The accessory gene regulator (agr) is a quorum sensing cluster of genes which orchestrate the expression of cell-secreted and virulence factors, and several metabolic pathways in Staphylococcus aureus in a growth dependant fashion [1, 2]. It has been hypothesized that S. aureus which exhibit agr dysfunction may possess an intrinsic survival advantage: these strains have demonstrated vancomycin tolerance and a proclivity to develop heterogeneous resistance under vancomycin selective pressure[3, 4]. However, agr dysfunction has primarily been evaluated in strains of healthcare-associated (HA) MRSA where a down-regulated agr locus has been associated with prolonged and persistent bactaeremia [5, 6].

Worldwide, the incidence of community-associated (CA) MRSA is rapidly increasing with the emergence of highly virulent strains in Canada [7–9]. While a large proportion of HA-MRSA display dysfunctional in the agr loci, the prevalence of agr dysfunction among CA-MRSA is relatively low from 3.5 to 9% [10, 11]. Although this may account for the enhanced virulence of CA-MRSA as compared with HA-MRSA, whether dysfunction in agr would hamper vancomycin bactericidal activity among Canadian CA-MRSA has not been fully elucidated. Therefore, the objectives of this current study were to compare vancomycin pharmacodynamics of agr dysfunctional versus functional CA-MRSA and HA-MRSA clinical isolates from the Canadian Nosocomial Infection Surveillance Program at low and high initial inoculum.

Vancomycin MICs against HA-MRSA 7, 9, were 1.0, 2.0 mg/L and for CA-MRSA 26 and 20 were 1.0, and 1.0 mg/L. 15% (6 of 40) of isolates were dysfunctional in agr which was higher among HA-MRSA (26.3%, 5 of 19) vs. CA-MRSA (4.76%, 1 of 21, USA 400/Canadian MRSA 7). The activity of vancomycin against a low (10⁶ cfu/ml) and high (10⁸ cfu/ml) initial inoculum of bacterial strains is depicted in Figure 1. agr functional strains (CA-MRSA 26) displayed greater killing versus agr dysfunctional MRSA (CA-MRSA 20). Maximal bacterial reductions at 64 times the MIC for CA-MRSA 20 and CA-MRSA 26 were 3.30 and 3.01 log₁₀ cfu/ml at 24 h, respectively. However, against 10⁶ cfu/ml inoculum differences in vancomycin killing of HA-MRSA with respect to agr function were noted with less activity versus agr dysfunctional strains. Interestingly, against a high initial inoculum of MRSA, vancomycin did not achieve bactericidal activity against any isolate. Most notably, against 10⁸ cfu/ml, the killing activity of vancomycin against agr functional versus dysfunctional strains consistently differed for both CA-MRSA and HA-MRSA strains. Greater vancomycin killing activity occurred against agr functional (HA-MRSA 7, CA-MRSA 26) versus dysfunctional (HA-MRSA 9, CA-MRSA 20) strains. Reductions in bacterial counts at 24 h were greater against HA-MRSA 7 versus HA-MRSA 9 with 1.87 versus 1.24 log₁₀ CFU/ml reduction at 24 h. Against CA-MRSA 20, killing approached the bactericidal threshold with a 2.63 log₁₀ cfu/ml reduction as compared with CA-MRSA 26 where a decrease of only 1.25 log₁₀ cfu/ml at 24 h was demonstrated.

Analysis of pharmacodynamics (PD) revealed excellent model fits of the data to the Hill model. R² for was >0.99 for all MRSA isolates at both initial inoculum. The concentration effect profile is depicted in Figure 2 with maximal likelihood fitted parameters from equation 2. Reduction in bacterial counts with vancomycin occurred in a concentration-dependent manner, as seen with the steep sigmoidicity constant (H) and low EC50 against most isolates. All pharmacodynamic parameter estimates consistently differed for both HA-MRSA and CA-MRSA when comparing agr functional versus dysfunctional isolates at the higher starting inoculum (10⁸). There were significant differences in regards to vancomycin activity in reducing bacterial counts when comparing agr function and dysfunctional strains among HA-MRSA and CA-MRSA. HA-MRSA isolates displayed a 2.89 fold decrease in the maximal reduction in bacterial counts (Emax) which was 2.91 log₁₀ CFU/ml for agr functional vs. 2.45 log₁₀ CFU/ml for agr dysfunctional MRSA log₁₀ CFU/ml (p = 0.0163).

CA-MRSA displayed an even greater trend with a 20.0 fold decrease in the maximal reduction in bacterial counts (Emax) which was 3.71 log₁₀ CFU/ml for agr functional vs. 2.41 log₁₀ CFU/ml for agr dysfunctional MRSA (p = 0.0007). The sigmoidicity constant (H) was nearly three times higher and EC₅₀ values were consistently lower in agr functional versus dysfunctional isolates, but did not achieve significance. Model fitted for each isolate parameter estimates from Equation 2 in Figure 2.

The epidemiology of Staphylococcus aureus is rapidly changing. Highly virulent strains of CA-MRSA have emerged worldwide and have been associated with fatal infections in a plethora of recent outbreaks including severe necrotizing pneumonia and fasciitis and in both children and healthy adults [8, 9, 14]. A number of virulence determinants have recently been characterized in CA-MRSA including alpha-type phenol soluble modules (PSMs), the arginine catabolic mobile element (ACME), the panton- valentine leucocidin cytotoxin and a variety of virulence factors not common in HA-MRSA[15–17]. Interestingly, many of these virulence determinants are regulated via the two component agr system, the master regulator of S. aureus, controlling growth, colonization and virulence[1, 17, 18]. However, although agr dysfunction in S. aureus may potentially decrease virulence, the loss of agr function may increase the proclivity to develop vancomycin tolerance and resistance, although this has not been previously evaluated in strains of CA-MRSA. Therefore, we sought to determine if dysfunctional agr strains in either HA-MRSA and CA-MRSA would impact vancocmycin pharmacodynamics.

In the current investigation, agr dysfunction resulted in attenuation of vancomycin killing activity with decreases in maximal effect and lack of bactericidal activity against both HA-MRSA and CA-MRSA. We also determined that strains which exhibited agr dysfunction were far less common among CA-MRSA vs. HA-MRSA, with only one out of 18 isolates demonstrating this phenomena. Taken together, these findings may have potential implications for the use of vancomycin for the treatment of CA-MRSA infections. First, although reports of persistent bacteraemia, vancomycin treatment failure and heterogeneous resistance have been primarily in HA-MRSA, the findings of the present study highlight similarities of CA-MRSA and HA-MRSA when exposed to vancomycin [5, 6]. Similar to previous reports in HA-MRSA, limited bacterial killing was evident by vancomcyin against CA-MRSA, especially in the context of agr dysfunction. Second, with reports of CA-MRSA as an emerging pathogen in infections that are sequestered or deep-seated, such as endocarditis, the potential for attenuation of bactericidal activity should be considered when vancomycin is utilized against a high initial inoculum of CA-MRSA [19]. Therefore, the inoculum effect displayed by vancomycin displays may be a potential explanation for the lack of bactericidal activity shown in this study. This is especially important to consider since eradication of S. aureus from sites of infection that are at high bacterial density or in a biofilm matrix at may be particularly difficult for vancomycin due to its pharmacodynamics properties, including slow bactericidal activity and the heterogeneous nature of vancomycin resistance. Finally, as the antimicrobial susceptibility profile of CA-MRSA is commonly greater than HA-MRSA and with initial reports of heterogeneous vancomycin-intermediate resistance in CA-MRSA emerging[20], the treatment of agr dysfunctional CA-MRSA may favor the use of combination therapy or alternative agents rather than glycopeptide monotherapy.

Potential limitations of this study include the limited number of strains including only two CA-MRSA and two HA-MRSA isolates which were utilized to evaluate vancomycin efficacy. We also acknowledge the lack of inclusion of a USA300 strain in time kill experiments because there were no agr dysfunctional CA-MRSA strains among the 40 isolates which we screened. Interestingly, these data corroborate our previous work which also shows an exceptionally low prevalence (2.5%) of agr dysfunction in community isolates of MRSA. Therefore, further in vitro and in vivo studies are necessary to confirm these findings in additional CA-MRSA before these results are applied to clinical practice.

This study is the first to demonstrate that vancomycin activity was impaired against a high inoculum of both community and healthcare associated strains of MRSA that were dysfunctional in agr. Additionally this study characterized the function on agr among MRSA strains which were genetically defined according to community and hospital original and determined that agr dysfunction were far less common among CA-MRSA as compared with HA-MRSA (11). These result are important to consider particularly in the treatment of infections of high bacterial density due to MRSA when vancomycin is utilized against a high initial inoculum of CA-MRSA