Role of wall teichoic acids in Staphylococcus aureus endophthalmitis.

Bacterial endophthalmitis is a severe and sight-threatening ocular infection.1 It is characterized by massive inflammation and tissue damage caused both by bacterial infection and subsequent immune response. Bacterial endophthalmitis usually occurs in the context of ocular surgery, trauma, microbial keratitis, or the hematogenous spread of the organism to the eye. Postoperative endophthalmitis is a severe complication of ocular surgery, such as cataract surgery, glaucoma surgery, or vitrectomy.1 Although the technical procedures used in surgery are improving, the incidence of endophthalmitis has not changed and may be increasing.2 Staphylococcus aureus is a commensal species that colonizes the mucosa and skin adjacent to the eye, and as a result, often contaminates surgical sites and leads to endophthalmitis.1,3 Methicillin resistance in S. aureus is on the rise in the community, which in the United States has been associated with the proliferation of the USA300 MRSA (methicillin-resistant S. aureus) lineage,4,5 which likely has contributed to the increased incidence of endophthalmitis caused by MRSA.6,7 Although fluoroquinolones and cephalosporins are widely used for the treatment and prevention of S. aureus endophthalmitis, they are less efficacious against MRSA.6,7 Moreover, S. aureus recently acquired resistance to vancomycin, a drug of last resort.8–10 Because of the rapid emergence of resistant strains, new antibacterial targets that lack cross-resistance with drugs currently in use to treat S. aureus infection are urgently needed. A possible therapeutic target that is now being explored is S. aureus wall teichoic acid (WTA) biosynthesis. WTAs are anionic polymers composed of repeating units of ribitol-phosphate that are covalently linked to the bacterial cell wall in many Gram-positive pathogens.11 They influence critical properties of the cell envelope, including charge, cation binding, tensile strength, rigidity, and permeability.11 WTAs are not required for the growth of S. aureus in vitro, but are needed to establish infection in most animal models.12–15 S. aureus WTA polymer biosynthesis is performed by the Tar enzymes.11,16 Of interest, this biosynthetic pathway contains two classes of targets: antivirulence targets and antibacterial targets. The first two genes in the pathway (tarO or tarA) can be knocked out, and the resulting mutants are viable in vitro, making the products of these genes potential antivirulence targets. However, genes downstream of tarA in the WTA pathway cannot be deleted in a wild-type background,16 making them antibacterial targets in wild-type strains. However, if flux through the pathway is diverted, either through genetic mutation (of tarO or tarA) or pharmacologic inhibition of TarO by the natural product tunicamycin, these downstream enzymes become expendable, suggesting that the essentiality of these downstream functions results from the accumulation of WTA biosynthetic intermediates in the pathway. The characterization of the first small-molecule inhibitor of an antibacterial target within the WTA biosynthetic pathway was recently reported by one of our groups (SW).17 The compound, 1835F03, inhibits TarG, the transmembrane component of the ABC transporter that exports WTAs to the cell surface.17 A structure–activity relationship study of 1835F03 led to the discovery of a second-generation analogue, targocil, which is 10 times more potent.18 Because blocking polymer synthesis at the beginning of the pathway renders TarG nonessential, these WTA inhibitors generate mutants in which WTAs are no longer expressed. Thus, they can be regarded as both antibiotics and antivirulence factor agents.17 Although the roles of WTAs have been investigated in S. aureus colonization of epithelial and endothelial cells,12–15 the role of WTAs in vivo in eye infection and the extent to which this pathway could represent a viable treatment target have not yet been explored. In the present study, we assessed the necessity for WTA biosynthesis during S. aureus endophthalmitis, both in vitro and in vivo. The WTA transport inhibitor, targocil, was previously shown to halt S. aureus growth through a bacteriostatic mechanism.17,18 In this study, we showed the surprising finding that targocil treatment killed S. aureus in vitro in vitreous humor (VH). Further, we found that a WTA-deficient mutant cannot survive in VH in vitro or in vivo, and as a result WTA-deficient mutants are highly attenuated, with little capacity to cause endophthalmitis.