Investigation into the Efficacy of Formic Acid Fumigation for Inactivating Clinically Isolated Filamentous Fungi

Inactivating filamentous fungi (molds) reduce toxin production and transmission. This study evaluated the efficacy of formic acid (FA) fumigation to identify optimal conditions and its effects on fungal morphology. We aim to develop an efficient inactivation protocol for molds and provide valuable insights into laboratory techniques for fungal inactivation. Initially, 238 clinical isolates from 23 species of molds were evaluated for the inactivation efficacy of formic acid, formaldehyde, and peracetic acid. After identifying the optimal sterilizing agent, 188 isolates from 16 clinically significant species were tested. Four concentrations (30%, 50%, 70%, and 100%) and six-time intervals (4 to 14 hours) were tested to optimize fumigation conditions. Morphological changes were analyzed using Aspergillus fumigatus ATCC MYA-3626 as the standard strain. The inactivation rates of 238 molds fumigated with formic acid, formaldehyde, and peracetic acid for 24 hours were 99.16%, 89.08%, and 90.35% (P < 0.05), indicating that formic acid was more effective than formaldehyde and peracetic acid. Subsequently, 188 isolates from 16 species were fumigated with formic acid at concentrations of 30%, 50%, 70%, and 100% for 24 hours, resulting in inactivation rate of 73.9%, 98.4%, 100%, and 100% (P < 0.05). Inactivation rate increased significantly with longer exposure times: 51.1% at 4 hours, 80.9% at 6 hours, 89.4% at 8 hours, 94.7% at 10 hours, 100% at 12 hours, and 100% after 14 hours (P < 0.05). After fumigation, A. fumigatus ATCC MYA-3626 growth ceased. Electron microscopy revealed irregular folds and roughness on spore surfaces, as well as collapsed or dead spores. Formic acid demonstrated superior inactivation efficacy compared to formaldehyde and peracetic acid. At a concentration of 70% and a fumigation duration of 12 hours, the inactivation of molds was most effective.