What are Bacteria?
Bacteria are prokaryotic microorganisms and are unicellular in nature. Since they
lack a membrane-bound nucleus, they are less complex than other living creatures.
Although only some bacteria can be seen by our naked eyes while the rest are
microscopic in nature, they display a wide range of shapes, sizes, and structures.
The bacteria capsule is a large structure common to many bacteria. It is a polysaccharide layer
that lies outside the cell envelope, and is thus deemed part of the outer envelope of a bacterial
cell. It is a well-organized layer, not easily washed off, and it can be the cause of various
diseases.
It is important to note that not all bacteria have a cell wall. Having said that though, it is also
important to note that most bacteria (about 90%) have a cell wall and they typically have one
of two types: a gram positive cell wall or a gram negative cell wall.
A flagellum is a hairlike appendage that protrudes from certain plant and animal sperm cells,
and from a wide range of microorganisms to provide motility. Many protists with flagella are
termed as flagellates. A microorganism may have from one to many flagella.
Pili or fimbriae are protein structures that extend from the bacterial cell envelope for a
distance up to 2 μm . They function to attach the cells to surfaces. E. coli cells can have
up to 300 of these organelles. The pili are involved in conjugation. This is the transfer of
genetic material between cells, and pili have a hollow core for this purpose. The conjugation
pilus, also called sex pilus or F pilus, has receptors to recognize recipient cells to receive the
donor's genetic material.

The plasma membrane, or the cell membrane, provides protection for a cell. It also
provides a fixed environment inside the cell. And that membrane has several different
functions. One is to transport nutrients into the cell and also to transport toxic substances out
of the cell.
The cytoplasm of bacterial cells is where the functions for cell growth, metabolism, and
replication are carried out. It is a gel-like matrix composed of water, enzymes, nutrients,
wastes, and gases and contains cell structures such as ribosomes, a chromosome, and
plasmids.
Ribosomes are large ribozymes that synthesize all cellular proteins. As protein synthesis is
rate-limiting for bacterial growth and ribosomes can comprise a large portion of the cellular
mass, elucidation of ribosomal turnover is important to the understanding of cellular
physiology.
Plasmid is small in size, circular in shape and it is a piece of DNA that is not the same as
chromosomal DNA. Its ability to replicate is independent of chromosomal DNA. They are
usually found in bacteria, but they are also present in multicellular organisms.
Nucleoid is a region within a prokaryotic cell that contains most or all of the genetic material.
This material is called a prokaryotic chromosome. DNA is found in both the nucleus and
nucleoid. And hence, the functions of the nucleus and nucleotide are almost the same.
What are the Characteristics of Bacteria?
Some of the basic characteristics of bacteria are as follows:
I. Bacteria are microorganisms with only one cell and without a nucleus or
membrane-bound organelles.
II. A plasmid is an additional circular of genetic material found in some bacteria.
III. They do not possess internal organisation and their DNA is often sequestered
into a region of the bacterial cell known as the nucleoid.
IV. Bacteria have a cell wall that is often made of peptidoglycan, which is a complex
molecule consisting of sugars and amino acids.
V. Bacteria may be found in all types of environments on Earth, including soil, rock,
seas, and even polar snow.
VI. The binary fission method is used by bacteria to reproduce. The bacteria, which
is a single cell, splits into two identical daughter cells throughout this procedure.
VII. These bacteria can be pathogenic or non-pathogenic in nature.
What is the Morphology of Bacteria?
i. A bacteria’s greatest distinguishing characteristic is its morphology or shape. It’s a
certain species’ distinguishing feature.
ii. The shape of a bacterium cell is determined by its morphology, which also
determines the pathogenicity of the bacteria.
iii. The adaptation and evolution of bacteria are influenced by their morphological
characteristics.
iv. The peptidoglycan (murein) in the bacterial cell wall and is a polymer of sugars
that alternates NN-acetylglucosamine (NAG) connected to NN-acetylmuramic acid
(NAM) and amino acids in a peptide chain.

v. The varying morphology and form of bacteria are due to changes in the polymer’s
structure and thickness.
Different Sizes of Bacteria
i. Bacteria are about a tenth of the size of eukaryotic cells. Bacteria range in size
from 0.50.5 to 5μm5�m on average.
ii. They can, however, be as little as 0.3m0.3m and as big as 0.7mm.0.7mm.
iii. The unaided eye’s resolution limit is around 200200 microns, and because many
bacteria are smaller than this, they are invisible to the human eye.
iv. Thiomargarita namibiensis, which may grow to be half a millimetre long,
and Epulopiscium fishelsoni, which can grow to be 0.750.75 millimetres long, are
among the biggest bacteria.
v. Members of the genus Mycoplasma are the smallest bacteria, measuring
about 0.3μm0.3�m and being as small as the smallest viruses.
vi. The diameter of common bacteria such as Escherichia coli ranges
from 1.11.1 to 1.5μm.
Different Shapes of Bacteria
i. The majority of bacteria have a rigid cell wall that gives them a defined form while
also protecting their internal components.
ii. Even while the majority of bacteria have this characteristic, they vary in shape,
allowing them to be categorised into distinct categories depending on their
morphologies.
iii. The bacterial cell wall and cytoskeleton are responsible for a broad range of
forms.
iv. The common categories of bacteria based on their shapes are as follows:
A. Cocci
a. The bacteria that are oval or spherical in shape are known as cocci bacteria.
b. These can be solitary or connected to one another in a group. When grouped
together, they seem flattened.
c. Coccoid forms are thought to have evolved from rod-shaped creatures throughout
evolutionary time.
Arrangements of Cocci
a. Cocci bacteria can be organised individually, in pairs, in four-cell groups, in
chains, in clusters, or in eight-cell cubes.
b. During cell division, these cells stay together. The bacteria’s form is altered by the
plane of cell division.
c. Cocci have a gram-positive cell wall with a thick peptidoglycan layer or a gram-
negative cell wall with a thin peptidoglycan layer.
d. According to the arrangement of cells, the cocci are again divided into the
following subtypes:
1. Monococcus – It is a bacterial species that consists of a single cell.

2. Diplococcus
i. When two bacterial cells form a pair, this configuration occurs (joined together).
ii. Some cells in this arrangement may be spherical, while others may be flattened,
elongated, or bean-shaped.
iii. Examples: Streptococcus pneumoniae.
3. Streptococcus
i. In this type, the bacteria are organised in long chains here.
ii. These bacteria belong to the Streptococcaceae family, which is characterised by
Gram-positive bacteria and a lack of motility.
iii. Streptococcus pneumonia, Streptococcus pyogenes, and Streptococcus mutans are
some examples.
4. Tetrads
i. Tetrad bacteria are organised in a group of four cells that remain connected
during cell division and development in the attachment.
ii. When the cells split into two planes, this pattern occurs.
iii. Aerococcus, Pediococcus, and Tetragenococcus are some examples.
5. Staphylococcus
i. Bacteria organised in grape-like clusters make up this kind of arrangement.
ii. This is caused by cell division in both planes and is characterised by immotile and
Gram-positive organisms.
iii. Staphylococcus haemolyticus, Staphylococcus capitis, Staphylococcus aureus, and
other bacteria are examples.
6. Sarcinae
i. The bacterial cells form an eight-cell cluster in this configuration.
ii. This happens when the cells divide in a perpendicular plane.
iii. The fact that these organisms are strictly anaerobic in nature.
iv. Sarcina lutea, Sarcina aurantiaca, and Sarcina ventriculi are other examples.

B. Bacilli (Rod-shaped)
These are rod-shaped cells that, like cocci, can exist alone or in association with
other cells. Bacilli bacteria were among the earliest to emerge, and their form is
considered to be less favourable than that of other bacteria.
Arrangement of Bacilli
1. Bacillus
i. Bacilli are bacteria that are rod-shaped and exist as solitary cells.
ii. These bacteria are facultative anaerobes that may generate endospores
iii.Salmonella enterica subsp., Bacillus cereus and Salmonella choleraesuis are some
examples.
2. Diplobacilli
i. Diplobacilli, like Diplococci, is found in pairs.
ii. The two cells do not divide and develop in an associated configuration after cell
division.
iii. Coxiella burnetii, Klebsiella rhinoscleromatis, and Moraxella bovis are some
examples.
3. Streptobacilli
i. Bacteria in this category are arranged in chains.
ii. This happens when a single chain of cells divides.
iii. Streptobacillus moniliformis, Streptobacillus felis, Streptobacillus Levaditi,
and Streptobacillus hongkongensis are examples of streptobacilli.
4. Coccobacilli
i. As the name indicates, Coccobacilli is similar to both cocci and bacilli.
ii. Because they are smaller, they look stumpy.
iii. Chlamydia trachomatis, Gardnerella vaginalis, and Haemophilus influenzae are
some of the examples.
5. Palisades
i. Palisades are bacilli bacteria that have a picket fence-like shape due to a bend at
the site of division during cell division.
ii. They have the appearance of Chinese characters.
iii. Example – Corynebacterium diphtheria.

C. Spiral
This group of microorganisms includes bacteria that are either helical-
shaped or curved or comma-shaped. The bacterium might have a corkscrew-like
spiral or be slightly bent.
1. Vibrio
i. These are the comma-shaped bacteria that are slightly bent.
ii. Vibrio mytili, Vibrio anguillarum, Vibrio parahaemolyticus, and Vibrio cholera are
some examples.
Fig: Vibrio
2. Spirochetes
i. Spirochetes are spiral bacteria that have a helical shape.
ii. These organisms are flexible and have an axial filament which helps in motility.
These filaments are a key feature that distinguishes spirochetes from other bacteria.
iii. These filaments travel the length of the bacterium, aiding in the twisting of the
bacteria’s motility.
iv. Examples include Leptospira interrogans, Treponema pallidum, etc.
Fig: Spirochetes

3. Spirilla (Helical-shaped/Corkscrew form)
i. Spirochetes have a similar structure to these bacteria, although they are more
rigid.
ii. They, too, have a flagellum, but unlike spirochetes, they lack the endoflagella.
iii. Helicobacter pylori, Campylobacter jejuni, and Spirillum winogradskyi are some of
the examples.
Fig: Spirillum