Onychomycosis has a prevalence of approximately 10% in the United States, which makes it an important public health problem. Accurate diagnosis depends on the findings of physical examination, direct microscopy, and fungal culture. Identification of the causative fungal organism can be complicated by the presence of Pseudomonas aeruginosa. On several occasions, we have made the in vitro observation that dermatophytes and other fungi grow poorly or not at all in the presence of P aeruginosa. To better understand this phenomenon, we analyzed debris from 90 nails with onychomycotic features including nail plate thickening and opacification, nail bed hyperkeratosis, onycholysis, altered nail surface texture, nail discoloration, and distal subungual crumbly debris. A potassium hydroxide preparation with calcofluor stain (KOH/calcofluor) identified fungal elements in half of the 90 specimens. An analysis of cultures derived from each specimen showed that KOH/calcofluor–positive nail specimens were statistically more likely to yield P aeruginosa than KOH/calcofluor-negative specimens (13% vs 0%, respectively; P = .03). Taken together, these observations suggest that fungal infection may potentiate colonization by or growth of P aeruginosa within the nail. Subsequent analyses of fungal cultures, however, showed that the likelihood of fungal growth was significantly higher in specimens testing negative for P aeruginosa than in nail debris coinfected with P aeruginosa and a dermatophyte or nondermatophyte mold (77% vs 33%, respectively; P = .05). These observations suggest that P aeruginosa exerts a growth-inhibitory effect on dermatophytes and nondermatophyte molds within the infected nail.
Foster KW, Thomas L, Warner J, Desmond R, Elewski BE. A Bipartite Interaction Between Pseudomonas aeruginosa and Fungi in Onychomycosis. Arch Dermatol. 2005;141(11):1467-1468. doi:10.1001/archderm.141.11.1467