What are Bacteria?
Properties of Bacteria
- Prokaryotic (no membrane-enclosed nucleus).
- No mitochondria or chloroplasts.
- a single chromosome
- A closed circle of double-stranded DNA
- With no associated histones
- If flagella are present, they are made of a single filament of the protein flagellin; there are none of the “9+2” tubulin-containing microtubules of the eukaryotes.
- Ribosomes differ in their structure from those of eukaryotes.
- Have a rigid cell wall made of peptidoglycan.
- The plasma membrane (in Gram-positive bacteria) and both membranes in Gram-negative bacteria are phospholipid bilayers but contain no cholesterol or other steroids.
- No mitosis.
- Mostly asexual reproduction.
- Any sexual reproduction very different from that of eukaryotes; no meiosis
- Many bacteria form a single spore when their food supply runs low. Most of the water is removed from the spore and metabolism ceases. Spores are so resistant to adverse conditions of dryness and temperature that they may remain viable even after several years of dormancy.
Classification of Bacteria
Until recently classification has done on the basis of such traits as:
- Ability to form spores
- Method of energy production (glycolysis for anaerobes, cellular respiration for aerobes)
- Nutritional requirements
- Reaction to the Gram stain
|Gram-positive bacteria are encased in a plasma membrane covered with a thick wall of peptidoglycan. Gram-negative bacteria are encased in a triple-layer. The outermost layer contains lipopolysaccharide (LPS).|
The Gram stain is named after the 19th century Danish bacteriologist, Hans Christian Gram (1853–1938) 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.
Although the Gram stain might seem an arbitrary criterion to use in bacterial taxonomy, it does, in fact, distinguish between two fundamentally different kinds of bacterial cell walls and reflects a natural division among the bacteria.