Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Colony and colony formation

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Rhinovirus has been successful in causing colds for

such as long time because of the large number of antigenic

types of the virus that exist. Producing a vaccineagainst the

virus would require the inclusion of hundreds of antibodies to

the hundreds of different possible antigens. This is not practi-

cal to achieve. Furthermore, not all the Rhinovirus antigens

that are important in generating a cold are exposed at the sur-

face. So, even if a corresponding antibodywere present, neu-

tralization of the antigenvia the binding of the antibody with

the antigen would not occur. Another factor against vaccine

development is the difficulty in being able to grow Rhinovirus

in the laboratory.

Another virus that causes colds are members of the

Coronavirus family. The name of the virus derives from the

distinctive flexible shape and appearance of the virus particle.

Surface projections give the virus a crown-like, or corona,

appearance. There are more than 30 known strains of

Coronavirus. Of these, three or four from the genus

Coronaviruscan infect humans. Cattle, pigs, rodents, cats,

dogs, and birds are also hosts. Members of the genus Torovirus

can also cause gastroenteritis.

Coronavirus has been known since 1937, when it was

isolated from chickens. It was suspected of being a cause of

colds, but this could not be proven until the 1960s, when tech-

niques to grow the virus in laboratory cultures were devised.

Like Rhinovirus, Coronavirus also contains RNA. However,

in contrast to the same amount of genetic material carried in

Rhinoviruses, the genome of the Coronavirus is the largest of

all the RNA-containing viruses.

Other viruses account for 10–15% of colds in adults.

These adenoviruses, coxsackieviruses, echoviruses, orthomyx-

oviruses (including the influenzaA and B viruses), paramyx-

oviruses, respiratory syncytial virus and enteroviruses can also

cause other, more severe illnesses.

Aside from vaccines, various “home remedies” to the

common cold exist. Larger than normal doses of Vitamin C

have been claimed to lessen the symptoms or prevent the

common cold. The evidence for this claim is still not defini-

tive. Another remedy, mythologized as an example of a

mother’s care for her children, is chicken soup. Studies have

demonstrated that chicken soup may indeed shorten the

length of a cold and relieve some of the symptoms. The active

ingredient(s), if any, that are responsible are not known, how-

ever. For now, the best treatment for a cold is to attempt to

relieve the symptoms via such home remedies and over the

counter medications. Nasal decongestants decrease the secre-

tions from the nose and help relieve congestion. Anti-

histamines act to depress the histamineallergic response of

the immune system. This has been claimed to help relieve

cold symptoms. Analgesics relieve some of pain and fever

associated with a cold.

Some so-called alternative medications may have some

benefit. For example, lozenges composed of zinc can some-

times reduce the duration of the common cold, perhaps due to

the need for zinc by the immune system. Echinacea is known

to stimulate white blood cell activity.

See alsoVirology

COLIFORM BACTERIA•seeESCHERICHIA COLI

(E. COLI)

CColony and colony formationOLONY AND COLONY FORMATION

A colony is population of a single type of microorganism that

is growing on a solid or semi-solid surface. Bacteria, yeast,

fungi, and molds are capable of forming colonies. Indeed,

when a surface is available, these microbes prefer the colonial

mode of growth rather than remaining in solution.

On a colonized solid surface, such as the various growth

media used to culture microorganisms, each colony arises

from a single microorganism. The cell that initially adheres to

the surface divides to form a daughter cell. Both cells subse-

quently undergo another round of growth and division. This

cycle is continually repeated. After sufficient time, the result is

millions of cells piled up in close association with each other.

This pile, now large enough to be easily visible to the unaided

eye, represents a colony.

The appearance of a colony is governed by the charac-

teristic of the organism that is the building block of that

colony. For example, if a bacterium produces a color (the

organism is described as being pigmented), then the colony

can appear colored. Colonies can be smooth and glistening,

rough and dry looking, have a smooth border or a border

that resembles an undulating coastline, and can have fila-

mentous appearance extensions sticking up into the air above

the colony.

The visual appearance of a colony belies the biochemi-

cal complexities of the population within. For example, in a

bacterial colony, the organisms buried in the colony and those

near the more aged center of the colony are not as robustly

growing as those bacteria at the periphery of the colony.

Indeed, researchers have shown that the various phases of

growth found when bacteria grow in a liquid growth medium

in a flask (fast-growing and slower-growing bacteria, dying

bacteria, and newly forming bacteria) all occur simultaneously

in various regions of a colony. Put another way, within

colonies, cells will have different phenotypes (structure) and

genotypes (expression of genes).

Variations of phenotypeand genotypehave been ele-

gantly demonstrated using a variant of Escherichia colithat

differentially expresses a genefor the metabolismof a sugar

called lactose depending on the growth rate of the bacteria.

Growth on a specialized medium produces a blue color in

those cells were the gene is active. Colonies of the variant will

have blue-colored sectors and colorless sectors, corresponding

to populations of bacteria that are either expressing the lac-

tose-metabolizing gene or where the gene is silent.

The nature of the solid surface also affects the formation

of a colony. For example, nutrients can diffuse deeper into a

semi-solid growth medium than in a very stiff medium.

Colonies of Bacillus subtilisbacteria tend to form more wavy,

fern-like edges to their colonies in the semi-solid medium.

This is because of uneven distribution of nutrients. Those bac-

teria in a relatively nutrient-rich zone will be able to grow

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