Microbiology and Immunology

Indicator species WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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are manipulating living systems, and many of the biological

processes involved take place inside the living egg and sperm.

This procedure is considered an in vitroprocess in order to

distinguish it from the natural fertilization of the egg in the

intact body of the female.

In vivo experimental research became widespread with

the use microorganismsand animal models in genetic manip-

ulation experiments as well as the use of animal models to

study drug toxicity in pharmacology. Geneticists have used

prokaryotic, unicellular eukaryotes like yeast, and whole

organisms like Drosophila, frogs, and mice to study genetics,

molecular biologyand toxicology. The function of genes has

been studied by observing the effects of spontaneous muta-

tionsin whole organisms or by introducing targeted mutations

in cultured cells. The introduction of gene cloning and in vitro

mutagenesis has made it possible to produce specific muta-

tions in whole animals thus considerably facilitating in vivo

research. Mice with extra copies or altered copies of a gene in

their genome can be generated by transgenesis, which is now

a well-established technique. In many cases, the function of a

particular gene can be fully understood only if a mutant ani-

mal that does not express the gene can be obtained. This is

now achieved by gene knock-out technology, which involves

first isolating a gene of interest and then replacing it in vivo

with a defective copy.

Both in vitroand in vivoapproaches are usually com-

bined to obtain detailed information about structure-function

relationships in genes and their protein products, either in cul-

tured cells and test tubes or in the whole organism.

See also Immunogenetics; Immunologic therapies;

Immunological analysis techniques; Laboratory techniques in

immunology; Laboratory techniques in microbiology;

Molecular biology and molecular genetics

IIndicator speciesNDICATOR SPECIES

Indicator organisms are used to monitor water, food or other

samples for the possibility of microbial contamination. The

detection of the designated species is an indication that harm-

ful microbes, which are found in the same environment as the

indicator species, may be present in the sample.

Indicator organisms serve as a beacon of fecal contami-

nation. The most common fecal microorganism that is used

have in the past been designated as fecal coliforms. Now, with

more specific growth media available, testing for Escherichia

colican be done directly. The detection of Escherichia coli

indicates the presence of fecal material from warm-blooded

animals, and so the possible presence of disease producing

bacteria, such as Salmonella, Shigella,and Vibrio.

To be an indicator organism, the bacteria must fulfill

several criteria. The species should always be present in the

sample whenever the bacterial pathogens are present. The

indicator should always be present in greater numbers than the

pathogen. This increases the chances of detecting the indica-

tor. Testing directly for the pathogen, which can be more

expensive and time-consuming, might yield a negative result

if the numbers of the pathogen are low. Thirdly, the indicator

bacterial species should be absent, or present in very low num-

bers, in clean water or other uncontaminated samples. Fourth,

the indicator should not grow more abundantly than the

pathogen in the same environment. Fifth, the indicator should

respond to disinfectionor sterilizationtreatments in the same

manner as the pathogen does. For example, Escherichia coli

responds to water disinfection treatments, such as chlorina-

tion, ozone, and ultra-violet irradiation, with the same sensi-

tivity as does Salmonella.Thus, if the indicator organism is

killed by the water treatment, the likelihood of Salmonella

being killed also is high.

Another indicator bacterial species that is used are of

the fecal Streptococcusgroup. These have been particularly

useful in salt water monitoring, as they persist longer in the

salt water than does Escherichia coli. In addition, the ratio of

fecal coliform bacteria to fecal streptococcican provide an

indication of whether the fecal contamination is from a human

or another warm-blooded animal.

The use of indicator bacteria has long been of funda-

mental importance in the monitoring of drinking water.

Similar indicator organisms will be needed to monitor water

against the emerging protozoan threats of giardiaand cryp-

tosporidium.

See alsoAntibiotic resistance, tests for; Water quality

INDUSTRIAL MICROBIOLOGY•seeECONOMIC

USES AND BENEFITS OF MICROORGANISMS

IInfection and resistanceNFECTION AND RESISTANCE

Infection describes the process whereby harmful microorgan-

ismsenter the body, multiply, and cause disease. Normally the

defense mechanisms of the body’s immune systemkeep infec-

tious microorganisms from becoming established. Those

organisms, however, that can evade or diffuse the immune sys-

tem and therapeutic strategies (e.g., the application of antibi-

otics) are able to increase their population numbers faster than

they can be killed. The population increase usually results in

host illness.

There are a variety of ways by which harmful microor-

ganisms can be acquired. Blood contaminated with microbes,

such as the viral agents of hepatitisand acquired immunodefi-

ciencysyndrome, is one source. Infected food or water is

another source that causes illness and death to millions of peo-

ple around the world every year. A prominent example is the

food and water-borne transmission of harmful strains of

Escherichia colibacteria. Harmful microbes can enter the

body through close contact with infected creatures.

Transmission of the rabiesvirus by an infected raccoon bite

and of encephalitis virus via mosquitoes are but two examples.

Finally, breathing contaminated air can cause illness. Bacterial

spores of the causative bacterial agent of anthraxare readily

aerosolized and inhaled into the lungs, where, if sufficient in

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