Introduction to Fungi, Third Edition

EXAMPLES OF MUCORALES
181
Fig 7.13 Azygospore of Mucor azygospora.Originalimage
kindly provided byT.W.K.Young.
many species being widespread in soil or
on substrates in contact with soil. Most species
are mesophilic (growing at 10 40°C with an
optimum 20 35°C), but some, e.g. M. miehei or
M. pusillus (sometimes classified as species of
Rhizomucor; see Mouchacca, 1997, 2000) are
thermophilic, with a minimum growth temperature
of about 20°C and a maximum extending up
to 60°C (Cooney & Emerson, 1964; Maheshwari
et al., 2000). Mucor indicus and M. circinelloides are
used as starters in food processing to break down
starchy polysaccharides in rice, cassava and
sorghum, releasing simple sugars for the
preparation of fermented foods or alcohol
production (Hesseltine, 1991).
Most species of Mucor grow rapidly on agar
at room temperature, filling a Petri dish in
2 3 days with their coarse aerial mycelium.
When incubated in liquid culture under semianaerobic
conditions, several species grow in
a yeast-like state. The ability to switch between
the yeast-like and filamentous state is termed
dimorphism, a phenomenon which has been
studied in greatest detail in M. rouxii (see
Fig. 7.14), but also occurs in M. circinelloides,
M. fragilis, M. hiemalis, M. lusitanicus and in other
Mucorales (Orlowski, 1991, 1995). Sporangia are
globose and borne on branched and unbranched
sporangiophores growing into the air. The
columella is large and typically elongated
(Figs. 7.2 and 7.3). Zygospores are rarely formed
in agar culture because most species are heterothallic.
Amongst the most common species from
soil are M. hiemalis, M. racemosus and M. spinosus
(Domsch et al., 1980). Several species of Mucor,
e.g. M. mucedo, fruit on dung (Ellis & Ellis, 1998;
Richardson & Watling, 1997), and they are the
earliest fungi to appear in the succession of
fungal fruit bodies on this substrate (Dix &
Webster, 1995). The sporangiospores of coprophilous
Mucor spp. survive digestion by herbivorous
mammals.
A few species of Mucor are human pathogens.
The term mucormycosis, however, usually refers
to conditions caused by Mucorales generally
rather than the genus Mucor (Rinaldi, 1989;
Eucker et al., 2001) because it is not possible to
identify species by the microscopic appearance
of their coenocytic mycelium within diseased
tissue. Diagnosis is dependent on the isolation
and identification of the suspected pathogen in
culture, sometimes post mortem. By these means,
several ubiquitous species of Mucor have been
associated with disease symptoms, including
M. circinelloides, M. hiemalis and M. racemosus.
Infections are opportunistic, derived from sporangiospores
present in the soil or air, and are
usually associated with patients suffering from
other diseases such as diabetes, leukaemia,
AIDS and post-operative conditions. There are
no records of person-to-person transmission.
Mucormycoses are serious, even fatal in immunocompromised
patients, although some can be
successfully treated by surgery and antibiotics
such as amphotericin B (Kwon-Chung & Bennett,
1992).
Schipper (1978) has given a key to 49 species
of Mucor, and Watanabe (1994) has described the
six homothallic and two azygosporic species.