Differentiation of Rhizomucor Species on the Basis of Their Different Sensitivities to Lovastatin

Abstract

The opportunistic pathogens Rhizomucor pusillus and Rhizomucor miehei may be agents of frequently fatal mycotic diseases. In the present study, the susceptibilities of 27 clinical and environmental isolates of R. miehei and R. pusillus to lovastatin under different culturing conditions were investigated. Most of the R. miehei strains grew at lovastatin concentrations as high as 64 to 128 g/ml. In contrast, the inhibitory effect of lovastatin on all of the R. pusillus strains was evident at lovastatin concentrations as low as 1 to 2 g/ml. A simple and reliable method for species-level differentiation, based on the significantly higher sensitivity of R. pusillus to lovastatin than that of R. miehei, was elaborated. According this, on malt extract agar containing 6 g of lovastatin/ml, R. pusillus is not able to produce colonies, while R. miehei will form compact colonies.A growing number of susceptible hosts form a high-risk population for opportunistic fungal infections. Among these, zygomycosis comprises a diverse group of increasingly recognized mycotic diseases caused by some members of the orders Entomophthorales and Mucorales (e.g., Absidia, Mortierella, Rhizomucor, and Rhizopus [5]). In spite of the fact that these mycoses are in most cases limited to patients with a challenged and weakened immune system, they are attracting even more attention because of their frequently fatal outcome. The limited success of the current therapies mainly stems from the rapidly progressive nature of these infections and the low number of antimycotics that can be applied (3,5,10).An expanding body of data is becoming available that demonstrates the opportunistic pathogenic role of the members of the genus Rhizomucor (1,2,4,5,9,11,16). This genus involves two ubiquitous species: R. pusillus and R. miehei. Their thermophilic nature and characteristic morphological features, such as the presence of stolons, rhizoids, and repeatedly branched sporangiophores, clearly distinguish them from most members of the Mucorales (7). At the same time, the identification of Rhizomucor isolates at a species level seems to be problematic, frequently resulting in misidentifications, or the species names remain to be determined.There are several approaches that may be of help in the identification of Rhizomucor species, but all of these methodologies involve unresolved problems and limitations. Determination of the number of nuclei in the sporangiospores (12) or morphological characterization of the zygospores (the diameter is Ͻ50 m and Ͼ50 m for R. miehei and R. pusillus, respectively) (7, 15) has some potential in providing characters for species delimitation, but the results are not clear-cut in most cases. It is also known that homo-and heterothallic Rhizomucor strains do not group according to species: R. miehei involves only homothallic strains, but both hetero-and (rarely) homothallic R. pusillus isolates are known (7). Assimilation differences can also be used to differentiate these organisms: an inability to assimilate sucrose, glycine, ...

Mucormycosis Caused by Unusual Mucormycetes, Non-Rhizopus, -Mucor, and -Lichtheimia Species

“…We found four other cases in which the etiology of the Mucormycetes species involved in mucormycosis resulted from the fact that these cases were epidemiologically related to other proven R. pusillus infection cases during nosocomial outbreaks (83,106,120). Several publications mention the potential for human R. miehei infection (130,198,279,351), but we found only one case of invasive R. miehei infection, in a transplant recipient (244,337); the researchers did not provide a clinical description of this case (337).…”

“…Zygospore production is useful for differentiating R. pusillus from R. miehei, as the former produces heterothallic zygospores (279,360). The higher tolerance of R. miehei to lovastatin can be useful for differentiation of it from other species (198), but its reliability in differentiating Rhizomucor from other genera is unknown (143). DNA sequencing to compare the sequence to those of known type strains in international databases is currently considered the best way to identify and classify Mucormycetes species (13,68,72,78,143,341,342,351).…”

Molecular Identification of Rhizomucor pusillus as a Cause of Sinus-Orbital Zygomycosis in a Patient with Acute Myelogenous Leukemia

“…Zygospore production may be used also to differentiate the two Rhizomucor species associated with human disease, R. pusillus and R. miehei, because the latter has homothallic zygospore production (20). Lukács et al showed that these two species could be reliably differentiated by the higher tolerance of R. miehei to lovastatin (16). Since no other zygomycetes were included in this evaluation, it is unknown whether this method would help to differentiate Rhizomucor from other genera, a problem that has been observed in several instances (13,22).…”

“…Mating studies and specialized phenotypic assays have been used for the identification of Rhizomucor species and uncommon zygomycetes (16,27,29). Weitzman et al showed that mating studies leading to the production of zygospores could be used reliably to confirm the identity of the three Rhizomucor isolates demonstrating atypical growth characteristics, but they also recognized that this method may be limited to reference laboratories due to the need to maintain a library of testing strains (29).…”

Identification of fungal pathogens in a patient with acute myelogenic leukemia using a pathogen detection array technology

“…18 It is a rare disease, often found in the patients with a weakened immune system and can often lead to a fatal outcome. 18 It occurs most often in patients with hematological malignancies, [19][20][21][22][23][24][25][26] and diabetes mellitus. 27 Zygomycosis presents a major clinical challenge, both during diagnosis and patient treatment.…”