Bradley D. Jett

Virulence of enterococci.

Enterococci are commensal organisms well suited to survival in intestinal and vaginal tracts and the oral cavity. However, as for most bacteria described as causing human disease, enterococci also possess properties that can be ascribed roles in pathogenesis. The natural ability of enterococci to readily acquire, accumulate, and share extrachromosomal elements encoding virulence traits or antibiotic resistance genes lends advantages to their survival under unusual environmental stresses and in part explains their increasing importance as nosocomial pathogens. This review discusses the current understanding of enterococcal virulence relating to (i) adherence to host tissues, (ii) invasion and abscess formation, (iii) factors potentially relevant to modulation of host inflammatory responses, and (iv) potentially toxic secreted products. Aggregation substance, surface carbohydrates, or fibronectin-binding moieties may facilitate adherence to host tissues. Enterococcus faecalis appears to have the capacity to translocate across intact intestinal mucosa in models of antibiotic-induced superinfection. Extracellular toxins such as cytolysin can induce tissue damage as shown in an endophthalmitis model, increase mortality in combination with aggregation substance in an endocarditis model, and cause systemic toxicity in a murine peritonitis model. Finally, lipoteichoic acid, superoxide production, or pheromones and corresponding peptide inhibitors each may modulate local inflammatory reactions. Images

Bacterial Endophthalmitis: Epidemiology, Therapeutics, and Bacterium-Host Interactions

SUMMARY Endophthalmitis is a severe inflammation of the interior of the eye caused by the introduction of contaminating microorganisms following trauma, surgery, or hematogenous spread from a distant infection site. Despite appropriate therapeutic intervention, bacterial endophthalmitis frequently results in visual loss, if not loss of the eye itself. Although the pathogenicity of bacterial endophthalmitis has historically been linked with toxin production during infection, a paucity of information exists as to the exact mechanisms of retinal toxicity and the triggers for induction of the intraocular immune response. Recently, research has begun to examine the bacterial and host molecular and cellular events that contribute to ocular damage during endophthalmitis. This review focuses on the causative agents and therapeutic challenges of bacterial endophthalmitis and provides current data from the analysis of the role of bacterial virulence factors and host inflammatory interactions in the pathogenesis of eye infections. Based on these and related studies, a hypothetical model for the molecular pathogenesis of bacterial endopthalmitis is proposed. Identifying and understanding the basic mechanisms of these bacterium-host interactions will provide the foundation for which novel, information-based therapeutic agents are developed in order to prevent vision loss during endophthalmitis.

Internalization of Staphylococcus aureus by human corneal epithelial cells: role of bacterial fibronectin-binding protein and host cell factors.

ABSTRACT Wild-type Staphylococcus aureus was observed to be capable of invading human corneal epithelial cells (HCEC) in vitro. Internalization of S. aureus required expression of fibronectin-binding proteins (FnBPs); the capacity of an FnBP-deficient isogenic strain to invade HCEC was reduced by more than 99%. The binding of S. aureus to HCEC did not require viable bacteria, since UV-killed cells were observed to adhere efficiently. Invasion of HCEC by S. aureus involved active host cell mechanisms; uptake was nearly completely eliminated by cytochalasin D and genistein. These data suggest that FnBPs play a key role in host-parasite interactions and may serve as an important adhesin or invasin in ulcerative keratitis caused by S. aureus.

Biofilm Formation by Enterococcus faecalis on Intraocular Lens Material

Purpose: To compare biofilm formation by Enterococcus faecalis on different intraocular lens (IOL) materials. Methods: E. faecalis biofilms were cultivated on disks of IOL material (silicone, PMMA [polymethylmethacrylate], or acrylic). Biofilms were stained with crystal violet (CV), which served as an index of biofilm formation. The bacterial population was enumerated after biofilm homogenization. Biofilms were also examined by confocal microscopy. Results: At 24 hr, there was no significant difference in biofilm formation, or the population within biofilms, among the three materials tested. However, after 48 and 72 hr, it was observed that silicone supported the least amount of biofilm formation (p < 0.05); biomass on PMMA and acrylic IOLs continued to increase, with acrylic demonstrating the greatest amount of stainable biofilm (p < 0.0005). The population on PMMA was significantly greater than the other IOL materials (p < 0.005). Confocal microscopy confirmed the lack of biomass on silicone. Conclusions: Among three IOL materials, E. faecalis biofilms formed more readily on PMMA and acrylic compared to silicone.

Host-Parasite Interactions in Staphylococcus aureus Keratitis

Ulcerative keratitis is among the leading ocular bacterial infections, and Streptococcus aureus accounts for approximately 25% of cases in some surveys. Although S. aureus expresses numerous virulence factors, many of which are under the control of staphylococcal global regulatory genes, their pathophysiologic roles in keratitis are largely unknown. Similarly, the nature of the host response during S. aureus keratitis is unclear. Following a review of previously published research on the pathophysiology of S. aureus ocular infection, we present the results of a study designed to assess the host-parasite relationship between S. aureus and human corneal epithelial cells (HCECs) in vitro. In this model system, a wild-type S. aureus strain and its isogenic mutants harboring mutations in agr and sar global regulatory genes or fibronectin-binding proteins A and B (fnbAB) were tested for their ability to bind and invade confluent HCEC monolayers. The contribution of host cell factors was assessed by preincubating HCECs with various inhibitory agents. These studies demonstrated that S. aureus not only adhered to the surface of HCECs but was also internalized, as has been previously observed in other nonocular cell lines. Adherence and invasion of HCECs was saturable at 1 h of incubation in the presence of approximately 10(7) CFU per HCEC monolayer (multiplicity of infection approximately 10). A mutant defective in both agr and sar global regulators was not significantly different in invasive capacity compared to its isogenic wild-type parent strain. In contrast, mutations in fibronectin-binding proteins A and B (fnbAB) reduced the invasiveness of S. aureus by 99% compared to the wild-type strain. Pretreatment of HCECs with colchicine had little effect on S. aureus invasion. In sharp contrast, cytochalasin D and genistein were each capable of inhibiting invasion by >99%. In summary, the results of this study point to fibronectin-binding protein as a key S. aureus surface adhesin facilitating invasion of HCECs in vitro. Furthermore, these results suggest an active mechanism for S. aureus internalization by HCECs, likely involving actin polymerization and tyrosine kinase activity. Additional studies are warranted to determine the applicability of these findings in vivo, and to facilitate the rational design of therapeutic agents aimed at blocking the establishment and progression of S. aureus keratitis.

Host/Parasite Interactions in Bacterial Endophthalmitis

Bacterial infections within the eye arise as complications of intraocular surgery, penetrating injury, or hematogenous spread from distant anatomical sites. Because: 1) the interior surfaces of the eye are lined with sensitive, nonregenerating tissues, 2) the inner chambers of the eye are relatively sequestered from circulating immunological components, 3) the integrity of blood-ocular barriers provides poor penetration of systemically administered antibiotics, and 4) aqueous and vitreous humor represent rich, relatively acellular culture media; endophthalmitis often progresses rapidly and total loss of vision frequently results. Years of clinical experience have shown that current therapies for endophthalmitis, including antimicrobials, antiinflammatory agents, and vitrectomy, are frequently unsuccessful in ameliorating destruction of intraocular tissues. While bacterial and host factors were thought to play key roles in the course and severity of endophthalmitis, it is only recently that their contributions have been experimentally defined. Molecular-based techniques are gaining increased use in the study of infectious eye diseases. Current findings regarding the host/parasite interactions within the eye are reviewed, and a resulting integrative model of the natural course of endophthalmitis proposed. A molecular-level understanding of the roles of both bacterial and host factors during endophthalmitis will likely reveal potential targets for therapeutic intervention aimed at salvaging vision.