Microbiology and Immunology

Protists WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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identify proteins that are unique to the infection. Some of

these could become targets for diagnosis, therapy, or for pre-

vention of the infection.

The study of proteins is difficult. The amount of protein

cannot be amplified as easily as can the amount of DNA, mak-

ing the detection of minute amounts of protein challenging.

The structure of proteins must be maintained, which can be

difficult. For example, enzymes, heat, light, or the energy of

mixing can break down some proteins.

With the advent of the so-called DNA chips, the expres-

sion of thousands of genes can be monitored simultaneously.

But DNA is static. It exists and is either expressed or not.

Moreover, the expression of a protein does not necessarily

mean that the protein is active. Also, proteins can be modified

after being produced. Proteins can adopt different shapes,

which can determine different functions and levels of activity

after they have been produced. These functions provide the

structural and operational framework for the life of the bac-

terium. Proteomics represents the next step after gene expres-

sion analysis

Proteomics utilizes various techniques to probe protein

expression and structure. The migration of proteins can

depend on their net charge and on the size of the protein mol-

ecule. When these migrations are in two dimensions, as in 2-

D polyacrylamide gel electrophoresis, thousands of proteins

can be distinguished in a single experiment. A technique called

mass spectrometry analyzes a trait of proteins known as the

mass-to-charge ratio, which essentially enables the sequence

of amino acids comprising the protein to be determined.

Techniques exist that detect modifications after protein manu-

facture, such as the addition of phosphate groups. Analogous

to DNA chips, so-called protein microarrays have been devel-

oped. In these, a solid support holds various molecules (anti-

bodies and receptors, as two examples) that will specifically

bind protein. The binding pattern of proteins to the support can

help determine what proteins are being made and when they

are synthesized.

Proteomics typically operates in tandem with bioinfor-

matics, which is an integration of mathematical, statistical,

and computational methods to unravel biological data. The

vast amount of protein information emerging from a single

experiment would be impossible to analyze by manual com-

putation or analysis. Accordingly, comparison of the data with

other databases and the use of computer modeling programs,

such as those that calculate three-dimensional structures, are

invaluable in proteomics.

The knowledge of protein expression and structure, and

the potential changes in structure and function under different

conditions, could allow the tailoring of treatment strategies.

For example, in the lungs of those afflicted with cystic fibro-

sis, the bacterium Pseudomonas aeuruginosaforms adherent

populations on the surface of the lung tissue. These popula-

tions, which are enclosed in a glycocalyxthat the bacteri pro-

duce, are very resistant to treatments and directly and

indirectly damage the lung tissue to a lethal extent. Presently,

it is known that the bacteria change their genetic expression as

they become more firmly associated with the surface. Through

proteomics, more details of the proteins involved in the initial

approach to the surface and the subsequent, irreversible sur-

face adhesion could be revealed. Once the targets are known,

it is conceivable that they can be blocked. Thus, biofilms

would not form and the bacteria could be more expeditiously

eliminated from the lungs.

See alsoBiotechnology; Molecular biology and molecular

genetics

PProtistsROTISTS

The kingdom Protista is the most diverse of all the five

Eukaryotic kingdoms. There are more than 200,000 known

species of protists with many more yet to be discovered.

Protists can be found in countless colors, sizes, and shapes.

They inhabit just about any area where water is found some or

all of the time. They form the base of ecosystems by making

food, as is the case with photosynthetic protists, or by them-

selves being eaten by larger organisms. They range in size

from microscopic, unicellular organisms to huge seaweeds

that can grow up to 300 ft (100 m) long.

The German zoologist Ernst Haeckel (1834–1919) first

proposed the kingdom Protista in 1866. This early classifica-

tion included any microorganism that was not a plant or an

animal. Biologists did not readily accept this kingdom, and

even after the American botanist Herbert F. Copeland again

tried to establish its use 90 years later, there was not much sup-

port from the scientific community. Around 1960, R. Y.

Stanier and C. B. van Neil(1897–1985) proposed the division

of all organisms into two groups, the prokaryotes and the

eukaryotes. Eukaryotes are organisms that have membrane-

bound organelles in which metabolic processes take place,

while prokaryotes lack these structures. In 1969, Robert

Whittaker proposed the five-kingdom system of classification.

The kingdom Protista was one of the five proposed kingdoms.

At this time, only unicellular eukaryotic organisms were con-

sidered protists. Since then, the kingdom has expanded to

include multicellular organisms, although biologists still dis-

agree about what exactly makes an organism a protist.

Protists are difficult to characterize because of the great

diversity of the kingdom. These organisms vary in body form,

nutrition, and reproduction. They may be unicellular, colonial,

or multicellular. As eukaryotes, protists can have many differ-

ent organelles, including a nucleus, mitochondria, contractile

vacuoles, food vacuoles, eyespots, plastids, pellicles, and fla-

gella. The nuclei of protists contain chromosomes, with DNA

associated with proteins. Protists are also capable of sexual, as

well as asexual reproduction, meiosis, and mitosis. Protists can

be free-living, or they may live symbiotically with another

organism. This symbiosis can be mutualistic, where both part-

ners benefit, or it can be parasitic, where the protist uses its host

as a source of food or shelter while providing no advantage to

the other organism. Many protists are economically important

and beneficial to mankind, while others cause fatal diseases.

Protists make up the majority of the planktonin aquatic sys-

tems, where they serve as the base of the food chain. Many pro-

tists are motile, using structures such as cilia, flagella, or

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