Evaluation of Accessory Gene Regulator ( agr ) Group and Function in the Proclivity towards Vancomycin Intermediate Resistance in Staphylococcus aureus

Abstract

Simulated therapeutic vancomycin exposures were evaluated against agr wild-type and knockout Staphylococcus aureus groups I, II, III, and IV using an in vitro pharmacodynamic model. All agr groups developed intermediate resistance to vancomycin after subtherapeutic exposure. The free unbound fraction of the area under the concentration-time curve (fAUC/MIC) required to suppress resistance was fourfold higher (P < 0.001) in agr dysfunctional strains (112 to 169) than that in parent wild-type strains (28).The accessory gene regulator is a quorum-sensing operon which coordinates the expression of secreted and cell-associated virulence factors and controls several metabolic pathways in Staphylococcus aureus in a growth phase-related fashion (4,5,7,8). S. aureus strains which exhibit a dysfunction accessory gene regulator (agr) locus may possess an intrinsic survival advantage under vancomycin-selective pressure (11)(12)(13)(14). However, the correlation between vancomycin exposure using the area under the concentration-time curve (AUC/MIC) necessary to suppress the development of vancomycin-intermediate resistance and agr function or group has not been investigated. We examined the relationship between vancomycin exposure and the development of intermediate-level resistance in agr groups I, II, III, and IV S. aureus using an in vitro pharmacodynamic model. RN6390, RN6607, RN3984, and RN4850 represent agr-positive S. aureus prototype strains carrying agr groups I, II, III, and IV, respectively, and were obtained from the Network on Antimicrobial Resistance in Staphylococcus aureus (NARSA), supported under NIAID/NIH contract N01-AI-95359, and Richard P. Novick. RN6911, RN9120, and RN9121 are the agr-null derivates of groups I, II, and IV (NARSA), respectively. RN3984-M is the null derivate of RN3984, which demonstrated a loss of agr function by a lack of production of delta-hemolysin.Vancomycin analytical-grade powder was commercially purchased (Sigma, St. Louis, MO). Stock solutions were freshly prepared at the beginning of each week and kept frozen daily at Ϫ4°C. Mueller-Hinton broth (Difco, Detroit, MI) supplemented with 25 g/ml calcium and 12.5 g/ml magnesium (SMHB) was used for in vitro pharmacodynamic models and susceptibility testing involving vancomycin. Colony counts were determined using tryptic soy agar (TSA; Difco, Detroit, MI) plates. MICs were determined by broth microdilution in SMHB according to CLSI guidelines (2). The function of the agr operon was qualitatively assessed in all isolates using S. aureus RN4420 (NARSA) for delta-hemolysin expression, as previously described (13).An in vitro pharmacodynamic model was utilized as previously described for the collection of bacterial and antimicrobial dosing and sampling (1). All model simulations were conducted over 72 h and were performed in triplicate to ensure reproducibility. Vancomycin regimens of 62.5, 125, 250, 500, 750, and 1,000 mg every 12 h (free unbound fraction of the AUC [fAUC/MIC/24] exposures of 14 to 225 g/ml/h; half-life 6 h) were simula...

Reduced Vancomycin Susceptibility inStaphylococcus aureus, Including Vancomycin-Intermediate and Heterogeneous Vancomycin-Intermediate Strains: Resistance Mechanisms, Laboratory Detection, and Clinical Implications

“…In addition, it was demonstrated that S. aureus from agr groups I to IV all develop intermediate vancomycin resistance upon in vitro exposure to the drug (358). It was demonstrated that when hVISA or VISA develops from VSSA, there is a reduction in levels of agr expression in the resistant isolates (121,269).…”

“…In a number of in vitro studies, reduced agr function has been shown to favor the development of vancomycin resistance (295,296,358); however, an association between agr function and teicoplanin resistance development was not demonstrated (281). Assuming that the relationship between agr function and the development of vancomycin resistance exists, it is worth noting that in one study, 48% of hospital MRSA strains were found to have reduced agr function, compared to 3.5% of community-associated MRSA strains (357), suggesting a possible higher tendency for hospital MRSA strains to develop into hVISA or VISA strains.…”

Polymyxin B in combination with doripenem against heteroresistant Acinetobacter baumannii : pharmacodynamics of new dosing strategies

“…22 Cellulosic hollow-fibre cartridges (C3008; Fiber Cell System, Inc., Fredrick, MD, USA) were utilized in all experiments. Twenty-three samples (0.5 mL) were serially collected over 240 h to determine bacterial counts for each experiment.…”

Multicenter Evaluation of Vancomycin Dosing: Emphasis on Obesity

“…However, recent literature has created an increased awareness of Staphylococcus aureus isolates that are heterogeneously resistant to vancomycin otherwise known as hVISA. 16,17 The development of hVISA is associated with low vancomycin concentrations making the appropriate empiric dosing of vancomycin critical if clinicians want to have vancomycin available as an option for the treatment of MRSA in the future.…”