BACTERIA

Bacteria are a large domain of procariotic microorganism. Bacteria are often maligned as the causes of human and animal disease (like this one, Leptospira, which causes serious disease in livestock). However, certain bacteria, the actinomycetes, produce antibiotics such as streptomycin and nocardicin; others live symbiotically in the guts of animals (including humans) or elsewhere in their bodies, or on the roots of certain plants, converting nitrogen into a usable form. Bacteria put the tang in yogurt and the sour in sourdough

bread; bacteria help to break down dead organic matter; bacteria make up the base of the food web in many environments. Bacteria are of such immense importance because of their extreme flexibility, capacity for rapid growth and reproduction, and great age - the oldest fossils known, nearly 3.5 billion years old, are fossils of bacteria-like organisms.

Properties of Bacteria

prokaryotic (no membrane-enclosed nucleus)
no mitochondria or chloroplasts
a single chromosome
If flagella are present, they are made of a single filament of the protein flagellin
ribosomes differ in their structure from those of eukaryotes

Classification of Bacteria

shape

bacilli: rod-shaped
cocci: spherical
spirilla: curved walls

The Gram stain is named after the 19th century Danish bacteriologist who developed it.

The bacterial cells are first stained with a purple dye called crystal violet.
Then the preparation is treated with alcohol or acetone.
This washes the stain out of Gram-negative cells.
To see them now requires the use of a counterstain of a different color (e.g., the pink of safranin).
Bacteria that are not decolorized by the alcohol/acetone wash are Gram-positive.

Bacterial Cell Structure

Internal Structure: Bacteria have a very simple internal structure, and no membrane-bound organelles.

1. Nucleoid

DNA in the bacterial cell is generally confined to this central region. Though it isn't bounded by a membrane, it is visibly distinct (by transmission microscopy) from the rest of the cell interior.

2. Ribosomes

Ribosomes give the cytoplasm of bacteria a granular appearance in electron micrographs. Though smaller than the ribosomes in eukaryotic cells, these inclusions have a similar function in translating the genetic message in messenger RNA into the production of peptide sequences (proteins).

3. storage granules

(not shown) Nutrients and reserves may be stored in the cytoplasm in the form of glycogen, lipids, polyphosphate, or in some cases, sulfur or nitrogen.

4. Endospore

(not shown) Some bacteria, like Clostridium botulinum, form spores that are highly resistant to drought, high temperature and other environmental hazards. Once the hazard is removed, the spore germinates to create a new population.

Surface Structure: Beginning from the outermost structure and moving inward, bacteria have some or all of the following structures:

1. Capsule

This layer of polysaccharide (sometimes proteins) protects the bacterial cell and is often associated with pathogenic bacteria because it serves as a barrier against phagocytosis by white blood cells.

2. outer membrane

(not shown) This lipid bilayer is found in Gram negative bacteria and is the source of lipopolysaccharide (LPS) in these bacteria. LPS is toxic and turns on the immune system of , but not in Gram positive bacteria.

3. cell wall

Composed of peptidoglycan (polysaccharides + protein), the cell wall maintains the overall shape of a bacterial cell. The three primary shapes in bacteria are coccus (spherical), bacillus (rod-shaped) and spirillum (spiral). Mycoplasma are bacteria that have no cell wall and therefore have no definite shape.

4. plasma membrane

This is a lipid bilayer much like the cytoplasmic (plasma) membrane of other cells. There are numerous proteins moving within or upon this layer that are primarily responsible for transport of ions, nutrients and waste across the membrane.