Microbiology and Immunology

Mold WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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notype(cell death, inability to generate energy, or a silent

mutation that has no phenotypic effect) is dependent on the

number and severity of mutationswithin tissues.

During fertilization, mitochondria within the sperm are

excluded from the zygote, resulting in mitochondria that come

only from the egg. Thus, mitochondrial DNA is inherited

through the maternal lineage exclusively without any recom-

binationof genetic material. Therefore, any trait coded for by

mitochondrial genes will be inherited from mother to all of her

offspring. From an evolutionary standpoint, Mitochondrial

Eve represents a single female ancestor from who our mito-

chondrial genes, not our nuclear genes, were inherited 200,000

years ago. Other women living at that time did not succeed in

passing on their mitochondria because their offspring were

only male. Although the living descendants of those other

females were able to pass on their nuclear genes, only

Mitochondrial Eve succeeded in passing on her mitochondrial

genes to humans alive today.

See alsoMitochondria and cellular energy; Mitochondrial

DNA; Molecular biology and molecular genetics; Molecular

biology, central dogma of

MMold OLD

Mold is the general term given to a coating or discoloration

found on the surface of certain materials; it is produced by the

growth of a fungus. Mold also refers to the causative organ-

ism itself.

A mold is a microfungus (as opposed to the macrofungi,

such as mushrooms and toadstools) that feeds on dead organic

materials. Taxonomically, the molds belong to a group of true

fungiknown as the Ascomycotina. The characteristics of the

Ascomycotina are that their spores, that is their reproductive

propagules (the fungal equivalent of seeds), are produced

inside a structure called an ascus (plural asci). The spores are

usually developed eight per ascus, but there are many asci per

fruiting body (structures used by the fungus to produce and

disperse the spores). A fruiting body of the Ascomycotina is

properly referred to as an ascomata. Another characteristic of

molds is their rapid growth once suitable conditions are

encountered. They can easily produce a patch visible to the

naked eye within one day.

The visible appearance of the mold can be of a soft, vel-

vety pad or cottony mass of fungal tissue. If closely observed,

the mass can be seen to be made up of a dense aggregation of

thread-like mycelia (singular, mycelium) of the fungus. Molds

can be commonly found on dead and decaying organic mate-

rial, including improperly stored food stuffs.

The type of mold can be identified by its color and the

nature of the substrate on which it is growing. One common

example is white bread mold, caused by various species of the

genera Mucorand Rhizobium.Citrus fruits often have quite

distinctive blue and green molds of Penicillium.Because of

the damages this group can cause, they are an economically

important group.

In common with the other fungi, the molds reproduce

by means of microscopic spores. These tiny spores are easily

spread by even weak air currents, and consequently very few

places are free of spores due to the astronomical number of

spores a single ascomata can produce. Once a spore has landed

on a suitable food supply, it requires the correct atmospheric

conditions, i.e., a damp atmosphere, to germinate and grow.

Some molds such as Mucorand its close relatives have

a particularly effective method of a sexual reproduction. A

stalked structure is produced, which is topped by a clear, spher-

ical ball with a black disc, within which the spores are devel-

oped and held. The whole structure is known as a sporangium

(plural, sporangia). Upon maturity, the disc cracks open and

releases the spores, which are spread far and wide by the wind.

Some other molds, such as Pilobolus,fire their spores off like

a gun and they land as a sticky mass up to 3 ft (1 m) away. Most

of these never grow at all, but due to the vast number produced,

up to 100,000 in some cases, this is not a problem for the fun-

gus. As has already been mentioned, these fungi will grow on

organic materials, including organic matter found within soil,

so many types of molds are present in most places.

When sexual reproduction is carried out, each of the

molds require a partner, as they are not capable of self-fertil-

ization. This sexual process is carried out when two different

breeding types grow together, and then swap haploid nuclei

(containing only half the normal number of chromosomes),

which then fuse to produce diploid zygospores (a thick-walled

cell with a full number of chromosomes). These then germi-

nate and grow into new colonies.

The Mucormold, when grown within a closed environ-

ment, has mycelia that are thickly covered with small droplets

of water. These are, in fact, diluted solutions of secondary

metabolites. Some of the products of mold metabolismhave

great importance.

Rhizopusproduces fumaric acid, which can be used in

the production of the drug cortisone. Other molds can produce

alcohol, citric acid, oxalic acid, or a wide range of other chem-

icals. Some molds can cause fatal neural diseases in humans

and other animals.

Moldy bread is nonpoisonous. Nevertheless, approxi-

mately one hundred million loaves of moldy bread are dis-

carded annually in the United States. The molds typically

cause spoilage rather than rendering the bread poisonous.

Some molds growing on food are believed to cause cancer,

particularly of the liver. Another curious effect of mold is

related to old, green wallpaper. In the nineteenth century, wall-

paper of this color was prepared using compounds of arsenic,

and when molds grow on this substrate, they have been known

to release arsenic gas.

The first poison to be isolated from a mold is aflatoxin.

This and other poisonous substances produced by molds and

other fungi are referred to as mycotoxins. Some mycotoxins

are deadly to humans in tiny doses, others will only affect cer-

tain animals. Aflatoxin was first isolated in 1960 in Great

Britain. It was produced by Aspergillus flavusthat had been

growing on peanuts. In that year, aflatoxin had been responsi-

ble for the death of 100,000 turkeys—a massive financial loss

that led to the research that discovered aflatoxin. From the

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