Microbiology and Immunology

Bacterial movement WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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When thin sections of bacteria are viewed in the trans-

mission electron microscope, the membranes appear visually

similar to a railroad track. There are two parallel thickly

stained lines separated by an almost transparent region. The

dark regions are the charged head groups of molecules called

phospholipids. Bacterial phospholipids consist of the charged,

hydrophilic (“water-loving”) head region and an uncharged,

hydrophobic(“water-hating”) tail. The tail is buried within the

membrane and forms most of the electron-transparent region

evident in the electron microscope.

Phospholipids make up the bulk of bacterial mem-

branes. In Gram-positive bacteria and in the inner membrane

of Gram-negative bacteria the phospholipids are arranged

fairly evenly on either “leaflet” of the membrane. In contrast,

the outer membrane of Gram-negative bacteria is asymmetric

with respect to the arrangement of phospholipids. The major-

ity of the phospholipids are located at the inner leaflet of the

membrane. The outer leaflet contains some phospholipid, and

also proteins and a lipid molecule termed lipopolysaccharide.

The asymmetrical arrangement of the Gram-negative

outer membrane confers various functions to the bacterium.

Proteins allow the diffusion of compounds across the outer

membrane, as long as they can fit into the pore that runs through

the center of the protein. In addition, other proteins function to

specifically transport compounds to the inside of the bacterium

in an energy-dependent manner. The lipopolysaccharide com-

ponent of the outer membrane is capable of various chemical

arrangements that can influence the bacterium’s ability to elude

host immune defenses. For example, when free of the bac-

terium, lipopolysaccharide is referred to as endotoxin, and can

be toxic to mammals, including humans.

The presence of the outer membrane makes the existence

of the periplasm possible. The periplasm was once thought to

be just functionless empty space. Now, however, the periplasm

is now known to have very important functions in the survival

and operation of the bacterium. The region acts as a buffer

between the very different chemistries of the external environ-

ment and the interior of the bacterium. As well, specialized

transport proteins and enzymesare located exclusively in this

region. For example, the periplasm contains proteins that func-

tion to sense the environment and help determine the response

of a bacterium to environmental cues, such as occurs in the

directed movement known as chemotaxis.

Not all bacteria have such a cell wall structure. For

example the bacteria known as mycobacteria lack a peptido-

glycan and have different components in the cell membrane.

Specifically, a compound called mycolic acid is present. Other

bacteria called Mycoplasmalack a cell wall. They need to

exist inside a host cell in order to survive.

The synthesis of the cell wall and the insertion of new

cell wall material into the pre-existing wall is a highly coordi-

nated process. Incorporation of the new material must be done

so as not to weaken the existing wall. Otherwise, the bac-

terium would lose the structural support necessary for shape

and survival against the osmotic pressure difference between

the interior and exterior of the bacterium. Wall synthesis and

insertion involves a variety of enzymesthat function in both

the mechanics of the process and as sensors. The latter stimu-

late production of the cell wall as a bacterium readies for divi-

sion into two daughter cells.

See alsoBacterial ultrastructure; Bacterial surface layers

BBacterial movementACTERIAL MOVEMENT

Bacterial movement refers to the self-propelled movement of

bacteria. This movement is also referred to motility. The jig-

gling movement seen in some nonmotile bacteria that are inca-

pable of self-propelled movement is due to the bombardment

of the bacteria by water molecules. This so-called Brownian

motion is not considered to represent bacterial movement.

There are several types of bacteria movement. The most

common occurs by the use of appendages called flagella. A

bacterium can contain a single flagellum, several flagella

located at one or both poles of the cell, or many flagella dis-

persed all over the bacterial surface. Flagella can rotate in a

clockwise or counterclockwise direction. When the motion is

counterclockwise, even multiple flagella can unite into a fla-

gellar bundle that functions as a propeller. This occurs when

the bacterium is moving towards a chemical attractant or away

from a repellent in the behavior known as chemotaxis. If the

flagella turn in the opposite direction, the coordinated motion

of the flagella stops, and a bacterium will “tumble,” or move

in an undirected and random way.

Spirochaete bacteria have flagella that are internal.

These so-called axial filaments provide the rigidity that

enables the spiral bacterium to twist around the axis of the fil-

ament. As a result, the bacterium literally screws itself through

the fluid. Reversal of the twist will send the bacterium in a

reverse direction. Examples of bacteria that move in this man-

ner include Treponema pallidumand Rhodospirillum rubrum.

The bacteria that are known as gliding bacteriaexhibit

another type of bacterial movement. One example of a gliding

bacterium is the cyanobacterium Oscillatoria. Gliding move-

ment is exactly that; a constant gliding of a bacterium over a

surface. The basis of this movement is still not clear, although

it is known to involve a complex of proteins.

In a human host, disease causing bacteria such as

Salmonella typhymuriumcan move along the surface of the

host cells. This movement is due to another bacterial

appendage called a pilus. A bacterium can have numerous pili

on its surface. These hair-like appendages act to bind to sur-

face receptors and, when retracted, pull the bacteria along the

surface. Movement stops when a suitable area of the host cell

surface is reached.

See alsoBacterial appendages

BACTERIAL SHAPES •seeBACTERIAL ULTRASTRUC-

TURE

BACTERIAL SMEARS•seeMICROSCOPE AND

MICROSCOPY

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