Bacteria are microbes with a cell structure simpler than that of many other organisms. Their control centre, containing the genetic information, is contained in a single loop of DNA. Some bacteria have an extra circle of genetic material called a plasmid rather than a nucleus. The plasmid often contains genes that give the bacterium some advantage over other bacteria. For example it may contain a gene that makes the bacterium resistant to a certain antibiotic. Bacteria are classified into five groups according to their basic shapes: spherical (cocci), rod (bacilli), spiral (spirilla), comma (vibrios) or corkscrew (spirochaetes). They can exist as single cells, in pairs, chains or clusters.

1. SUBMITTED BY- DR. SAMAKSHI VERMA
ASSISTANT PROFESSOR, DEPARTMENT OF
BIOTECHNOLOGY
FACULTY OF ENGINEERING AND TECHNOLOGY
RAMA UNIVERSITY
GT ROAD, MANDHANA, KANPUR (UTTAR PRADESH)
Bacterial Structure
and Morphology

2. Ultra structure of bacteria
Bacteria is a prokaryotic, unicellular organism. Bacteria are found
everywhere, they are the most abundant living organisms. They can
thrive in extreme conditions like hot springs, snow, deep ocean, where it
is difficult for other organisms to live.

3. Salient features of bacteria are the following:
•Unicellular prokaryotic cell
•Present in different shape, size and arrangement
•The cell lack nucleus and membrane-bound cell organelles
•Bacterial DNA is found in the cytoplasm and not packaged to form
chromatin as in eukaryotic cell
•Bacteria cell is 10 times smaller than the human cell
The diameter of a bacteria cell is ~1µm (10-6 m)
The outer covering of a bacteria cell is the cell wall, which is rigid
and provides structural integrity
The bacteria cell wall is made up of peptidoglycan or murein
Different shapes of bacteria cell are the characteristic feature of a
bacteria species
Bacteria cells may contain external appendages like cilia, flagella,
etc.
Bacteria can be photoautotrophs, chemoautotrophs or parasites

4. Morphology of a Bacteria Cell
Morphology or shape of a bacteria cell is the most distinguished property of
bacteria. It is the characteristic property of a particular species.
Morphology of bacteria cell not only tells the shape but also decides its
pathogencity. Morphological traits of a bacteria cell are an important factor in its
adaptability and evolution. There are many features like motility, mode of
nutrition, that are affected by the shape of bacteria.
The bacterial cell wall is made up of peptidoglycan (murein), which is a polymer of
sugars, alternating N-acetylglucosamine (NAG) linked to N-acetylmuramic acid
(NAM) and amino acids peptide chain. Change in the configuration of the polymer
and its thickness is responsible for the varied morphology and shape of a bacteria
cell.
Size of Bacteria
The size of bacteria can be averaged to 2 micrometres with a diameter of 0.5
micrometres. Bacteria are known to display a wide variety of size and shapes.
They are about one-tenth the size of a eukaryotic cell.

5. Useful Bacteria
Not all bacteria are harmful to humans. There are some bacteria which are beneficial in
different ways. Listed below are few benefits of bacteria:
1.Convert milk into curd – Lactobacillus or lactic acid bacteria
2.Ferment food products – Streptococcus and Bacillus
3.Help in digestion and improving the body’s immunity system – Actinobacteria,
Bacteroidetes, Firmicutes, Proteobacteria
4.Production of antibiotics, which is used in the treatment and prevention of bacterial
infections – Soil bacteria
Harmful Bacteria
•There are bacteria that can cause a multitude of illnesses. They are responsible for
many of the infectious diseases like pneumonia, tuberculosis, diphtheria, syphilis, tooth
decay. Their effects can be rectified by taking antibiotics and prescribed medication.
•However, precaution is much more effective. Most of these disease-causing bacteria
can be eliminated by sterilizing or disinfecting exposed surfaces, instruments, tools and
other utilities. These methods include- application of heat, disinfectants, UV radiations,
pasteurization, boiling, etc.

6. Different shapes of bacteria

7. Different shapes of bacteria are used to categorise bacteria. Different shapes of a bacterial cell are:
1.Spherical- Cocci
2.Rod-shaped- Bacilli
3.Spiral bacteria
4.Comma shaped- Vibrio
1. Spherical- Cocci:

8. Cocci can be single or multiple in a group of 2, 4, 8, etc. Cocci bacteria can be round, oval or
elongated or bean-shaped.
•The cell wall of coccus shaped bacteria may be gram-positive with thick peptidoglycan
layer, or gram-negative with the thin peptidoglycan cell wall
•The cells do not separate after cell division. The plane of cell division changes the shape of
the bacteria
According to the arrangement of cells, cocci bacteria are subdivided into various subtypes:
Monococcus: bacteria exist as a single spherical cell
Diplococcus: Cells are arranged in pairs after cell division. Examples of diplococcus bacteria are:
Gram-negative bacteria- Neisseria spp, Moraxella catarrhalis
Gram-positive bacteria- Streptococcus pneumoniae, Enterococcus spp, etc.
Streptococcus: the cocci are joined in a plane and arranged in a chain pattern. These are non-
motile, aerobic and gram-positive bacteria that cause many diseases. Examples of streptococci
are Streptococcus mutans, Streptococcus pyogenes, Streptococcus bovis, Streptococcus
agalactiae, etc.
Tetrads: Tetrads are arranged in a group of 4 cells. The cell division occurs in two different
planes. Examples of tetrads cocci bacteria are- Micrococcus spp, Pediococcus, etc.
Staphylococcus: Cells are arranged in an irregular cluster, which looks like grapes. This is due to
the division in three planes. Examples of staphylococcus are staphylococcus aureus,
Staphylococcus haemolyticus, etc.
Sarcinae: Sarcinae bacteria are anaerobic gram-positive bacteria. They occur as a group of 8
cells. It is found in the family Clostridiaceae. It is found in the large intestine and skin. Examples
of sarcinae are: Clostridium maximum, Sarcina auranatica, Sarcina ventriculi, Clostridium
maximum, Microcioccus luteus, etc.

9. 2. Rod-shaped- Bacilli:
These are rod shaped bacteria. They also occur singly or as multiple cells attached after the cell
division.
Bacilliform bacteria are found in many different taxonomic groups of bacteria
Bacilliform bacteria can be gram-positive, e.g. Actinomyces, Clostridium, Bacillus or gram-negative
bacteria, e.g. Escherichia, Klebsiella, Salmonella, Streptobacillus, etc.
Bacteria of genus Bacillus are gram-positive, rod shaped bacteria
•They are obligate aerobe or facultative anaerobic bacteria
•They can form endospores and live for years as dormant and resistant to heat, radiation and
disinfectants
•They can survive extreme heat and temp as high as 420 ℃
•They are the most abundant bacteria and found everywhere
•Mostly non-parasitic, free-living species
•Two parasitic and pathogenic bacillus species are Bacillus anthracis causing anthrax and Bacillus
cereus, which causes food poisoning

10. Based on the arrangement of rod shaped cells, bacilli are classified into various types:
1.Bacillus: Single unattached cell, that looks like a rod. Examples are- Bacillus cereus,
Salmonella enterica, etc.
2.Diplobacilli: Two rods are attached to each other and found in pairs after cell division.
Examples are- Moraxella bovis, Klebsiella rhinoscleromatis, etc.
3.Streptobacilli: Due to cell division in one plane, bacilli are arranged in a chain. Genus
Streptobacillus contains gram-negative, aerobic or facultative anaerobic bacteria. Examples
are Streptobacillus moniliformis, Streptobacillus felis, etc.
4.Coccobacilli: These are short compared to other bacilli and oval in shape, they appear like a
coccus. Examples are- Chlamydia trachomatis, Haemophilus influenzae, Gardnerella
vaginalis, etc.
5.Palisades: The bacilli after cell division bend and therefore arranged in a palisade, fence-like
structure. Example: Corynebacterium diptheria
3. Spiral bacteria:
These bacteria are spiral or helical in shape.
Based on the thickness, flexibility and motility of the cell, they are further divided into two types:
Spirillum: These are gram-negative, rigid bacteria having external flagella. Examples
are Spirillum, Campylobacter jejuni, Helicobacter pylori, etc.
Spirochete: These bacteria are spiral, thin and flexible. They have internal periplasmic flagella.
These are pathogenic species that cause various serious diseases. Examples are- Leptospira,
Treponema pallidum, etc.

12. 4. Comma shaped- Vibrio:
These are curved and appear like a comma.
These are mostly gram-negative bacteria
They are known to cause various foodborne diseases
Vibrio sp are facultative anaerobes and have 2 chromosomes which replicate independently
Examples are Vibrio cholerae, Vibrio parahaemolyticus, etc.
Other than the main 4 shapes, the bacterial cell possess various different morphology namely-
Filamentous: have a long filamentous structure. Example: Candidatus savagella
Star-shaped: resemble a star. Example: Stella humosa, Stella vacuolata
Rectangular: these are box or rectangular shaped. Example: halophilic bacteria such as Haloarcula
vallismortis
Pleomorphic: these have ability to change their shape and size due to various external factors like pressure,
environmental stress, etc. Example: Mycoplasma pneumonia, Mycoplasma genitalium
Appendaged: they are also known as budding bacteria. They can be non-motile or motile with flagella.
Example: Hypomicrobium, Rhodomicrobium
Trichome: trichomes are a chain of vegetative cell. This is often covered with slimy sheath as in
cyanobacteria. Example: Thiothrix nivea
Lobed: these have a lobed structure and mostly found in hot and volcanic springs. They are flagellated and
have an irregular shape. They are acidophiles and thermophiles. Example: Sulfolobus acidocaldarius,
Sulfolobus solfataricus
Fusiform: these are spindle-shaped, bulged in the middle and the end is tapered. Example: Fusobacterium
necrophorum
Stalked: a stalk develops at one end of bacteria due to asymmetrical cell division. Example: Caulobacter
crescentus
Sheathed: bacteria cell is covered with sheath. They are found in water. Example: Leptothrix, Clonothrix

14. Classification of Bacteria
Bacteria can be classified into various categories based on their features and characteristics. The
classification of bacteria is mainly based on the following:
•Shape
•Composition of the cell wall
•Mode of respiration
•Mode of nutrition
Classification of bacteria based on Shape
Type of Classification Examples
Bacillus (Rod-shaped) Escherichia coli (E. coli)
Spirilla or spirochete (Spiral) Spirillum volutans
Coccus (Sphere) Streptococcus pneumoniae
Vibrio (Comma-shaped) Vibrio cholerae
Classification of bacteria based on the Composition of the Cell Wall
Type of Classification Examples
Peptidoglycan cell wall Gram-positive bacteria
Lipopolysaccharide cell wall Gram-negative bacteria

15. Classification of bacteria based on the Mode of Nutrition
Type of Classification Examples
Autotrophic Bacteria Cyanobacteria
Heterotrophic Bacteria All disease-causing bacteria
Classification of bacteria based on the Mode of Respiration
Type of Classification Examples
Anaerobic Bacteria Actinomyces
Aerobic Bacteria Mycobacterium