Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Bacterial membranes and cell wall

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Septum formation must be coordinated with other cellu-

lar events, such as genetic replication. As well, the growth of

the cell wall is a coordinated process. The peptidoglycanis the

stress-bearing structure of a bacterium. Therefore, the inser-

tion of new material into the existing peptidoglycan must be

done in such a way that the strength of the peptidoglycan net-

work is maintained. Otherwise, the bacterium bursts.

While proteins important in bacterial growth and divi-

sion have been identified, such as the Min series of proteins

active in septum formation, the nature of their actions still

remains unresolved.

See alsoBacterial membranes and cell wall; Colony and

colony formation

BACTERIAL GROWTH CURVE•seeBACTERIAL

GROWTH AND DIVISION

BBacterial kingdomsACTERIAL KINGDOMS

Bacterial kingdoms are part of the classification scheme that

fits bacteriainto appropriate groupings based on certain crite-

ria. The kingdom is the broadest classification category.

There are two kingdoms of prokaryotes. These are the

bacteria (or eubacteria) and the archaebacteria (or the

Archaea). The members of these two kingdoms appear similar

in shape and appearance, even under the extreme magnifica-

tion of the electron microscope. However, they are very dif-

ferent from each other in a number of molecular and

biochemical aspects. It is these differences that have resulted

in the microorganismsbeing grouped into separate kingdoms.

For example, eubacteria contain the rigid, stress-bearing

network known as the peptidoglycan. The only exceptions are

the bacteria from the genera Mycoplasmaand Chlamydia.

Archaebacteria do not contain peptidoglycan. Instead, they

contain a different structure that is called pseudomurein.

Another major difference in the prokaryotic kingdoms

is in the sequence of a species of ribonucleic acid(RNA)

known as 16S ribosomal (r) RNA. The 16 S rRNA is found in

many prokaryotic and eukaryotic cells. The function it per-

forms is vital to the life of the cell. Hence, the RNA species

has not been altered very much over evolutionary time. The

16s rRNA species of eubacteria and Archaebacteria are very

different. Thus, these microorganisms must have taken differ-

ent evolutionary paths long ago.

Within the eubacterial kingdom are other divisions also

known as kingdoms. These divisions are again determined

based on the differences in the sequences of the 16S rRNA of

the various bacteria. These sequence differences within the

eubacterial kingdom are, however, not as pronounced as the

sequences differences between the eubacteria and Archaebac-

teria kingdoms.

The first eubacterial kingdom is referred to as protobac-

teria. This designation encompasses most of the bacteria that

are Gram-negative. Because a great many bacteria are Gram-

negative, the protobacterial kingdom is extremely diverse in

the shape and the biochemical characteristics of the bacteria.

Examples of protobacteria include the photosynthetic purple

bacteria, Pseudomonas, and bacteria that dwell in the intes-

tinal tract of warm-blooded animals (e.g., Escherichia coli,

Salmonella, and Shigella.

The second eubacterial kingdom is comprised of the

Gram-positive bacteria. This group is also diverse in shape

and chemical character. The kingdom is further split into two

major groups, based on the proportion of the nucleic acid that

is composed of two particular building blocks (guanosine and

cytosine). One group contains those bacteria whose DNAis

relatively low in G and C (e.g., Clostridium, Staphylococcus,

Bacillus, lactic acid bacteria, Mycoplasma). The other group

is made up of bacteria whose DNA is relatively enriched in G

and C (e.g., Actinomyces, Streptomyces, Bifidobacterium. The

latter group contains most of the antibiotic-producing bacteria

that are known.

The various eubacterial kingdoms, and the Archaebac-

terial kingdom, are markedly different in 16S rRNA sequence

from the eukaryotic kingdoms (plants, fungi, animals). Thus,

following the establishment of these life forms, the eukaryotes

began to diverge from the evolutionary paths followed by the

eubacteria and Archaebacteria.

See alsoLife, origin of; Microbial taxonomy

BACTERIAL MEMBRANE TRANSPORT•see

PROKARYOTIC MEMBRANE TRANSPORT

BBacterial membranes and cell wallACTERIAL MEMBRANES AND CELL WALL

Bacteriaare bounded by a cell wall. The cell wall defines the

shape of the microorganism, exerts some control as to what

enters and exits the bacterium, and, in the case of infectious

microorganisms, can participate in the disease process.

Many bacteria can be classified as either Gram-positive

or Gram-negative. The Gram stain is a method that differenti-

ates bacteria based on the structure of their cell wall. Gram-

positive bacteria retain the crystal violet stain that is applied to

the bacteria, and appear purple. In contrast, gram-negative

bacteria do not retain this stain, but are “counterstained” red

by the safranin stain that is applied later. The basis of these dif-

ferent staining behaviors lies in the composition of the cell

walls of each Gram type.

Gram-positive bacteria have a cell wall that consists of

a single membrane and a thick layer of peptidoglycan. Gram-

negative bacteria have a cell wall that is made up of two mem-

branes that sandwich a region known as the periplasmic space

or periplasm. The outermost membrane is designated the outer

membrane and the innermost one is known as the inner mem-

brane. In the periplasm lies a thin peptidoglycan layer, which

is linked with the overlaying outer membrane.

The cell wall of Gram-positive bacteria tends to be 2 to

8 times as thick as the Gram-negative wall.

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