid.…”