Bacteria
- Definition: Bacteria are unicellular, microscopic, prokaryotic organisms that reproduce mainly by binary fission and lack a true nucleus and membrane-bound organelles.
- They can be free-living or pathogenic (causing diseases in plants, animals, and humans).
General Characteristics of Plant Pathogenic Bacteria
|
Character |
Description |
|
Nature |
Prokaryotic, unicellular |
|
Size |
0.5–5 µm |
|
Cell Wall |
Present, made of peptidoglycan |
|
Nucleus |
Absent (nucleoid present) |
|
Reproduction |
Asexual – binary fission |
|
Movement |
Many are motile (flagella) |
|
Nutrition |
Heterotrophic |
|
Habitat |
Soil, water, plant surfaces |
|
Shape |
Cocci (spherical), Bacilli (rod), Spirilla (spiral) |
Shape and Arrangement of Bacteria
|
Shape |
Example |
Arrangement |
|
Coccus (spherical) |
Streptococcus |
Singly or chains |
|
Bacillus (rod) |
Bacillus subtilis |
Single rods |
|
Spirillum (spiral) |
Spirillum volutans |
Spiral or helical |
|
Comma-shaped |
Vibrio cholerae |
Curved rods |
Flagellar Arrangements (Motility Types)
|
Type |
Description |
Example |
|
Monotrichous |
Single flagellum |
Xanthomonas |
|
Lophotrichous |
Tuft of flagella at one end |
Pseudomonas |
|
Amphitrichous |
One or more flagella at both ends |
Spirillum |
|
Peritrichous |
Flagella all over |
Erwinia |
|
Atrichous |
No flagella (non-motile) |
Agrobacterium |
Reproduction
- By Binary Fission (asexual).
- No sexual reproduction like fungi, but genetic exchange occurs via:
- Conjugation
- Transformation
- Transduction
Detection of Bacteria
- Ooze Test: Milky or yellow ooze from infected tissue (bacterial streaming).
- Gram Staining:
- Gram +ve: Retains violet color (thick peptidoglycan)
→ Clavibacter michiganensis - Gram –ve: Red/pink (thin peptidoglycan)
→ Xanthomonas, Pseudomonas, Erwinia
- Gram +ve: Retains violet color (thick peptidoglycan)
Important Plant Diseases Caused by Bacteria
|
Disease |
Causal Organism |
Host |
|
Citrus canker |
Xanthomonas citri |
Citrus |
|
Black rot |
Xanthomonas campestris pv. campestris |
Crucifers |
|
Bacterial blight |
Xanthomonas oryzae pv. oryzae |
Rice |
|
Fire blight |
Erwinia amylovora |
Apple, pear |
|
Soft rot |
Erwinia carotovora |
Potato, vegetables |
|
Leaf spot |
Pseudomonas syringae |
Beans |
|
Crown gall |
Agrobacterium tumefaciens |
Many dicots |
|
Moko disease |
Ralstonia (Pseudomonas) solanacearum |
Banana |
|
Angular leaf spot |
Xanthomonas campestris pv. malvacearum |
Cotton |
Important Facts & MCQs
|
Question |
Answer |
|
Father of Bacteriology |
Louis Pasteur |
|
First bacterial plant disease discovered |
Fire blight of pear (Erwinia amylovora) |
|
Gram staining developed by |
Christian Gram (1884) |
|
Bacteria reproduce by |
Binary fission |
|
Ooze test used for detecting |
Bacteria |
|
Cell wall composition |
Peptidoglycan |
|
Storage material |
Glycogen |
|
DNA arrangement |
Circular, naked (no histones) |
|
Sensitive to Tetracycline |
Mycoplasma / Phytoplasma |
|
Shape of Xanthomonas |
Rod-shaped |
|
Causes gall formation |
Agrobacterium tumefaciens |
|
Disease suppressed by chlorine |
Downy mildew of Bajra |
|
Mycoplasma vs Bacteria |
Mycoplasma → No cell wall; Bacteria → Have cell wall |
MYCOPLASMA / PHYTOPLASMA
|
Feature |
Mycoplasma / Phytoplasma |
|
Nature |
Cell wall-less bacteria |
|
Sensitive to |
Tetracycline |
|
Shape |
Pleomorphic (variable) |
|
Habitat |
Phloem tissue |
|
Transmission |
Leafhoppers / planthoppers |
|
Examples |
Little leaf of brinjal, Phyllody of sesame, Grassy shoot of sugarcane |
