Fundamentals of Plant Pathology

GENERAL CHARACTERISTICS

Bacteria are second most important organisms which cause plant diseases.

• They are prokaryotic single celled mostly achlorophyllous organisms whose body is surrounded by cell wall and nuclear material is not surrounded by membrane.
• They lack membrane bound organelles such as mitochondria or plastids and also a visible endoplasmic reticulum.
• Most of the bacterial species are saprophytes living on dead organic matter. There are about 200 bacterial species which are plant pathogenic.
• Morphologically the bacteria are rod shaped (bacilli), spherical (cocci), spiral (spirilli), coma shaped (vibrios) or thread like (filamentous).
• Streptomyces has a filamentous branched hypha-like structure, sometimes mistakenly called as ray fungi; and mycoplasma have no definite shape due to lack of cell wall.
• In young cultures the rod shaped bacteria range from 0.6 to 3.5 µm in length and from 0.5 to 1µm in diameter (0.6-3.5 x 0.5-1 µm size).
• Single bacterium mostly appears as hyaline or yellowish white under the compound microscope, when grown on a medium, soon a colony is formed.
• The colonies of most of bacteria have a whitish or greyish appearance but some of them develop yellow, red or other colours.

Bacterial Cell Structure

• A bacterium has a thin, relatively tough, rigid cell wall, and a distinct three layered but thin cytoplasmic membrane.
• Most bacteria have a slime layer made up of viscous gummy material. Slime layer has bacterial immunological property.
• When the layer is thick and firm, it is called capsule.
• Generally plant pathogenic bacteria lack capsule but some of them like Pseudomonas and Xanthomonas produce slime.
• Slime layer is mostly composed of polysaccharides but may rarely contain amino sugars, sugar acids, etc.

Flagella

• Most of the plant pathogenic bacteria have delicate thread like flagella, which are usually longer than the cell
• They are the organs of locomotion.
• The arrangement of flagella on bacterial cell is an important taxonomic character that aid in bacterial classification.
• This arrangement may be
• Monotrichous- with one polar flagellum
• Lophotrichous -tuft of flagella at one end
• Amphitrichous- at both the ends
• Peritrichous - distributed all around the cell or surface.
• Atrichous- bacteria lacking flagella.

Gram Staining
Bacterial species are often distinguished from one another by Gram staining.

• In this process, a bacterial smear is heat fixed on glass slide, stained with crystal violet and mordanted with iodine and finally rinsed with ethanol.
• When the bacteria retain the crystal violet stain after rinsing, the bacteria are called gram positive; and those which do not retain the stain are called gram negative.
• The later are then counter stained with pink colour safranin.
• The ability of bacteria to retain crystal violet stain or not, depends upon fundamental structure of cell wall.

Gram positive vs Gram negative bacteria

Gram Positive bacteria	Gram Negative bacteria
1. Cell wall is thicker and homogemous. 2. Contains lower content of lipids (5-10%) 3. Peptidoglycan comprises up to 90% of the cell wall and hence maximum lipid. 4. Techoic acid present. 5. Cell wall has higher amino sugar content (10-20%) 6. Cell wall is simple in shape and is single layered. 7. Mesosomes more prominent. 8. Retains violet dye 9. Examples: Bacillus, Clavibacter, Streptomyces	Cell wall is thinner and usually thin layered. Contains higher content of lipids (up to 40%) Peptidoglycan comprises only 10%. Techoic acid absent. Low content of amino sugars Varying cell wall shape and is tripartite (3-layered). Mesosomes less prominent. Retains red dye Examples: Erwinia, Pseudomonas, Xanthomonas, Agrobacterium, Xylella