Analysis of phylogenetic relationship of Cylindrocarpon lichenicola and Acremonium falciforme to the Fusarium solani species complex and a review of similarities in the spectrum of opportunistic infections caused by these fungi

Abstract

An emerging pattern of similarity in medical case reports led to a project to compare the phylogenetic affinities of two well-known tropical fungal opportunistic pathogens, Cylindrocarpon lichenicola and Acremonium falciforme, to members of the Fusarium solani species complex. C. lichenicola and A. falciforme, despite their deviating conidial morphologies, were shown via sequencing of the ribosomal large subunit to be well instituted within a clade mainly consisting of typical F. solani strains and other species until recently considered variants of F. solani. The original name Fusarium lichenicola C. B. Massalongo is reestablished, and the new combination F. falciforme is made. Recognition of these species as fusaria is necessary for correct interpretation of current and future molecular diagnostic tests. Reevaluation of species morphology in light of the molecular findings showed that certain features, especially elongate filiform conidiophores with integrated terminal phialides, facilitate correct microscopic classification of these atypical Fusarium species. There is a strong and underrecognized overlap in the spectra of cases caused by members of the F. solani clade, particularly ocular infections, mycetomas, and, in the neutropenic host, disseminated and other serious systemic infections. A novel synthesis of case reports shows that patients from areas with warm climates may develop a distinctive fusarial intertrigo caused by F. solani, Fusarium lichenicola, or Fusarium oxysporum.

FIG. 1.

Phylogenetic relationships within the genus Fusarium showing the positions of the members of the Fusarium solani clade and its substituents Acremonium falciforme and Cylindrocarpon lichenicola in relation to those of other pathogenic and nonpathogenic species. One of 60 most parsimonious trees derived from parsimony analysis of positions 116 to 615 of the 28S rDNA is shown. See the text for methods, scores, and other details. Abbreviations for genus names: Alb., Albonectria; Acr., Acremonium; Cal., Calonectria; Ccl., Cylindrocladium; Cos., Cosmospora; Cyl., Cylindrocarpon; F., Fusarium; H., Haematonectria; L., Leuconectria; Nec., Nectria; Nen., Neonectria; Neo., Neocosmospora; P., Plectosphaerella; V., Verticillium.

FIG. 2.

A typical elongate, microconidium-producing, septate conidiophore of F. solani. Septa, which are in slightly different planes of focus, are marked with arrows. Magnification, ×1,400.

FIG. 3.

Macroconidia of Cylindrocarpon lichenicola produced from an elongate phialidic conidiophore (not fully shown). Magnification, ×1,400.

FIG. 4.

Conidiophores of A. falciforme. The conidiophore at the center has two septa (arrows); comparable structures in typical Acremonium species in the family Bionectriaceae (e.g., Acremonium kiliense, Acremonium strictum, Acremonium alternatum) would have no septum or, less commonly, one septum (10). Magnification, ×1,400.