Invasion and survival ofFusarium solaniin the dexamethasone-treated cornea of rabbits

Abstract

Electron microscopic observations, as well as in vitro experiments, on experimental Fusarium solani keratitis of rabbits were performed to study the mode of fungal invasion into the corneal stroma, the interactions between E solani and inflammatory cells under the influence of topical dexamethasone (DXM) treatment, and the survival mechanism of the fungi in the DXM-treated cornea. Electron microscopy showed that, while the fungus invaded into the corneal stroma, digestion of collagen fibrils occurred around the hyphae, where amorphous material was often noted. In DXM-nontreated cornea, the fungal hyphae were entrapped by pseudopodia of the neutrophils and destruction of the hyphae was noted on day 3 of infection, most hyphae having disappeared by day 7. In the DXM-treated cornea, however, neutrophils could not ingest and destroy the hyphae. In qualitative nitroblue tetrazolium (NBT)-reduction tests using rabbit peripheral blood neutrophils, DXM significantly suppressed the rate of NBT-reduction and the rate of adherence to the fungal microconidia. In the DXM-treated corneal lesions, a considerable increase in both number and size of fungal peroxisomes was noted. Furthermore, the hyphae, surrounded by neutrophils, showed double or triple cell wall formation or sometimes a hypha-in-hypha structure.Similar hypha-in-hypha structures were also observed when the organisms were treated in vitro with a fungistatic concentration of H20> We suggest that this special structure is a protective device produced for the survival of E solani when subject to neutrophil attack in the DXM-treated cornea.Fusarium species are widely distributed in nature, being detected mainly in soils and on plants. They are important plant pathogens [23] and are now recognized as opportunistic pathogens causing keratitis [11,21,27], infections of burns [1,28] and systemic infection in immunologically compromised patients [18,30]. Among the Fusarium species, Fusarium solani is the most commonly found etiologic agent of mycotic keratitis in Japan where its incidence has increased [15]. Corneal trauma is probably a major predisposing factor for E solani infection [11,21,27]. It is also well known that clinical use of corticosteroid eye drops exacerbates the infection [9,19].Inoculation of cultured E solani into rabbit cornea caused keratitis where the fungi grew in the corneal stroma [8,14]. In these lesions, the killing process by the infiltrating neutrophils was noted 3 days after inoculation and the lesions healed spontaneously thereafter. When corticosteroid eye solution was applied topically, the fungi grew much more rapidly and survived much longer in the cornea [14]. However, the mechanisms of invasion of the fungi, killing of the fungi by neutrophils, and exacerbation of the experimental keratitis by topical corticosteroids, are still not clear. To investigate these

Pathogenesis of microbial keratitis

“…This permits fungi to survive in the presence of anti-fungal drugs and resist anti-microbial therapy. The same intrahyphal hyphae or hypha-in-hypha and thickened fungal cell walls are seen in corneal tissues infected with L. theobromae and in corneas infected with F. solani keratitis treated earlier with corticosteroids, as seen in the presence of antifungal drugs [94,193]. This suggests that these morphological alterations may allow fungi to evade host defenses and present a barrier against antifungal drugs [77] demonstrated the role of EFG1-regulated SAP6 gene of C. albicans which encodes for a unique secreted aspartyl proteinase.…”

Fungal infections of the cornea

“…1 It has been hypothesised that rapid progression of mycotic keratitis in the early phases is mainly by agent factors, such as a large fungal inoculum and penetration deep into the corneal stroma, while progression in the later phases involves a combination of agent and host factors and resistance to antifungals. 17 Invasiveness of fungi in corneal tissue may be aided by formation of intrahyphal hyphae, 57,58 or by liberation of fungal proteinases 59 or toxins. 60 Progression of mycotic keratitis may be augmented by the activation of resident corneal cells or inflammatory cells, particularly polymorphonuclear leucocytes, which release proteinases of different molecular weights, resulting in extensive degradation of the corneal tissue matrix.…”

Current Perspectives on Ophthalmic Mycoses

“…The presence of "intrahyphal hyphae" or "hypha-in-hypha," and thickened fungal cell walls (Table 9) may reflect such morphogenesis occurring in fungi invading corneal tissue; these morphological alterations may constitute a barrier against antifungal drugs or host defenses (392,393) or may be a virulence factor for fungi in corneas where the defense mechanisms have been compromised by the application of corticosteroids (190). Rigorous experimental and other studies are required to elucidate these aspects.…”