Members of the Fusarium solani species complex that cause infections in both humans and plants are common in the environment

Abstract

Members of the Fusarium solani species complex (FSSC) are increasingly implicated as the causative agents of human mycoses, particularly in the expanding immunocompromised and immunosuppressed patient populations. Best known as ubiquitous plant pathogens and saprotrophs, the FSSC comprises over 45 phylogenetically distinct species distributed among three major clades. To identify which species are associated with human infections, we generated multilocus haplotypes based on four partial gene sequences from 471 isolates. Of these, 278 were from human patients, 21 were from hospital environments, and 172 were from other sources. Phylogenetic trees inferred from an ergosterol biosynthesis gene (erg-3) were highly discordant with those inferred from the three other partial gene sequences; therefore, this partition was analyzed separately. Multilocus analysis showed that isolates from humans were restricted to but spread throughout clade 3 of the FSSC phylogeny, comprising at least 18 phylogenetically distinct species. The majority (74.5%) of the clinical isolates, however, were associated with four major lineages, designated groups 1 to 4. Groups 1 and 2 were strongly supported as phylogenetic species, whereas groups 3 and 4 were not. Although isolates from ocular infections were found in all four groups, they had a significant tendency to belong to group 3 (P < 0.001). Human clinical isolates shared identical multilocus haplotypes with isolates from plants, other animals, and from hospital environments, suggesting potential nosocomiality. The major finding of this study is that FSSC-associated mycoses of humans and other animals have origins in a broad phylogenetic spectrum, indicating widespread ability to cause infection in this diverse species complex.

FIG. 1.

One of 1,270 most parsimonious (MP) trees showing the distribution of different sources among the 218 three-locus haplotypes together with the four major clinical groups, designated groups 1 to 4. The size of each pie chart is proportional to the number of isolates within each of the four groups. Mosaics indicate that an identical multilocus haplotype was shared by isolates from more than one source. Branches with black squares received ≥85% bootstrap support. CI, consistency index; RI, retention index.

FIG. 2.

Histogram showing the tailed distribution of isolates from human, hospital environment, and other sources in each haplotype class defined by number of isolates per multilocus haplotype. The number above 12 of the haplotype classes identifies those classes represented by between 2 and 108 multilocus haplotypes.