Taxonomy and epidemiology of Mucor irregularis, agent of chronic cutaneous mucormycosis

Abstract

Mucormycosis usually presents as a progressive infection with significant angio-invasion. Mucormycosis due to Mucor irregularis (formerly Rhizomucor variabilis var. variabilis), however, is exceptional in causing chronic cutaneous infection in immunocompetent humans, ultimately leading to severe morbidity if left untreated. More than 90 % of the cases known to date were reported from Asia, mainly from China. The nearest neighbour of M. irregularis is the saprobic species M. hiemalis. The aim of this study was to evaluate the taxonomic position, epidemiology, and intra- and inter-species diversity of M. irregularis based on 21 strains (clinical n = 17) by multilocus analysis using ITS, LSU, RPB1 and RPB2 genes, compared to results of cluster analysis with amplified fragment length polymorphism (AFLP) data. By combining MLST and AFLP analyses, M. irregularis was found to be monophyletic with high bootstrap support, and consisted of five subgroups, which were not concordant in all partitions. It was thus confirmed that M. irregularis is a single species at 96.1-100 % ITS similarity and low recombination rates between populations. Some geographic structuring was noted with some localised populations, which may be explained by limited air-dispersal. The natural habitat of the species is likely to be in soil and decomposing plant material.

Keywords: Mucor hiemalis; Mucor irregularis; Mucormycosis; biodiversity; chronic cutaneous infection; epidemiology; taxonomy.

Fig. 1.

Phylogeny of Mucor irregularis obtained from a Neighbour-Joining analysis of Internal Transcribed Spacer (ITS) using Mega 5 software. Bootstrap support values were estimated based on 500 replicates, and are shown by the numbers near the branches (Neighbour-Joining bootstrap values / Bayesian posterior probabilities, boldface branch for values ≥ 80). Strains forming well-supported subgroups are marked in colour. Three Mucor species were taken as outgroup. Species names are given only for ex-type strains. Geographic origins of isolates are given with strain numbers. T = ex-type strain; NT = ex-neotype strain; = strains from clinical samples; = isolates from soil; = isolates from plants; = isolates from animals or animal excretion.

Fig. 2.

Neighbour-Joining phylograms of 21 isolates of Mucor irregularis based on four different markers: a. LSU; b. RPB1; c. RPB2; d. multilocus, combining sequences of LSU, ITS, RPB1 and RPB2 by Mega 5 software. Branches with bootstrap values, estimated based on 500 replicates in the Neighbour-Joining analysis, are printed in bold when 70 or higher. Branch support values are indicated by the numbers near the branches (Neighbour-Joining bootstrap values / Bayesian posterior probabilities). Group colours used in four phylograms per strain are the same as in the ITS phylogram. CBS 243.35 and CBS 201.65 were taken as outgroup. T = ex-type strain; NT = ex-neotype strain.

Fig. 3.

Dendrogram of amplified fragment length polymorphism (AFLP) profiles of 21 Mucor irregularis isolates. Mucor hiemalis (CBS 201.65) and Mucor luteus (CBS 243.35) are taken as outgroup. The scale bar on the top indicates the percentage similarity. Strain colours: blue = ITS subgroup 1; green = ITS subgroup 2; yellow = ITS subgroup 3; red = ITS subgroup 4; ochre = ITS subgroup 5.

Fig. 4.

Macroscopic and microscopic morphology of Mucor irregularis. a. Colony on MEA after 4 days incubation at 30 °C; b. colony from backside; c, d, f–k. columella and sporangiospores; e. sporangia. — Scale bar = 10 μm.