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Mucor hiemalis, a commonly distributed fungi worldwide, is a member under the Mucor genus. This fungi, along with all its other members of this genus are soil-borne. M. hiemalis thrives best underground below 25°C,   causing contamination in plants, especially food that we consume including potatoes, tomatoes, and bananas to name a few. It causes rots by means of producing secondary metabolites like specific enzymes and lipids. While it's pathogenicity has yet to be reported, a rare case of primary cutaneous infection caused by zygomycosis was discovered.

History and taxonomy
While there is very little known about the complete historical origin of M. hiemalis, it is reported that it was first described in 1907 by Wehmer. M. hiemalis is a member of the Mucor genus, under the Zygomycota phylum. Out of the other 50 species under this genus, M. hiemalis is the most variable and the most common.

Closely resembled species
Members within this genus are closely related. More specifically, M. hiemalis highly resembles Mucor circinelloides. They, however, differ at which temperature allows them to grow more rapidly. The former can optimally grow in a wider range temperature from 5°C to 25°C  while the former only grows optimally in a narrow range temperature from 5°C to 10°C. In addition, they also differ at which one species is able to be pathogenic. M. circinelloides is considered to be pathogenic in humans and animals while M. hiemalis does not.

Growth and morphology
M. hiemalis, a heterothallic species is an environmental organism.

Physical properties
Its sporangia are small such that they do not exceed over 80 um in size. They acquire a round, spherical shape and have a yellow to dark brown colour. Their sporangia are held at the terminal end of the sporangiophore. With regards to color appearance, the M. hiemalis sporangiophores are reddish-yellow and the sporangia are yellow. Sporangiophores are initially unbranched then grow to have a few branches that can be up to 15 mm tall and 14 um wide. Their sporangiospores are smooth-walled and are narrowly ellipsoid to cylindrical with dimensions of 5.7-8.7 x 2.7-5.5 um. All spores are uninucleate and sometimes reniform (i.e. acquiring a kidney shape). Oida is present in the substrate hyphae and its columellae are globose. Sometimes, the formation some chlamydospores are found in mycelium but not in abundance, compared to other common species of the Mucor genus, such as Mucor racemosus.

Sporulation on agar medium
Depending on the type of agar M. hiemalis sporulates in, different colours of the colonies are exhibited. When it is grown on malt extract agar (MEA), colonies are greyish-yellow. When it is grown on chapel yeast autolysate (CYA), colonies are grey. When it is grown on potato dextrose agar (PDA), colonies are yellow. Moreover, depending on the time of day, different colours are also exhibited, such that during the day it is a creamy-yellow and during the night, it is more grey. (Smith Despite the different colours of sporulation on different agar environments, all M. hiemalis colonies will spread across the entire Petri dish.

Secondary metabolites
M. hiemalis is known for producing a diverse set of enzymes and lipids. Inducible extracellular lipase is an example of an enzyme this fungi produces. This lipase manifests to food contamination, specifically in cameroonian palm fruit. Under certain conditions, they are even known to produce rhizoids. More research on the production of various secondary metabolites by M. hiemalis and their mechanistic pathways need to be done.

Temperature
As previously mentioned, M. hiemalis is best grown from 5°C - 25°C. Best sporulation also occurs between these said temperatures. Stunted growth is exhibited at 30°C while 37°C is the temperature at which no growth can occur. However, specific isolates that can thrive in higher temperatures may cause human infections. It has been reported that some isolates can actually grow at temperatures well below 0°C.

Habitat and ecology
One of M. hiemalis’ distinguishing features is that it is a soil-borne fungi. Recall that the optimal temperature for M. hiemalis can be up to 25°C, the ideal soil temperature for plant growth. This is probably why it is commonly found in soil-contaminated products. Indoors, it is found on the floor, carpet and mattress dust. M. hiemalis can also be a saprotroph or a parasite from plants and animals, such that they can inhabit dung, decaying vegetation and stored grains.

Food contamination
Specifically, M. hiemalis is known to cause Mucor rots in root vegetables such as carrots and cassava. It also causes rots in tomatoes and guavas. Other food products include fruit, beans, bananas, sugar cane, corn, yogurt. Food product contamination manifests from the enzymes and lipids produced by M. hiemalis. Food product contamination manifests from the enzymes and lipids produced by M. hiemalis.

Human infection
In contrast, it is a rare cause of human infection since it is rarely found in humans and warm-blooded animals. The high bodily temperatures of humans and warm-blooded animals are unfavourable for M. hiemalis to survive in. As a result, there is very scare information regarding its medical significance. However, M. hiemalis may cause zygomycosis (synonymous to mucormycosis), an opportunistic infection. Infections in mucous membranes, nasal sinuses and passages, skin, brain and lungs may occur as a result. People who appear to be most susceptible to zygomycosis are those that suffer from diabetes, immunosuppression symptoms associated with AIDS and those who take drugs intravenously suffering.

Case Study
In a rare finding, M. hiemalis was once isolated from a primary subcutaneous infection. In a case report done from1991, researchers looked at a healthy 10-year old girl who had primary subcutaneous zygomycosis. She first visited the clinic when she noticed an insect bite located on her cheek. A skin biopsy soon after revealed that she had zygomycosis. They then hypothesized that sporangiospores were inoculated into her skin via the insect. Physical characteristics of this patient manifested in erythematous, scaly, infiltrated, sharply demarcated plaque on her right cheek. Her biopsy demonstrated epidermal spongiosis and both a superficial and deep granulomatous dermatitis. The granulomas consisted of lymphocytes, histiocytes and some plasma cells. Treatment with topical and oral griseofulvin was unsuccessful, but the infection resolved following intravenous amphotericin B therapy. This special case of primary subcutaneous infection is the least common form, accounting for less than a tenth of reported cases.

Overall, further exploration regarding this area of pathogenicity research needs to be conducted.