Mucor brunneogriseus Sarbhoy   
(Mucor plumbeus group)

Kingdom: Fungi
Phylum: Zygomycota
Subphylum: Mucoromycotina
Order: Mucorales

British distribution: Unknown.
World distribution: Unknown.

Yet another spoilage mould resulting from the author's somewhat relaxed attitude towards keeping his refrigerator tidy, and illustrating the advisability of consuming food by the 'eat by' date. In this instance we have somewhat passé bacon and cheese rolls.

The fungus proved to be a Mucor and its mostly globose, minutely ornamented spores should immediately have been diagnostic of Mucor plumbeus, a common, opportunistic mould on all kinds of organic substrata and with no likely aversion to bacon and cheese rolls. However, a characteristic feature of M. plumbeus is the presence of spiny projections from the surface of the columella apex, whereas in the fungus shown here, the columella apex lacks such projections and is at most uneven (see photograph below). It fits the description of Mucor brunneogriseus, described as a distinct species by Sarbhoy (1968).

Sarbhoy based the species on just one collection and it may well be that it falls within the range of variation of M. plumbeus, a view taken in the key work on identification of the Mucorales (Zycha, et al., 1969) (under the name, M. spinosus). However, since the collection shown here also had brown tinges to the mycelium and spores, rather small sporangia, and frequent giant, irregular spores - all given as particular features of M. brunneogriseus by Sarbhoy - I am following his concept of it as a separate species. It is probably widespread and, like M. plumbeus, an opportunistic, generalist saprotroph.

M. brunneogriseus, sporangiophores and sporangia:
view under the medium power of the microscope (original photograph at × 125). The mycelium is coenocytic (generally aseptate), though septa are formed where sporangiophores branch. In this species, sporangia are formed predominantly on short lateral branches, as well as terminally on the sporangiophore.
sporangium: structure containing asexual spores (plural: sporangia)
sporangiospore: asexual spore produced within a sporangium
sporangiophore: aerial hypha on which the sporangia are formed
columella: the colourless 'core' of the sporangium (plural: columellae).
Sporangium: view under high power (original photograph at × 500, Nomarski DIC at blue setting). The sporangial membrane has dissolved in the mounting medium (10% ammonia) and spores are beginning to break away, exposing the columella.
In this collection, sporangia were 80-100(-110) µm in diameter.
Columellae: two columellae are shown here (original photograph at × 500) after the spores have been released. The remnants of the sporangial membranes are visible as collars around the base of each columella.
As noted above, the apices of the columellae lack the spine-like projections that would be expected in the closely related M. plumbeus.
Spores (sporangiospores): original photograph at × 500. Most are spherical, but a few of the irregular, giant spores are also shown.
Description of spores in this collection:- spores 7-10(-14) µm diameter, mostly spherical, but some larger and ellipsoid to phaseoliform or ellipsoid-triangular. Surface very finely echinulate.
Regrettably, slight spore movement during exposure means that the definition in this photograph is insufficient to show the fine ornamentation.

Mucor is a well known genus, with in excess of 600 different published names, though the true number of accepted species is around 50 or 60. They are mostly generalist saprotrophs in soil or on decaying fungi and other organic matter, and are frequent food-spoilage moulds. They exploit sugars and other simple decomposition products. A number are thermophilic.

Because of their simple structure and ease of growth in culture, they are commonly used as experimental organisms, notably in the cases of M. mucedo and M. hiemalis. They give their name to 'mucormycoses' - serious, deep-seated fungal infections in humans and other animals (typically when the immune system is damaged or suppressed), but such infections are, in the main, caused by species in related genera, e.g. Absidia corymbifera.

Most Mucor species, presumably including M. brunneogriseus, are heterothallic, i.e. opposite mating types are required for sexual reproduction - fusion of hyphal outgrowths (gametangia) to form a zygospore. Since the material photographed here represents (presumably) a single strain, only asexual reproduction - production of sporangiospores within sporangia - is shown.

Photographs: Paisley, Scotland, 1999.

•   Sarbhoy, A.K., (1968). Revision of the Sphaerosporus group of Mucor. Trans. Brit. mycol. Soc. 51: 25-32.
•   Zycha, H., Siepmann, R., & Linnemann, G., (1969). Mucorales, eine Beschreibung aller Gattungen und Arten dieser Pilzgruppe. J.Cramer, Lehre.

© A.J. Silverside
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