Association between fungal growth temperature and corneal lesions in fungal keratitis: a case series

Fungal keratitis is a refractory disease, and cases caused by filamentous fungi are particularly challenging to manage. Considering that the depth of corneal infiltration influences both treatment and visual prognosis, the ability of causative fungi to grow at corneal and anterior chamber temperatures was hypothesized to contribute to the progression of lesions from the deep cornea into the anterior chamber. This study investigated the relationship between the growth temperature of filamentous fungi isolated from keratitis and the depth of corneal infiltration.

Eight cases of filamentous fungal keratitis diagnosed at Ehime University Hospital between January 2020 and April 2024 were retrospectively reviewed. Causative organisms were identified from corneal scrapings via morphological analysis, MALDI-TOF mass spectrometry, or sequencing of the internal transcribed spacer region. Growth temperature testing was performed on Sabouraud agar at 28 °C, 35 °C, and 37 °C, assessing colony formation after 5 days. The depth of corneal infiltration was clinically assessed, and its relationship with fungal growth temperatures was analyzed.

The causative fungi were Fusarium spp. (n = 4), Paecilomyces spp. (n = 2), Beauveria bassiana (n = 1), and Colletotrichum sp. (n = 1). Beauveria grew only at 28 °C, confined to the superficial corneal stroma. Isolates capable of growth up to 35 °C showed variable clinical courses, with two cases limited to the superficial stroma and one extending into the deep stroma. All Fusarium isolates grew at 37 °C, including two cases wherein the lesion extended through Descemet's membrane and reached the anterior chamber, thus necessitating therapeutic keratoplasty.

In this case series, isolates that grew only at 28 °C were restricted to the superficial corneal stroma, whereas those capable of growth at 37 °C were associated with deeper stromal or anterior chamber involvement. Growth temperature testing may therefore provide additional insight into the potential depth of lesion progression in filamentous fungal keratitis.