Less-Frequent Fusarium Species of Clinical Interest: Correlation between Morphological and Molecular Identification and Antifungal Susceptibility

Forty-eight Fusarium isolates morphologically identified as belonging to seven species of clinical interest (i.e., Fusarium chlamydosporum, Fusarium dimerum, Fusarium incarnatum, Fusarium napiforme, Fusarium nygamai, Fusarium proliferatum, and Fusarium sacchari) were characterized molecularly by the analysis of the sequences of the TUB region of the β-tubulin gene. F. chlamydosporum and F. dimerum were the most genetically heterogeneous species. A high degree of correlation between the morphological and molecular identification was shown among the isolates studied. A table with the key morphological features for the identification of these Fusarium species is provided. The antifungal susceptibilities of the Fusarium isolates to 11 antifungal drugs were tested; terbinafine was the most active drug against all the species tested with the exception of F. incarnatum, for which amphotericin B was the most active.The most frequent species causing fusariosis are Fusarium solani, Fusarium oxysporum, and Fusarium verticillioides (1, 16, 47), but several other species are also found to cause human infections, although less frequently. Some of these species are Fusarium chlamydosporum, Fusarium dimerum, Fusarium incarnatum, and the following other species that are included into the Gibberella fujikuroi species complex: Fusarium napiforme, Fusarium nygamai, Fusarium proliferatum, and Fusarium sacchari (30, 31). These species have been associated with different types of infection, in particular with keratomycoses and other ocular infections (10) and with disseminated infections in immunocompromised patients (2, 6, 17, 20, 23, 24, 26, 39, 41, 43, 44). The real incidence of these species is unknown since they are poorly known and laboratorians and clinical microbiologists are not generally aware of their possible presence in human infections.Since the species of Fusarium are generally resistant to all the available antifungal drugs (40), it could be considered that speciation of Fusarium is necessary only for epidemiological purposes. However, some in vitro data concerning particular species seem to be very promising and deserve to be investigated clinically. For instance, F. verticillioides isolates were susceptible to posaconazole and terbinafine and Fusarium thapsinum isolates to terbinafine (4). The identification of fusaria to the species level is not easy, and in numerous clinical cases the etiological agent is reported as being a Fusarium sp. However, several recent studies have demonstrated the usefulness of molecular methods for the identification of those Fusarium species that are difficult to distinguish morphologically (1, 4, 47). In recent years, the in vitro antifungal susceptibilities of the most frequent species of Fusarium have been evaluated (1, 3, 4, 40, 47), but only a few isolates of the less-common species have been studied. The objectives of our study were (i) to evaluate the correlation between the morphological and the molecular identification of less-frequent Fusarium species isolates received by our laboratory and (ii) to determine the antifungal susceptibilities of isolates representative of those less-common Fusarium species of clinical interest identified molecularly.