This study distinguishes itself by being the first to describe covariates that best predict the presence of agr dysfunction among MRSA isolates in patients with MRSA bloodstream infections across two major academic hospitals. It is of greatest clinical interest that a relationship between isolates with agr dysfunction and prior antibiotic exposure was observed in the multivariate analysis. Given the results of the bivariate analysis, it is reasonable to surmise that this observed relationship was largely driven by prior administration of beta-lactams and fluoroquinolones in patients with an LOS of ≥72 h. Overall, patients who received a prior beta-lactam or fluoroquinolone were approximately twice as likely to be infected with a dysfunctional versus functional agr isolate.
Although this study was not designed to assess causality, these findings are biologically plausible. The prior use of fluoroquinolones and beta-lactam agents has been known to cause collateral damage through the selection of antibiotic-resistant populations in MRSA (1
). This phenomenon may be due to the counterselection of agr
dysfunctional strains (23
) and the eradication of agr
functional strains. In particular, subinhibitory concentrations of fluoroquinolones have been shown to induce mutational mechanisms in S. aureus
that promote survival, the development of resistance, and bacterial tissue adherence and colonization (2
). It also has been shown that agr
dysfunctional, slow-growing strains that are small colony variants of MRSA display increased resistance to fluoroquinolones and beta-lactam agents (33
). Along with recent evidence that suggests that the loss of agr
function is associated with increased biofilm production, persistence, and prolonged bacteremia, the potential associations with prior exposure to fluoroquinolones and beta-lactams is of interest and suggests a selective survival advantage for agr
dysfunctional strains which may be driven by these agents (31
Contrary to the prevailing thinking, we did not find an association between agr
dysfunction and prior exposure to vancomycin or other anti-MRSA antibiotics. Of the 19 patients with a documented prior exposure to vancomycin, only 3 were agr
dysfunctional. It has been shown that subinhibitory concentrations of vancomycin select for other resistance phenotypes, including decreased autolysis, hVISA, and VISA. This lack of a relationship may be an artifact of our institutions' empirical dosing strategies; our institutions began targeting trough values of 15 to 20 mg/liter in 2007 (20
). Alternatively, our findings may be a result of incomplete information on prior exposure outside the hospital setting, particularly among those receiving dialysis or those who developed bacteremia within 72 h of admission. Although we cannot exclude the possibility of a type II error given the sample size, our data suggest there may not be a relationship between agr
dysfunction and prior exposure to vancomycin, particularly in a setting where higher trough concentrations are targeted.
While a relationship between agr dysfunction and higher MICs has been presumed, this was not observed in our study. Previous studies also have suggested that defects in the agr locus contribute to hVISA development. However, we did not find an association between these variables. Our findings do not refute the relationship between agr dysfunction and the development of higher-MIC MRSA isolates and hVISA. Rather, our data suggest that there are other things involved in the causal pathway. In addition, we cannot exclude the possibility of a type II error given the sample size. As an example, there was a nonsignificant trend in the association between agr dysfunction and high Etest MICs (PR = 1.3; 95% CI = 0.9 to 1.8; P = 0.21). While the definitive explanations for these findings remain unclear, they underscore the need for additional studies to better describe the mechanisms responsible for the development of higher-MIC isolates and hVISA.
Several limitations of this study should be noted. First, we had incomplete information on antibiotic exposure prior to admission, therefore it is possible that subjects who presented with an MRSA bloodstream infection on admission were misclassified on prior antibiotic exposure. We also did not capture the vancomycin troughs maintained in patients previously exposed to vancomycin. Second, we saw a low proportion of hVISA isolates in our study, which may explain why we did not detect an association between this phenotype and agr dysfunction. Third, we acknowledge that hVISA is a relatively unstable phenotype, which is no exception concerning the clinical isolates that were included in our study. Fourth, the retrospective laboratory analyses employed in this study may have underestimated the prevalence of the hVISA phenotype at our institutions. Fifth, we did not take into account bacterial density in hVISA detection, as we utilized standard inocula of MRSA according to the manufacturer's recommendations. It has been hypothesized that stationary-phase growth, defective autolysis profiles, biofilm production, and the production of thicker cell walls is facilitated by quorum-sensing mechanisms at high bacterial density, which may increase the proportion of the heteroresistant subpopulations which were not directly studied. Sixth, limited sample size also may have played a role in the lack of association found between agr dysfunction and clinical covariates, including comorbidities, critical illness, prior antibiotic exposures, and health care exposure history. Seventh, the source of infection was not delineated in our study. Finally, the influences of the following were not considered: genotype, susceptibility to thrombin-induced platelet microbicidal proteins, and autolytic profile. The presence of different genotypes (e.g., agr group, staphylococcal cassette chromosome mec type, and pulsed-field gel electrophoresis pattern) among our isolates may have confounded the associations (or lack thereof) seen between different covariates and agr dysfunction.
In conclusion, MRSA isolates with agr
dysfunction have been described recently as an important clinical factor in outcomes of patients with S. aureus
). Our data support the major role of prior antibiotic exposure as a predictor of agr
dysfunctional isolates, particularly prior exposure to beta-lactams or fluoroquinolones. While clinicians presume there is a relationship between agr
dysfunction, higher MICs, and hVISA status, we were not able to verify this in our study. Our results do not refute the relationship between agr
dysfunction and the development of higher-MIC MRSA isolates and hVISA. Rather, they suggest that there are other things involved in the causal pathway and underscore the importance of rigorous studies specifically designed to pinpoint the mechanisms underlying the relationship between MICs and outcomes.