Microbiology and Immunology

Culture WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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toms of cryptosporidiosis developed. The malady is most

severe in immunocompromised people, such as those infected

with HIV(the virus that causes AIDS), or those receiving

chemotherapyfor cancer or after a transplant. For those who

are diabetic, alcoholic, or pregnant, the prolonged diarrhea can

be dangerous.

In another avenue of infection, some of the merozoites

grow bigger inside the host epithelial cell and form two other

types of cells, termed the macrogametocyte and microgameto-

cyte. The macrogametocytes contain macrogametes. When

these combine with the microgametes released from the

microgametocytes, a zygote is formed. An oocyst wall forms

around the zygote and the genetic process of meiosis results in

the creation of four sporozoites inside the oocyst. The oocyst

is released to the environment in the feces and the infectious

cycle is started again.

The cycle from ingestion to the release of new infectious

oocytes in the feces can take about four days. Thereafter, the

production of a new generation of parasitestakes as little as

twelve to fourteen hours. Internally, this rapid division can cre-

ate huge numbers of organisms, which crowd the intestinal

tract. Cryptosporidiosis can spread to secondary sites, like the

duodenum and the large intestine. In people whose immune sys-

tems are not functioning properly, the spread of the organism

can be even more extensive, with parasites being found in the

stomach, biliary tract, pancreatic ducts, and respiratory tract.

Detection of Cryptosporidium in water is complicated

by the lack of a culturemethod and because large volumes of

water (hundreds of gallons) need to be collected and concen-

trated to collect the few oocytes that may be present. Presently,

oocysts are detected using a microscopic method involving the

binding of a specific fluorescent probe to the oocyte wall.

There are many other noninfectious species of Crypto-

sporidium in the environment that react with the probe used in

the test. Furthermore, the test does not distinguish a living

organism from one that is dead. So a positive test result is not

always indicative of the presence of an infectious organism.

Skilled analysts are required to perform the test and so the

accuracy of detection varies widely from lab to lab.

See alsoGiardia and giardiasis; Water quality; Water purifi-

cation

CCultureULTURE

A culture is a single species of microorganism that is isolated

and grown under controlled conditions. The German bacteri-

ologist Robert Kochfirst developed culturing techniques in the

late 1870s. Following Koch’s initial discovery, medical scien-

tists quickly sought to identify other pathogens. Today bacte-

riacultures are used as basic tools in microbiology and

medicine.

The ability to separate bacteria is important because

microorganismsexist as mixed populations. In order to study

individual species, it is necessary to first isolate them. This

isolation can be accomplished by introducing individual bac-

terial cells onto a culture medium containing the necessary

elements microbial growth. The medium also provides condi-

tions favorable for growth of the desired species. These con-

ditions may involve pH, osmotic pressure, atmospheric

oxygen, and moisture content. Culture media may be liquids

(known broths) or solids. Before the culture can be grown, the

media must be sterilized to prevent growth of unwanted

species. This sterilizationprocess is typically done through

exposure to high temperatures. Some tools like the metal loop

used to introduce bacteria to the media, may be sterilized by

exposure to a flame. The media itself may be sterilized by

treatment with steam-generated heat through a process known

as autoclaving.

To grow the culture, a number of the cells of the

microorganism must be introduced to the sterilized media.

This process is known as inoculation and is typically done by

exposing an inoculating loop to the desired strain and then

placing the loop in contact with the sterilized surface. A few of

the cells will be transferred to the growth media and under the

proper conditions, that species will begin to grow and form a

pure colony. Cells in the colony can reproduce as often as

every 20 minutes and under the ideal conditions, this rate of

cell division could result in the production of 500,000 new

Liquid cultures of luminescent bacteria.

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