Mucor racemosus Fresen. 1850

M. racemosus has a worldwide distribution and is reported frequently all over Europe, and in the Americas from Alaska to Brazil. In the tropics it is found at the higher altitudes. M. racemosus is capable of surviving various environmental conditions, thus having a large scale of habitats and hosts. Though it is mainly discovered in the soil, it has been found elsewhere such as plants residues, grains, vegetables and even in houses. ^【1】Typically, it is often seen on plant-based materials such as soft fruit, fruit juice and marmalade but it has also been isolated from non-plant sources like soft camembert cheese. ^【2】 Recently, it has been stated in a study that M. racemosus can also be isolated from human gutmicrobiomeof non-obese individuals.^【3】 Another interesting thing about M. racemosus is that it is capable of biosynthesizing chitin and chitosan, which has been assumed as the reason why it can switch between the yeast-like form and mold phases. The in vivo differential rates of chitin-plus-chitosan biosynthesis in Mucor racemosus were determined under a variety of conditions, leading to yeast cell or mycelial morphology.^【4】 Filamentous fungi grow by apical extension at the hyphal tip^{【5】【6】}. This property is believed to be the result of the fusion of vesicles containing wall synthesizing enzymes with the plasma membrane at the apex^{【7】【8】【9】}. Bartnicki- Garcia and co-workers^{【10】【11】}have purified chitin synthetase-containing vesicles (chitosomes) from several diverse fungi. A few fungi are dimorphic, i.e., they are capable of growth as unicellular yeast cells or in a filamentous (mycelial) morphology. In dimorphic Mucor spp., yeast cell and mycelial walls are chemically similar^【12】.In the yeast form, wall polymers are laid down uniformly over the entire cell surface^【13】, leading to a spherical morphology. Presumably in yeast cells, chitosome fusion with the plasma membrane is not restricted to specific points as in the mycelial form.

M. racemosus has hyphae without septa and is multi-core and branched-like. It can spread both inside and outside of the substrate. It does not possess rhizoid or stolon and does not produce shaped colonies. Sporangiophores directly germinate from hymenium in the form of monopodial, racemose or false diachotomous branching. There are different shapes of columella inside of sporangia that are located on the tips of each branch. The sporangia produce a large number of sporangiospores that are smooth and have thin walls. Sporangia will release the spores once they become mature. Sexual reproduction can be heterothallic or homothallic. M. racemosus is a dimorph fungi, possessing the ability to form both filamentous and yeast-like morphologies.^【1】 The differences between these two forms are significant and the environment conditions have huge impact on the phases of M. racemosus. When given anaerobic conditions, it may exhibit yeast-like morphology. Anaerobic conditions and 30% carbon dioxide presence stimulate conversion to yeast form. It has also been reported that adding Tween 80,ergosterol when culturing M. racemosus and supplied with 100% nitrogenalso helps it convert to yeast-like form.^【2】Conversely, increasing oxygen concentration may lead to the yeast-like form to the mold form.^【3】 In the laboratory, the fungus forms dark grey or light grey colonies on most common laboratory media.^【4】 Like many other fungi, M. racemosus can produce both sexual and asexual spores depending on environmental conditions. ^【3】During sexual reproduction, hyphae of compatible mating types touch and fuse, ultimately giving rise to a thick-walled zygosporangium containing a single zygospore. Germination from the zygospore leads to growth of new hyphae that give rise to asexual spores of both + and - mating type.Germination of these spores produces new haploid hyphae of the same mating type. ^【3】 It is easily recognizable microscopically by its tall (up to 2 cm) needle-like sporangiophores and large sporangium.

·Human pathogen

Mucoris an uncommon opportunistic pathogen. Usually it will not attack human beings, but sometimes it will attack individuals with significantly compromised immune systems such as children, elderly and diseased patients like those with HIV. M. racemosushas also been associated with causation of extrinsic allergic alveolitis ^【1】. The symptoms, which may occur 6 to 8 hours after exposure, include: elevated temperature, flu-like symptoms, general malaise, and difficulty in breathing followed by asthma later.

·Commercial use

M. racemosus can grow as a yeast, thus it has the ability to form biochemicals which can be used in industry and our daily lives. For instance, it can produce a high yield ofphytase, an important industrial enzyme. Phytase may be produced efficiently by M. racemosus in solid-state fermentation on optimized coconut oil cake at 71% moisture level, pH 5.5, incubation temperature 25 °C. By optimized nutrient supplementation and selecting the most appropriate carbon and nitrogen sources production nearly doubled (from 14.5 IU/g DM to 26 IU/g DM). The quality of carbon and nitrogen sources appears to be critical factors for maximal phytase production^【2】.

Mucor includes at least 39 species, belonging to phylum zygomycota, order Mucorales, Mucoraceae family. M. racemosus was first described and named by Bulliard in 1791^.【1】 It is one of the earliest species people discovered in soil and was isolated long time ago——as early as 1886.^【2】 M. racemosus has several synonyms such as Calyptromyces globosus, Circinomucor sphaerosporus, M.dimorphosporus f. sphaerosporus, M. globosus, M. macrosporus, M. sphaerosporus and so on. This species is closely related to M. mucedo and M.rouxianus, since they all belong to the same genus and share lots of similar characters.