Mycelium WORLD OF MICROBIOLOGY AND IMMUNOLOGY406•
(linkers) can subsequently be introduced at the staggered cut
site, to alter the sequence of the DNA following its repair.
Cassette mutagenesis can be used to introduce selectable
genes at the specific site in the DNA. Typically, these are drug-
resistance genes. The activity of the insert can then be moni-
tored by the development of resistance in the transformed cell.
In deletion formation, DNA can be cut at more than one
restriction site and the cut regions can be induced to join, elim-
inating the region of intervening DNA. Thus, deletions of
defined length and sequence can be created, generating tailor-
made deletions. With site-directed mutagenesis, DNA of
known sequence that differs from the target sequence of the
original DNA, can be chemically synthesized and introduced
at the target site. The insertion causes the production of a
mutation of pre-determined sequence. Site-directed mutagen-
esis is an especially useful research tool in inducing changes
in the shape of proteins, permitting precise structure-function
relationships to be probed. Localized mutagenesis, also known
as heavy mutagenesis, induces mutations in a small portion of
DNA. In many cases, mutations are identified by the classical
technique of phenotypic identification—looking for an alter-
ation in appearance or behavior of the mutant.
Mutagenesis is exploited in biotechnologyto create
new enzymeswith new specificity. Simple mutations will
likely not have as drastic an effect as the simultaneous alter-
ation of multiple amino acids. The combination of mutations
that produce the desired three-dimensional change, and so
change in enzyme specificity, is difficult to predict. The best
progress is often made by creating all the different mutational
combinations of DNA using different plasmids, and then
using these plasmidsas a mixture to transform Escherichia
colibacteria. The expression of the different proteins can be
monitored and the desired protein resolved and used for fur-
ther manipulations.See alsoCell cycle (eukaryotic), genetic regulation of; Cell
cycle (prokaryotic), genetic regulation of; Chemical mutagen-
esis; Chromosomes, eukaryotic; Chromosomes, prokaryotic;
DNA (Deoxyribonucleic acid); Laboratory techniques in
immunology; Mitochondrial DNA; Mitochondrial inheri-
tance; Molecular biology and molecular geneticsMMyceliumYCELIUM
Mycelium (plural, mycelia) is an extension of the hyphaeof
fungi. A hyphae is a thread-like, branching structure formed by
fungi. As the hyphae grows, it becomes longer and branches
off, forming a mycelium network visually reminiscent of the
branches of tree.
The mycelium is the most important and permanent
part of a fungus. The mycelia network that emanates from a
fungal spore can extend over and into the soil in search of
nutrients. The ends of some mycelia terminate as mushrooms
and toadstools.
Mycelium have been recognized as fungal structures for
a long time. The author Beatrix Potter provided accurate
sketches of mycelium over 100 years ago. At the time herobservations were considered irrelevant and the significance
of mycelium was lost until some years after her work.
The growth of mycelia can be extensive. A form of
honey fungus found in the forests of Michigan, which began
from a single spore and grows mainly underground, now is
estimated to cover 40 acres. The mycelia network is thought to
be over 100 tons in weight and is at least 1,500 years old.
More recently, another species of fungus discovered in
Washington State was found to cover at least 1,500 acres
The initial hyphae produced by a fungus has only one
copy of each of its chromosomes. Thus, it is haploid. The
resulting mycelium will also be haploid. When one haploid
mycelium meets another haploid mycelium of the same
species, the two mycelia can fuse. The fused cells then contain
two nuclei. In contrast to plants and animals, where the nuclei
would fuse, forming a functional nucleuscontaining two
copies of each chromosome (a diploid state), the two nuclei in
the fugal cell remain autonomous and function separate from
one another.
Fusion of the nuclei does occur as a prelude to spore
formation. Several duplications and shuffling of the genetic
material produces four spores, each with a unique genetic
identity.
At any one time, part of a mycelia network may be
actively growing while another region may be dormant, await-
ing more suitable conditions for growth. Mycelium is able to
seek out such suitable conditions by moving towards a partic-
ular food source, such as a root. Also mycelium can change
their texture, for example from a fluffy state to a thin com-
pressed state or to thicker cord-like growths. All these attrib-
utes enable the mycelium to ensure the continued growth of
the fungus.See alsoArmillaria ostoyae; Fungal geneticsMMycobacterial infections, atypicalYCOBACTERIAL INFECTIONS, ATYPICAL
Atypical mycobacteria are species of mycobacteria that are
similar to the mycobacteria that are the cause of tuberculosis.
Like other mycobacteria, they are rod-like in shape and they
are stained for observation by light microscopy using a spe-
cialized staining method called acid-fast staining. The need for
this staining method reflects the unusual cell wall chemistry of
mycobacteria, relative to other bacteria. In contrast to other
mycobacteria, atypical mycobacteria do not cause tuberculo-
sis. Accordingly, the group of bacteria is also described as
nonpneumoniae mycobacteria. This group of bacteria is also
designated as MOTT (mycobacteria other than tuberculosis).
Examples of atypical mycobacteria include Mycobacterium
kansasii, Mycobacterium avium, Mycobacterium intracellu-
lare, Mycobacterium marinum, and Mycobacterium ulcerans.
The atypical mycobacteria are widely present in the
environment. They inhabit fresh and salt water, milk, soil, and
the feces of birds. Other environmental niches, which so far
have not been determined, are possible. The nature of their
habitats suggests that transmission to people via soiled or dirty
hands, and the ingestion of contaminated water or milk wouldwomi_M 5/7/03 7:53 AM Page 406