Download Lecture 10 Bacterial morphology – structure and arrangement of

Lecture 10
Bacterial morphology – structure and arrangement of cells
Learning objectives:
This lecture gives you basic understanding of how miniature the bacterial cell is in
comparison with other life and molecules that are even smaller than a virus. You are
also expected to know different shapes of bacteria with their arrangements and an
outline structure with its various components present in a bacteria. This lecture will
be a foundation for future classes to know all about bacteria.
Size, shape and arrangement
Size: Bacteria are very small, most being
approximately 0.5 to 1.0 m diameter.
Smallest bacteria are Mycoplasmas, as
small as 0.2 micrometers (almost as small
as largest poxviruses!) Accepted wisdom
is
that bacteria are smaller than eukaryotes.
But certain cyanobacteria are quite large;
Oscillatoria cells are 7 micrometers
diameter, size of red blood cells. And
certain eukarotes (e.g. Nanochlorum
eukaryotum) are very small, only 1 to 2
micrometers,
but
true
eukaryotes
(nucleus, chloroplast, mitochondrion are
present). So size difference is, like many
generalizations, only a useful yardstick,
not an absolute truth. Epulopiscium
fishelsoni, discovered in 1985 in intestinal
tract of sturgeonfish, is an enormous,
cigar-shaped cell, as large as 80 x 600
micrometers (that's 0.6 mm, large enough
to be seen by the naked eye). Amazingly,
this cell is prokaryotic! Initial evidence by EM was hard to believe, but confirmed
rRNA comparisons with other organisms, a cousin of Gram-positive Clostridium
genus. Later, largest cells of the colorless sulfur bacteria, Thiomargarita namibiensis,
with a diameter of 750 m was discovered from the
Namibian coast is considered as BIG bacteria.
Shape: The shape of the bacterium is governed by
rigid cell wall. Typical bacterial cells are spherical
(Cocci and Coccus) or straight rod shaped (Bacilli
Bacillus) or helically curved rods (Spirilli and
Spirillum). Few bacteria can exhibit verity of
shapes that are referred as Pleomarphic. (Ex.
Arthrobacter).
its
and
Cocci appear in several characteristic arrangements, depending on the plane of
cellular division and whether the daughter cells stay together or not.
Spherical is called coccus.
Division along the same plane forms chains; 2 cocci together – Diplococcus
4 - 20 in chains - Streptococcus.
Division along 2 different planes – Tetrads or Tetracoccus
Division along 3 planes regularly – Sarcinae
Division along 3 planes irregularly – Staphylococcus
Bacilli are not arranged in patterns as complex as those of cocci and mostly occur in
single or in pairs. The following are some arrangements of rod shaped bacteria.
Most bacilli appear as single rods.
Diplobacilli appear in pairs after division.
Streptobacilli appear in chains after division.
Some bacilli are so short and fat that they look
like cocci and are referred to as coccobacilli.
Palisades - Rods side by side arrangement like
match sticks or in X, V or Y figures. Ex.
Corynebacterium diphtheria
Spiral bacteria
have one or
more twists
called
Spirillium.
(Azospirillum).
Spirals with less
than one complete twist are called as Vibrioid
shapes (Ex. Vibrio).
.
If the organism is flexible and undulating, it is called Spirochete.
Other shapes:
Stella are star-shaped. Haloarcula, a genus of halophilic archaea, are
rectangular.
Filamentous shaped – Streptomyces sp. form long, multinucleated, branched
filaments called as hyphae
Actinomycetes - hyphae
Pear shaped – Ex. Pasteuria
Lobed spheres - Ex. Sulfolobus
Rod with square end – Ex. Bacillus anthracis
Bacteria - Morphology
Plasmid
Mesosome
Bacteria are unicellular organisms of relatively simple construction, especially
if compared to eukaryotes. Whereas eukaryotic cells have a preponderance of
organelles with separate cellular functions. Prokaryotes carry out all cellular
functions as individual units.
A prokaryotic cell has five essential structural components: a genome (DNA),
ribosomes, cell membrane, cell wall and a surface layer. Other than enzymatic
reactions, all the cellular reactions incidental to life can be traced back to the
activities of these macromolecular structural components. Thus, functional aspects of
prokaryotic cells are related directly to the structure and organization of the
macromolecules in their cell make-up, i.e., DNA, RNA, phospholipids, proteins and
polysaccharides. Diversity within the primary structure of these molecules accounts
for the diversity that exists among bacteria.