Definition
A bacterium is a prokaryotic [pro=before, karyo=nucleus] (containing small ribosomes (70S)) organism. The ribosomes are not organized into a nucleus (i.e., the DNA is not bound my a membrane. Bacteria are unicellular, micrscopic organisms that lack chlorophyll and multiply (repoduce) by binary fission (simple cellular division).
Etymology. 1847, from Mod.L. pl. of bacterium, from Gk. bakterion "small staff," dim. of baktron "stick, rod," from PIE *bak- "staff used for support." So called because the first ones observed were rod-shaped. Introduced as a scientific word 1838 by Ger. naturalist Christian Gottfried Ehrenberg (1795-1876).
Discussion
Fossil prokaryotes appear in rocks about 3.5 million years of age.
Plant pathogenic bacteria are heterotrophs (they require external source of organic carbon). Bacteria have cell membranes and rigid cell walls. They are much smaller than fungi, but can be seen with a light microscope. A typical bacterial cell is about 1 micrometer in diameter.
Bacteria can divide very rapidly: under proper conditions, some bacteria can divide every 20 minutes. At this rate, a single bacterium would produce about 3 X 1011 progeny in 24 hours (about 300 billion individuals). Bacteria often have one or more flagella (i.e., they are mobile).
About 1600 bacterial species are known, most are strictly saprophytic. About 80 species of bacteria cause plant diseases. Bacterial shapes are: rod-shaped (bacilli), spherical (cocci), ellipsoidal, comma shaped, or filamentous/coiled (spirilli).
All plant pathogenic bacteria are rod-shaped (with the exception of Streptomyces). Spore-forming bacteria exist, but are only associated with storage rots, not with field diseases.
There are two general types of bacteria:
-facultative saprophytes (may be cultured on artificial media)
-fastidious vascular bacteria (some are not culturable)
Major types of bacterial plant diseases are:
-blight and cankers (e.g., blast, black arm, wild fire)
-vascular wilt (where xylem vessels are invaded and wilting is the most obvious symptom)
-soft rot (host separation, or maceration, leading to tissue collapse in which the host tissue remains soft and moist.)
Some characteristic symptoms produced by plant pathogenic bacteria are:
1) Leaf lesions -'shot hole', small and angular necrotic spots, necrotic spots surrounded by a water-soaked area and a large chlorotic halo (e.g., halo blight), black veins, large necrotic areas (blight).
2) Stem lesions -crown gall, vascular wilt, stem canker, stem rot
3) Rot of fruits, tubers, etc.
4) Venial necrosis (blackened veins)
5) Blight
6) Hydrosis
7) Lesions with chlorotic halos
“Fastidious prokaryotes ” are small rod-shaped bacteria (less than 5 micrometers in diameter) whose taxonomic position is unclear.