Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Autoimmunity and autoimmune disorders

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Invented by Gerd Binnig and Christoph Gerber in Zurich,

Switzerland, and Calvin Quate (1923– ) in California, the AFM

uses a tiny needle made of diamond, tungsten, or silicon, much

like those used in the STM. While the STM relies upon a sub-

ject’s ability to conduct electricity through its needle, the AFM

scans its subjects by actually lightly touching them with the nee-

dle. Like that of a phonograph record, the AFM’s needle reads

the bumps on the subject’s surface, rising as it hits the peaks and

dipping as it traces the valleys. Of course, the topography read

by the AFM varies by only a few molecules up or down, so a

very sensitive device must be used to detect the needle’s rising

and falling. In the original model, Binnig and Gerber used an

STM to sense these movements. Other AFM’s later used a fine-

tuned laser. The AFM has already been used to study the super-

microscopic structures of living cells and other objects that

could not be viewed with the STM.

American physicist Paul Hansma (1946– ) and his col-

leagues at the University of California, Santa Barbara, conduct

various studies using AFM research. In 1989, this team suc-

ceeded in observing the blood-clotting process within blood

cells. Hansma’s team presented their findings in a 33–minute

movie, assembled from AFM pictures taken every ten seconds.

Other scientists are utilizing the AFM’s ability to remove sam-

ples of cells without harming the cell structure. By adding a bit

more force to the scanning needle, the AFM can scrape cells,

making it the world’s most delicate dissecting tool.

Scientists continue to apply this method to the study of

living cells, particularly fragile structures on the cell surface,

whose fragility makes them nearly impossible to view without

distortion.

See alsoBacterial membranes and cell wall; Bacterial surface

layers; Bacterial ultrastructure; Microscope and microscopy

ATTENUATION•seeVACCINE

AAttractants and repellentsTTRACTANTS AND REPELLENTS

Attractants and repellents are compounds that stimulate the

directed movement of microorganisms, in particular bacteria,

towards or away from the compound. The directed movement

in response to the presence of the attractant or repellent com-

pound is a feature of a bacterial behavior known as chemotaxis.

Various compounds can act as attractants. Overwhelm-

ingly, these are nutrients for the bacterium. Attractant com-

pounds include sugars, such as maltose, ribose, galactose, and

amino acids such as L-aspartate and L-serine.

Similarly, various compounds will cause a bacterium to

move away. Examples of repellents include metals that are

damaging or lethal to a bacterium (e.g., cobalt, nickel), mem-

brane-disruptive compounds such as indole, and weak acids,

which can damage the integrity of the cell wall.

The presence and influence of attractants and repellents

on the movement of bacteria has been known for over a cen-

tury. In the 1880s experiments demonstrated that bacteria

would move into capillary tubes filled with meat extract and

away from capillaries filled with acids.

Now, the molecular underpinning for this behavior is

better understood. The chemotaxis process has been particu-

larly well-studied in the related Gram-negative bacteria

Escherichia coliand Salmonellatyphimurium.

These bacteria are capable of self-propelled movement,

by virtue of whip-like structures called flagella. Movement

consists typically of a random tumbling interspersed with a

brief spurt of directed movement. During the latter the bac-

terium senses the environment for the presence of attractants

or repellents. If an attractant is sensed, the bacterium will

respond by exhibiting more of the directed movement, and the

movement will over time be in the direction of the attractant.

If the bacterium senses a repellent, then the periods of directed

movement will move the bacterium away from the compound.

Both of these phenomena require mechanisms in the bac-

terium that can sense the presence of the compounds and can

compare the concentrations of the compounds over time.

The detection of attractants and repellents is accom-

plished by proteins that are part of the cytoplasmic, or inner,

membrane of bacteria such as Escherichia coliand Salmonella

typhymurium. For example, there are four proteins that span

the inner membrane, from the side that contacts the cytoplasm

to the side that contacts the periplasmic space. These proteins

are collectively called the methyl-accepting chemotaxis pro-

teins (MCPs). The MCPs can bind different attractant and

repellent compounds to different regions on their surface. For

example, on of the MCPs can bind the attractants aspartate and

maltose and the repellents cobalt and nickel.

The binding of an incoming attractant or repellent mol-

ecule to a MCP causes the addition or removal of a phosphate

group to another molecule that is linked to the MCP on the

cytoplasm side. Both events generate a signal that is transmit-

ted to other bacterial mechanisms by what is known as a cas-

cade. One of the results of the cascade is the control of the

rotation of the flagella, so as to propel the bacterium forward

or to generate the random tumbling motion.

The cascade process is exceedingly complex, with at

least 50 proteins known to be involved. The proteins are also

involved in other sensory events, such as to pH, temperature,

and other environmental stresses.

The memory of a bacterium for the presence of an

attractant or repellent is governed by the reversible nature of

the binding of the compounds to the bacterial sensor proteins.

The binding of an attractant or a repellent is only for a short

time. If the particular compound is abundant in the environ-

ment, another molecule of the attractant or repellent will bind

very soon after the detachment of the first attractant or repel-

lent from the sensor. However, if the concentration of the

attractant or repellent is decreasing, then the period between

when the sensor-binding site becomes unoccupied until the

binding of the next molecule will increase. Thus, the bac-

terium will have a gauge as to whether its movement is carry-

ing the cell towards or away from the detected compound.

Then, depending on whether the compound is desirable or not,

corrections in the movement of the bacterium can be made.

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