The Use of Free-Living Amoeba in Treating Bacterial Infections: Thermotolerant Dictyostelids Are Avirulent and Do Not Cause Keratitis in Mice.

Purpose: Antimicrobial drug resistance is a significant threat to human health and additional drugs and therapeutic approaches are desperately needed. Amoebozoa is a large and diverse group whose primary mode of nutrition is phagocytosis of bacteria. This feeding behavior raises the possibility that dictyostelids could be used to topically treat bacterial infections. Dictyostelids are generally considered avirulent; however, there are two case reports of keratitis owing to Dictyostelium polycephalum, raising concerns about their therapeutic use. Our goal was to determine whether thermotolerant dictyostelids were pathogenic. Methods: One hundred dictyostelid strains from four phylogenetic groups were tested for their ability to feed, aggregate, and form spores at 32°C. Fifty-four strains tested positive for feeding on K. oxytoca and multicellular development at 32°C. Of these 54 strains, 3 strains of Dictyostelium mucoroides and one tropical strain of Heterostelium pallidum were examined for the potential to cause keratitis in mouse eyes. Mice were monitored for signs indicative of keratitis, including blepharitis, vascularization, stromal keratitis, and the development of antibodies to dictyostelid proteins. Results: We found that none of the four dictyostelid isolates tested induced irritation or keratitis when vegetative cells or spores were topically introduced to the eyes 5 times per day for 5 days. No pathological changes were identified. Application of spores or vegetative cells was not irritating, and none of the mice analyzed developed antibodies to vegetative cell proteins. Conclusions: These results suggest that dictyostelids are avirulent and may be safe for use in treating topical bacterial infections.