Rhinocladiella mackenziei

Rhinocladiella mackenziei was first identified in 1993 as Ramichloridium mackenziei by C.K. Campbell & Al-Hedaithy^[1] when it was identified as the cause of eight cases of human cerebral phaeohyphomycosis.^[6] All eight patients had abscess formation and six of the patients had aspirated pus with branching hyphae.^[7] Campbell and Al-Hedaithy considered different genera for the un-named fungus, including Zasmidium, Leptodontidium, Ramichloridium, and Rhinocladiella.^[7] They elected to place it in the genus Ramichloridium based on morphological similarity.^[7] In previous publications, Naim-Ur-Rahman misidentified the fungus in the genus Cladosporium, while Al-Hedaithy et al (1988) considered it synonymous with Fonsecaea pedrosoi.^[7]

The genus Ramichloridium encompasses asexual species that produce upright, darkly pigmented, apically elongating, zig-zag-shaped conidiophores that produce single-celled conidia.^[8] It was first described in 1937 by Stahel as Ramichloridium musae but the genus was considered invalid as Stahel's publication lacked a Latin diagnosis.^[8] The genus Ramichloridium was re-introduced by de Hoog in 1977 typified by R. apiculatum.^[8]

Rhinocladiella mackenziei was treated in the genus Ramichloridium until Arzanlou and coworkers explored the phylogeny of Ramichloridium and its related genera through partial sequences of the 28S (LSU) rRNA gene and the ITS region.^[8] The fungus was found to cluster in the Chaetothyriales clade along with Rhinocladiella species and was subsequently transferred to the genus Rhinocladiella.^[9] One of the main features that helped distinguish Ramichloridium and Rhinocladiella is the presence of Exophiala-like budding cells in Rhinocladiella, which were also seen in R. mackenziei.^[8] Additionally, the conidiophores of R. mackenziei resemble undifferentiated from the vegetative hyphae in contrast to those in Ramichloridium that are differentiated.^[8]

Rhinocladiella mackenziei is found in the hot and arid climates of temperate and tropical regions.^[5][10] It is considered endemic throughout the Middle East, specifically Saudi Arabia, Kuwait, and Qatar,^[10][5] and infections by this species have been observed in individuals from Afghanistan,^[11] Iran,^[10] and India.^[12] It is known less commonly to inhabit temperate and tropical regions.^[10] The environmental niche of R. mackenziei remains unknown.^[10] Another fungus associated with causing cerebral phaeohyphomycosis; Cladophialophora bantiana, has been isolated in one occasion from sawdust,^[10] which could also be a possible source of R. mackenziei. Due to the lack of knowledge about its environmental niche, it has been difficult to isolate R. mackenziei; and selective techniques such as the use of high temperatures, mouse vectors, alkyl benzenes and mineral oils are required.^[10] Enrichment of growth media with volatile aromatic hydrocarbons improves recovery of this agent,^[10] implying a role for aromatic hydrocarbon degradation in its environmental niche.^[10]

Rhinocladiella mackenziei is a black yeast-like fungus^[10] with holoblastic conidia (conidia that are produced by simple budding) that are broadly oval and more than 2μm wide.^[13] In vitro at 30 °C, R. mackenziei has smooth, pigmented, septate hyphae^[8][13] and narrower, pale brown aerial hyphae.^[8] Its conidiophores are undifferentiated or only slightly differentiated from vegetative hyphae,^[8] and produce brown, smooth-walled, oval conidia.^[13][8] Rhinocladiella mackenziei grows slowly in culture media.^[5] Colonies grown for a week at 30 °C on glucose peptone agar develop a dark grey-brown to black appearance with a black reverse, elevated center, and densely cottony texture.^[8][13] Rhinocladiella mackenziei grows poorly at 25 °C^[6] and is not to produce a sexual state.^[13]