PLANT PATHOGENIC BACTERIA
DR. RAJBIR SINGH
Department of Plant Pathology
Gochar Mahavidyalaya (Post Graduate College)
Rampur Maiharan, Saharanpur (UP), India
Affiliated to: CCS University, Meerut (UP), India
Note: All images have been taken form internet. I am grateful to all author of images.
History of Bacteria
• Anton van Leeuwenhoek (1632-1723) – ‘Father of Microbiology’. In 1676 observed bacteria
and Protozoa under microscope.
• Hooke (1820)- Under compound microscope seen bacteria and called “Small Microscopic
• Ehrenberg (1828)- He gave the term ‘Bacterium’.
• Louis Pasteur (1822-1895)- ‘Father of Bacteriology’. He gave terms: Sterlization, Fermentation,
Pasteurization, Immunization. He developed Rabies Vaccine & established Pasteur Institute.
• Koch’s Postulates (1843-1910)- ‘Father of Bacteriological Techniques’. In 1882 gave postulates.
• T. J. Burril (1878-82)- Reported first time that a plant disease ‘Fire Blight of Pear & Peach’ is
caused by Bcateria.
• Joseph Lister (1827-1912)- Give “Antiseptic and Aseptic Theory”.
• Winogradsky (1890)- ‘Father of Soil Bacteriology’. He describe NO2 & NO3 functioning.
Description of different parts of bacterial cell
(1). Cell Envelope
• It is outer covering & has 3 components— glycocalyx, cell wall
and cell membrane.
(i). Glycocalyx (Mucilage Sheath):
• Outermost mucilage layer & consists of non-cellulosic
polysaccharides with or without proteins.
• It may occur in the form of loose sheath then it is called ‘Slime
layer’. If thick and tough, the mucilage covering is called
• (a) Prevention of desiccation,
• (b) Protection from phagocytes, toxic chemicals and drugs & viruses,
• (e) Attachment,
• (f) Immunogenicity and Virulence.
(2). Cell Wall
• It is rigid solid covering , provides shape and structural support.
• In Gram + ve is 8-12 nm & in gram – ve 20-80 nm thick.
• It consists of lipopolysaccharides, lipids and proteins.
• Inner wall layer of Gram -ve is made up of pepidoglycan, proteins, non-
cellulosic carbohydrates, lipids, amino acids, etc.
• Peptidoglycan known as murein or mucopeptide. Peptidoglycan consists
of long glycan strands formed of repeating units of N-acetyl glucosamine
(NAG) and N-acetyl muranic acid (NAM). They are cross linked by small
• Peptidoglycan constitutes 70-80% of wall in Gram +ve bacteria. Lipid
content is little. 10-20% of wall in Gram -ve bacteria is formed of
peptidoglycan. Lipid content is 20-30%.
(3). Plasma Membrane
• It is selectively permeable covering of the cytoplasm.
• Plasma membrane or plasma lemma has a structure
similar to that of a typical membrane.
• It is made of a phospholipid bilayer with proteins of
• It holds receptor molecules for detection and
responding to different chemicals of the surroundings
• It is metabolically active as it takes part in respiration,
synthesis of lipids and cell wall components.
• Flagella are filamentous protein structures
attached to the cell surface.
• It provides the swimming movement.
Movement - 50 NM or 0.001/Second.
• Size is about 20 nm (0.02 µm) in diameter and
1-7µm in length.
• Made of 3 parts— basal body, hook and
• It is made up of protein called flagellin.
ii. Pili and Fimbriae
• longer, fewer and thicker tubular outgrowths which develop
in response to F+ or fertility factor in Gram +ve bacteria.
• Made up of protein pilin.
• Helpful in attaching to recipient cell and forming conjugation
tube. So called Sex Pili.
• Diameter is 3-10 nm while length is 0.5-1.5 µm.
• Some fimbriae cause agglutination of RBC. They also help in
mutual clinging of bacteria.
• It is crystallo-colloidal complex excluding its nucleoid.
• Cytoplasm is granular due to presence of a large number of ribosomes.
Various structures present in cytoplasm are as follows:
• It is a characteristic circular to villi form specialisation of cell membrane of
bacteria that develops as an in growth from the plasma membrane
• It takes part in replication of nucleoid by providing points of attachment to
the replicated ones.
• At the time of cell division, plasma membrane grows in the region wher
that most probably it provides membranes for rapid elongation.
• It contains respiratory enzymes and is, therefore, often called chondrioid.
• They are small membrane less, sub-microscopic ribo-
nucleoprotein entities having a size of 20 nm x 14-15
nm. Fixed ribosomes are attached to the plasma
• Each ribosome has two subunits, larger 50S and
• Ribosomes take part in protein synthesis. Free or
matrix ribosomes synthesize proteins for intracellular
use while fixed ribosomes synthesize proteins for
transport to outside.
• Ribosomes generally occur in helical groups called
polyribosomes or polysomes.
• It represents the genetic material of
• Nucleoid consists of a single circular strand of
DNA duplex which is supercoiled with the help
of RNA and polyamines to form a nearly oval or
• The folding is 250-700 times.
• Polyamines or nucleoid proteins are different
from histone proteins.
• DNA of prokaryotes is considered naked
because of its non-association with histone pro-
teins and absence of nuclear envelope around it.
• They are self-replicating, extra chromosomal
segments of double stranded, circular, naked
DNA. Plasmids provide unique phenotypic
characters to bacteria. They are independent
of main nucleoid.
• Some of them contain important genes like
fertility factor, nif genes, resistance factors and
colicinogenic factors. Plasmids which can get
associated temporarily with nucleoid are
known as episomes.
• They are internal membrane systems of
photosynthetic forms which possess
photosynthetic pigments. In purple bacteria
the membranes are typical while in green
bacteria they are non-unit, non-lipid and
proteinaceous. Chromatophores of green
algae are called chromosomes. Photosynthetic
pigments are bacteriochlorophyll,
bacteriophaeophytin (bacterioviridin) and
(iv). Inclusion Bodies:
• The inclusion bodies may occur freely inside
the cytoplasm or covered by 2-4 nm thick non-
lipids, non-unit protein membrane.
Types of Inclusion Bodies: 3 types base on nature
1. Gas vacuoles
2. Inorganic inclusions
3. Food reserve
Morphology of Bacteria
Morphology of bacteria include- size, shape, grouping
or aggregation of cells, flagellation and ultra structure
• Size of bacteria-
Generally diameter of bacteria is – 0.35 – 0.5 µm
Length is – 1-5 µm
• Shape of bacteria- three type of shapes:
1. Spherical bacteria
2. Straight rod shaped bacteria
3. Bent or curved shaped bacteria
1. Spherical bacteria
Spherical bacteria also known as Coccus pl. cocci. These bacteria are
oval, ellipsoidal shape but some may be pear shaped, bean shaped.
There diameter is about 0.2 – 4 µm. Base on their aggregation they
are of 6 types:
• Monococcus: A bacteria that lives as one cell. Exp. Micrococcus bicolor.
• Diplococcus: is a cocci that is found in pairs. Exp. Diplococcus pneumoniae.
• Streptococcus: the bacteria form long chains. Exp. Streptococcus lactis.
• Tetrad: A group of four cells forming a flat square. Exp. Micrococcus roseus.
• Sarcina: is a cube-like group of eight cocci. Exp. Sarcina lutea.
• Staphylococcus: bacteria form an irregular, grape-like cluster. Exp.
2. Straight Rod Shaped Bacteria
Rod shaped bacteria also known as Bacillus pl. bacilli. These bacteria are straight and
cylindrical like a rod with ends being flat rounded or cigar shaped. On the base of
aggregation they are following 4 types:
(1). Microbacillus: In this a rod shaped bacterium divide into two cells
and each divided bacterium live separate. Exp. Microbacterium.
(2). Diplobacillus: In this a rod shaped bacterium divide into two cells
and both divided bacterium attached to each other. Exp.
(3). Streptobacillus: This a rod shaped bacterium divide and make a
chain of divided bacteria. Exp. Streptothrix.
(4). Pailsade: In this rod shaped bacteria live in group and look like pole .
Exp. Corynebacterium diptheriae
3. Bent or Curved Rod Shaped Bacteria
These are two types:
1. Vibrio: These bacteria are comma ( , ) shaped. Exp.
2. Spiral or Helix or Spirillum: Spirllum is made of Greek
word Spira which meaning is Coil. These bacteria are
rigid spiral forms. Exp. Spirillum, Campylobacter.
Flagellation in Bacteria
The various forms of flagellation are as follows:
(a) Atrichous: Flagella absent.
(b) Monotrichous: A single flagellum occurs sat or near one
end of bacterium.
(c) Amphitrichous: A flagellum at each of the two ends.
(d) Lophotrichous: A group or tuft of flagella is found only
at one end.
(e) Cephalotrichous: A tuft or group of flagella occurs at
each of the two ends or poles.
(f) Peritrichous: A number of flagella are distributed all
over the surface.
Classification of Plant Pathogenic Bacteria
• According to David H. Bergey’s Manual of Determinative
Bacteriology (Last vol. published in 1994)
• Bergey divided plant pathogenic bacteria in 4 divisions and 7
Division- I- Gracilicutes (gram – ve bacteria)
Class- I- Scotobacteria
Class- II- Anoxyphytobacteria
Class- III- Oxyphytobacteria
Division –II- Firmicutes (Gram +ve bacteria)
Class –I- Firmibacteria
Class –II- Thallobacteria
Division –III- Tenricutes (Bacteria- lacking cell wall)
Class- I- Mollicutes
Division – IV – Mendosicutes (Bacteria with abnormal
Class – I- Archobacteria
Asexual Reproduction in Bacteria
Vegetative or Asexual reproduction in bacteria
is by following methods:
1. BY Binary fission
2. By Endospores
3. By Cysts
4. By Fragmentation
5. By Arthospores
6. By Conidia
1. Binary Fission: Most bacteria rely on binary
fission for propagation. In this a cell just needs to grow to
twice its starting size and then split in two.
2. Endospore: Spores are formed during unfavorable
environmental conditions. As the spores are formed within the
cell, they are called endospores. Only one spore is formed in a
bacterial cell. On germination, it gives rise to a bacterial cell.
3. Cysts: Cysts are formed by the deposition of
additional layer around the mother wall. These are the
resting structure and during favorable conditions they
again behave as the mother. Exp. Azotobacter.
4. Fragmentation: In this, body of a bacterium
break in several parts or fragments and each such
individual fragment develop into a bacterium.