Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Spirochetes

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occurs between states of the same spin, and phosphorescence if

the transition takes place between states of different spin.

In x-ray fluorescence, the term refers to the characteris-

tic x rays emitted as a result of absorption of x rays of higher

frequency. In electron fluorescence, the emission of electro-

magnetic radiation occurs as a consequence of the absorption

of energy from radiation (either electromagnetic or particu-

late), provided the emission continues only as long as the stim-

ulus producing it is maintained.

The effects governing x-ray photoelectron spectroscopy

were first explained by Albert Einstein in 1905, who showed

that the energy of an electron ejected in photoemission was

equal to the difference between the photon and the binding

energy of the electron in the target. In the 1950s, researchers

began measuring binding energies of core electrons by x-ray

photoemission. The discovery that these binding energies

could vary as much as 6 eV, depending on the chemical state

of the atom, led to rapid development of x-ray photoelectron

spectroscopy, also known as Electron Spectroscopy for

Chemical Analysis (ESCA). This technique has provided valu-

able information about chemical effects at surfaces. Unlike

other spectroscopies in which the absorption, emission, or

scattering of radiation is interpreted as a function of energy,

photoelectron spectroscopy measures the kinetic energy of the

electrons(s) ejected by x-ray radiation.

Mössbauer spectroscopy was invented in the late 1950s

by Rudolf Mössbauer, who discovered that when solids emit

and absorb gamma rays, the nuclear energy levels can be sep-

arated to one part in 10^14 , which is sufficient to reflect the

weak interaction of the nucleus with surrounding electrons.

The Mössbauer effect probes the binding, charge distribution

and symmetry, and magnetic ordering around an atom in a

solid matrix. An example of the Mössbauer effect involves the

Fe-57 nuclei (the absorber) in a sample to be studied. From the

ground state, the Fe-57 nuclei can be promoted to their first

excited state by absorbing a 14.4-keV gamma-ray photon pro-

duced by a radioactive parent, in this case Co-57. The excited

Fe-57 nucleus then decays to the ground state via electron or

gamma ray emission. Classically, one would expect the Fe-57

nuclei to undergo recoil when emitting or absorbing a gamma-

ray photon (somewhat like what a person leaping from a boat

to a dock observes when his boat recoils into the lake); but

according to quantum mechanics, there is also a reasonable

possibility that there will be no recoil (as if the boat were

embedded in ice when the leap occurred).

When electromagnetic radiation passes through matter,

most of the radiation continues along its original path, but a

tiny amount is scattered in other directions. Light that is scat-

tered without a change in energy is called Rayleigh scattering;

light that is scattered in transparent solids with a transfer of

energy to the solid is called Brillouin scattering. Light scatter-

ing accompanied by vibrations in molecules or in the optical

region in solids is called Raman scattering.

In vibrational spectroscopy, also known as Raman spec-

troscopy, the light scattered from a gas, liquid, or solid is

accompanied by a shift in wavelength from that of the incident

radiation. The effect was discovered by the Indian physicist C.

V. Raman in 1928. The Raman effect arises from the inelastic

scattering of radiation in the visible region by molecules.

Raman spectroscopy is similar to infrared spectroscopy in its

ability to provide detailed information about molecular struc-

tures. Before the 1940s, Raman spectroscopy was the method

of choice in molecular structure determinations, but since that

time infrared measurements have largely supplemented it.

Infrared absorption requires that a vibration change the dipole

moment of a molecule, but Raman spectroscopy is associated

with the change in polarizability that accompanies a vibration.

As a consequence, Raman spectroscopy provides information

about molecular vibrations that is particularly well suited to

the structural analysis of covalently bonded molecules, and to

a lesser extent, of ionic crystals. Raman spectroscopy is also

particularly useful in studying the structure of polyatomic

molecules. By comparing spectra of a large number of com-

pounds, chemists have been able to identify characteristic fre-

quencies of molecular groups, e.g., methyl, carbonyl, and

hydroxyl groups.

See alsoBiotechnology; Electron microscope, transmission

and scanning; Electron microscopic examination of microor-

ganisms; Electrophoresis; Enzyme-linked immunosorbant

assay (ELISA); Epidemiology, tracking diseases with technol-

ogy; Fluorescence in situ hybridization (FISH); Laboratory

techniques in immunology; Laboratory techniques in microbi-

ology; Microscope and microscopy

SPHEROPLASTS•seePROTOPLASTS AND SPHERO-

PLASTS

SPINAE• seeBACTERIAL APPENDAGES

SSpirochetesPIROCHETES

Spirochetes are a group comprised of six genera of bacteriain

a family known as Spirochaete. They are named because of

their spiral shape. Typically, spirochetes are very slender.

Their length can vary from about five microns (millionths of

an inch) to several hundred microns, depending on the species.

Under the light or electron microscope, the tight coiling that is

characteristic of spirochetes is readily visible. Spirochetes are

a significant health threat to humans. Both syphilisand Lyme

diseaseare caused by spirochetes. Beneficially, spirochetes

contribute to digestion in ruminants such as cows.

Besides their shape, another distinctive aspect of spiro-

chetes in the presence of what is essentially internal flagella.

These structures, called axial filaments, are embedded in the

cell wall of the bacterium. They are constructed very similarly

as flagella, having the characteristic arrangement of structures

that anchors the filament to the cell membrane. There can be

only a few to as many as 200 axial filaments present in a given

bacterium. The rigidity of an axial filament allows a bacterium

to move in a corkscrew type of motion. Axial filaments are

present in all spirochetes except Treponema.

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