A number of interesting observations emerged from the present investigations. Phenotypically, the current data extend upon past findings regarding antimicrobial peptide resistance; PB strains tend to be more resistant to key innate cationic host defense molecules from both neutrophils (e.g., hNP-1) and platelets (e.g., tPMP-1) [1
]. These findings suggest that PB isolates have an intrinsic capacity to survive interactions with 2 predominant host defense cells early in the course of bloodstream invasion (). The ability of PB strains to circumvent such innate immune defenses likely enhance their subsequent pathogenic potential [15
]. Furthermore, since these 2 innate defense molecules may be important in multiple stages of endovascular pathogenesis (e.g., tPMP-1–resistance fostering infective endocarditis progression) [28
], the ability of a strain to resist their microbicidal actions probably contributes to PB.
The present findings also demonstrated that PB isolates adhere better than RB isolates to host cells (i.e., endothelium) and matrix ligands relevant to endovascular pathogenesis (i.e., fibrinogen and fibronectin). Such capabilities may facilitate the colonization phases of PB infection (phase 2 colonization) (). Also, since fibronectin and fibrinogen binding are now considered integral to endothelial cell and vegetation persistence in experimental infective endocarditis [29
], increased binding of PB isolates to these ligands, compared with binding of RB isolates, would theoretically provide an advantage for PB pathogenesis.
Importantly, PB isolates exhibited substantially more fluidic membranes than RB isolates. A fluidic membrane phenotype has previously been linked to cationic antimicrobial peptide resistance in S. aureus
laboratory strains [14
]. This characteristic may facilitate persistent and progressive infective endocarditis due to PB isolates [14
]. The mechanism (or mechanisms) by which enhanced fluidity causes increased resistance to such peptides is not well understood but is postulated to be associated with reduced membrane binding or intramembrane organization of these cationic molecules.
Genotypically, a greater percentage of PB isolates were associated with SCCmec
II (95%, compared with 72% of RB isolates), CC30 (90% vs. 44%), and spa
type 16 (57% vs. 39%). This observation is in line with recent findings by Fowler et al. [19
], who demonstrated a significant trend toward more frequent hematogenous complications in strains exhibiting these genotypic profiles. Multiplex PCR revealed that PB isolates differed from RB isolates in terms of overrepresentation of capsule type 8 and cna
gene carriage. The increased presence of the adhesin gene, cna
, would theoretically allow PB strains to exploit specific anatomic targets, such as bones, joints, or endothelial substrata [30
]. This role could potentially contribute to enhancement of the colonization and/or persistence phases in the life cycle of PB isolates. In addition, overrepresentation of the tst-1
gene could ostensibly increase the incidence of “cytokine storm”-associated sepsis syndromes, leading to worse clinical outcomes for patients with PB [31
The net intrinsic virulence properties of these 2 strains were not different in the context of infective endocarditis induction or progression. In contrast, vancomycin therapy clearly divulged significant outcome differences between groups. Thus, isolates from the PB strain set were able to persist within cardiac vegetations to a greater extent than those from the RB strain set during vancomycin therapy. This result occurred despite identical vancomycin MICs and no hetero-VISA subpopulations in the PB and RB strain sets. A similar observation by Fowler et al. [1
] and Hawkins et al. [32
] demonstrated that vancomycin susceptibility was not decreased among PB isolates. However, these findings contrast with those of other studies in which the PB phenotype has been associated with reduced susceptibility to vancomycin [33
]. These in vivo data regarding vancomycin-induced disclosure of the PB outcome suggest several interesting possibilities. For example, PB and RB isolates may differ in other features not assessed in our profiling that may impact net responsiveness to vancomycin (e.g., cell wall perturbations, cell surface charge, and global metabolic pathway abnormalities) [7
]. Also, it is possible that vancomycin may itself differentially impact virulence pathways in PB isolates, compared with RB isolates.
Our study has several potential limitations. First, all strains were obtained from Duke University Medical Center, raising the possibility of single-center and/or geographic bias. Second, these strains emanated from 1994–1999 and therefore do not represent recent shifts. Third, we compared only the initial PB and RB isolates and did not screen for virulence signatures that may have adaptively evolved during treatment among follow-up blood isolates. Finally, we only examined a single PB-RB strain pair in vivo. Current studies are being designed to address these limitations.
In summary, the present data support our hypothesis that there are significant phenotypic and genotypic profiles that can distinguish PB isolates from RB isolates. Characterization of PB isolates may afford breakthrough discoveries in the treatment of life-threatening MRSA infections.