BacteriaBacteria

Bacteria are small: 0.6-3.5 microns in length, 0.5-1.0 microns in diameter http://www.cellsalive.com/howbig.htm

There are about several thousand species of bacteria currently identified.

Most are strictly saprophytic and are important in the decomposition of organic matter.

Several cause diseases in humans or animals, but only about 100 species are plant pathogens.

BacteriaBacteria

Morphology and Characteristics:Morphology and Characteristics:

1. Simple, unicellular

2. Shapes:

- coccus - spherical

- bacillus - rod shaped *** Most plant pathogens.

- spirillum - coiled

- filamentous - resemble fungal hyphae.

Ex. Streptomyces (see Fig. 12-4, page 411)

Bacteria associated with plants have several morphological shapes as can be seen with conventional microscopes at 400x to 1000x magnification.

Bacterial Bacterial

MorphologyMorphology

coccus bacillus

spirillum

filamentous

MorphologyMorphology

Morphology and Characteristics:Morphology and Characteristics:

3. Motility - all bacteria that are motile have delicate threadlike flagellae which are considerably longer than the length of the bacterial cell.

Flagellae typically are associated with bacilli.

Attachment of flagellae:a. monotrichous - one flagellum b. lophotrichous - 2 or more flagellae at one endc. amphitrichous -1 or more flagellae at both ends d. peritrichous - flagellae all over bacterium

Attachment of flagellae:Attachment of flagellae:

Multiplication:Multiplication: http://www.cellsalive.com/qtmovs/ecoli_mov.htm

Bacteria divide by binary fission (splitting into two equal parts). After the new DNA has replicated, the chromosomes attach to the cell wall and are pulled apart.

Bacteria can multiply very fast (e.g. 20 minutes). At this rate, one bacterial cell can produce 300 billion new cells in 24 hours. Most plant pathogenic bacteria thrive in warm, moist weather.

Morphology and Characteristics:Morphology and Characteristics:

4. They are prokaryotes - lack an organized nucleus.

5. Their cell walls lack cellulose.

6. They are surrounded by a protective layer of polysaccharide.

Believed to condition the immediate environment around the bacterial cell. Important for:

• Resistance to desiccation

• Resistance to attack (eg. by phagocyts in animals)

• Attachment to surfaces

• Pathogenicity

• Antigenic: Can elicit host defense mechanisms

Primary function is to give rigidity to the cell. Responsible for the shape of the cell.Responsible for the osmotic stability.

Cell Wall with Cell Membrane

Bacterial Cell WallBacterial Cell Wall

Identification of Bacteria:Identification of Bacteria:Generally rather difficult. In addition to morphology (shape and flagellae), is based on results from a large array of tests, such as:

1.Gram stain reaction - tests ability of cell wall to stain with crystal violet. Gm + stains with crystal violet, Gm - does not and must be counterstained with safranin (red color).

Most plant pathogenic bacteria are Gm -. Main exceptions are: Clavibacter, Streptomyces, Bacillus, Clostridium, and a few others.

2. Ability to liquefy gelatin3. Ability to utilize different carbon sources4. Production of acid or gas in culture5. etc, etc, etc

Gram + bacteria which have a thick peptidoglycan layer retain more crystal violet and appear purple.

Gram - bacteria that have an outer membrane and Mollicutes that lack a cell wall enabling the CM to stain, appear pink.

Identification of Bacteria:Identification of Bacteria:

Important Genera of Plant Pathogenic Bacteria:

1. Agrobacterium 2. Clavibacter 3. Erwinia 4. Streptomyces5. Pseudomonas

1/2 of all plant pathogens6. Xanthomonas

1/3 of all plant pathogens

** Images and lecture material were not entirely created by J. Bond. Some of ** Images and lecture material were not entirely created by J. Bond. Some of this material was created by others.**this material was created by others.**

Bacteria are more influenced by environmental conditions than any other plant pathogen.

This is why we generally have fewer epidemics caused by bacteria than by fungi.

Bacteria have no enzymatic or mechanical means of entering plants.

They enter through wounds or natural openings such as hydathodes, lenticels, stomates, nectaries, etc.

1) Bacteria lack resistance to environment and long-distance dispersal mechanisms

•Most are susceptible to desiccation. Very few bacteria form endospores.

•Most bacteria are also susceptible to damage by visible and UV radiation.

•Most are unable to disperse long distances without vectors.

•Some are unable to survive outside the plant or their insect vectors.

Physiological/Environmental Constraints

2) Bacteria lack active mechanisms to penetrate plants.

•May enter through natural openings including stomates, hydathodes, nectaries, and lenticels and natural junctions including at lateral roots, root hairs, leaves, stems, and flowers.

• May enter the plant via wounds caused by wind, hail, abrasion by surrounding branches, animal or insect feeding, cultural practices, plant development (e.g. leaf scars), lesions or wounds caused by other pathogens.

.

Constraints cont.

3. Once they get into the plant, they have entered a relatively humid but very nutrient poor environment surrounded by multiple polymers (e.g. cellulose, hemicellulose, pectin) that inhibit further spread.

Once inside the plant bacteria must increase the availability of nutrients.

1. By altering the membrane permeability of the plant cells.2. Agrobacterium reprograms the plant cells to produce nutrients

for themselves.2. By producing cell wall degrading enzymes and/or toxins. Lysis

of the plant cell releases

Constraints cont.