Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Membrane fluidity

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drugs is crucial to the development of new drugs. Work can be

at the research and development level, in the manufacture of

drugs, in the regulation and licensing of new antimicrobial

agents, and even in the sale of drugs. For example, the sale of

a product can be facilitated by the interaction of the sales asso-

ciate and physician client on an equal footing in terms of

knowledge of antimicrobial therapy or disease processes.

Following the acquisition of a graduate or medical

degree, specialization in a chosen area can involve years of

post-graduate or medical residence. The road to a university

lab or the operating room requires dedication and over a

decade of intensive study.

Careers in medical science and medical microbiology

need not be focused at the patient bedside or at the lab bench.

Increasingly, the medical and infectious disease fields are ben-

efiting from the advice of consultants and those who are able

to direct programs. Medical or microbiological training com-

bined with experience or training in areas such as law or busi-

ness administration present an attractive career combination.

See alsoBioinformatics and computational biology; Food

safety; History of public health; Hygiene; World Health

Organization

MMembrane fluidityEMBRANE FLUIDITY

The membranes of bacteriafunction to give the bacterium its

shape, allow the passage of molecules from the outside in and

from the inside out, and to prevent the internal contents from

leaking out. Gram-negative bacteria have two membranes that

make up their cell wall, whereas Gram-positive bacteria have

a single membrane as a component of their cell wall. Yeasts

and fungihave another specialized nuclear membrane that

compartmentalizes the genetic material of the cell.

For all these functions, the membrane must be fluid. For

example, if the interior of a bacterial membrane was crys-

talline, the movement of molecules across the membrane

would be extremely difficult and the bacterium would not sur-

vive.

Membrane fluidity is assured by the construction of a

typical membrane. This construction can be described by the

fluid mosaic model. The mosaic consists of objects, such as

proteins, which are embedded in a supporting—but mobile—

structure of lipid.

The fluid mosaic model for membrane construction was

proposed in 1972 by S. J. Singer of the University of

California at San Diego and G. L. Nicolson of the Salk

Institute. Since that time, the evidence in support of a fluid

membrane has become irrefutable.

In a fluid membrane, proteins may be exposed on the

inner surface of the membrane, the outer surface, or at both

surfaces. Depending on their association with neighbouring

molecules, the proteins may be held in place or may capable

of a slow drifting movement within the membrane. Some pro-

teins associate together to form pores through which mole-

cules can pass in a regulated fashion (such as by the charge or

size of the molecule).

The fluid nature of the membrane rest with the support-

ing structure of the lipids. Membrane lipids of microorgan-

isms tend to be a type of lipid termed phospholipid. A

phospholipid consists of fatty acid chains that terminate at one

end in a phosphate group. The fatty acid chains are not

charged, and so do not tend to associate with water. In other

words they are hydrophobic. On the other hand, the charged

phosphate head group does tend to associate with water. In

other words they are hydrophilic. The way to reconcile these

chemistry differences in the membrane are to orient the phos-

pholipidswith the water-phobic tails pointing inside and the

water-phyllic heads oriented to the watery external environ-

ment. This creates two so-called leaflets, or a bilayer, of phos-

pholipid. Essentially the membrane is a two dimensional fluid

that is made mostly of phospholipids. The consistency of the

membrane is about that of olive oil.

Regions of the membrane will consist solely of the lipid

bilayer. Molecules that are more hydrophobic will tend to dis-

solve into these regions, and so can move across the mem-

brane passively. Additionally, some of the proteins embedded

in the bilayer will have a transport function, to actively pump

or move molecules across the membrane.

The fluidity of microbial membranes also allows the

constituent proteins to adopt new configurations, as happens

when molecules bind to receptor portions of the protein. These

configurational changes are an important mechanism of sig-

naling other proteins and initiating a response to, for example,

the presence of a food source. For example, a protein that

binds a molecule may rotate, carrying the molecule across the

membrane and releasing the molecule on the other side. In

bacteria, the membrane proteins tend to be located more in one

leaflet of the membrane than the other. This asymmetric

arrangement largely drives the various transport and other

functions that the membrane can perform.

The phospholipids are capable of a drifting movement

laterally on whatever side of the membrane they happen to

be. Measurements of this movement have shown that the

drifting can actually be quite rapid. A flip-flop motion across

to the other side of the membrane is rare. The fluid motion of

the phospholipids increases if the hydrophobic tail portion

contains more double bonds, which cause the tail to be

kinked instead of straight. Such alteration of the phospho-

lipid tails can occur in response to temperature change. For

example if the temperature decreases, a bacterium may alter

the phospholipid chemistry so as to increase the fluidity of

the membrane.

See alsoBacterial membranes and cell wall

MEMBRANE TRANSPORT, EUKARYOTIC•

seeCELL MEMBRANE TRANSPORT

MEMBRANE TRANSPORT, PROKARYOTIC•

seePROKARYOTIC MEMBRANE TRANSPORT

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