THE BEECHMONT CREST GUIDE TO BIOLOGY
FACTS ABOUT BACTERIA
Prokaryote vs. eukaryote
Two basic classifications of cells
Bacteria and blue-green algae are prokaryotic cells.
The prokaryotic cell, unlike the eukaryotic cell, is not divided into compartments.
Most prokaryotes have a single circular chromosome. Prokaryotes do not possess a nuclear membrane. The chromosome is bound to a specific area called the mesosome, which is located on the cell membrane.
There are also differences between the ribosomes and ribosomal subunits of the two cell types.
Bacterial (prokaryote) membranes usually do not contain sterols like cholesterol. Sterols are common in the membranes of eukaryotic cells.
Prokaryotic cells have no nucleus; their genetic material is contained in a region of the cytoplasm called the nucleoid.
Prokaryotic cells do not have organelles (ex: mitochondria) that are found in eukaryotes.
The cell wall of prokaryotes allows bacteria to resist osmotic stress. (This cell wall is not found in eukaryotic animal cells.)
Bacteria are divided into two groups based on cell wall structure: gram-positive and gram-negative.
What difference does the cell wall make?
Differences between prokaryotes and eukaryotes due to cell wall (present in prokaryotes but not in eukaryotes):
Antibiotic sensitivities
Energy metabolism
Many bacteria can exist anaerobically
Classification of bacteria:
Bacteria taxonomy (classification) based on:
Bacterial structure
Metabolism
Bacterial evolution:
Bacteria evolve quickly due to
Mutational changes
Environmental pressures
Bacteria exchange genetic information between members of the same species as well as between members of different species.
Bacteriophages
Bacterial parasites are called bacteriophages. Bacteriophages multiply inside bacteria. Eventual result is lysis (cell death due to rupture of the cellular membrane).
Lysis releases new bacteriophage particles.
Eubacteria vs. archaebacteria
Bacteria that infect humans all belong to one kingdom (eubacteria/ bacteria).
Archaea are not human pathogens. Members of the kingdom Archaea (archaebacteria) are typically found in extreme environments. Archaea exist in environments characterized by temperature and pH extremes--- such as hot sulfur springs (thermoacidophiles). Other Archaea live in high-salt (halophiles) or methane-rich (methanogens) environments.
Both eubacteria and archaea
Have similar structure
Lack a nucleus
Evolved from similar ancestors
Key differences are biochemical; archaea are biochemically more similar to eukaryotes than to eubacteria.
Many genetic mechanisms of archaea are similar to eukaryotes. Protein synthesis mechanisms of eukaryotes and archaea seem to share similarities.
Gram positive vs. gram negative bacteria
Bacteria can be classified into two groups based on results of staining with the Gram stain.
Gram positive bacteria remain stained after washing. Gram negative bacteria do not.
What are the structural differences between gram positive vs. gram negative bacteria?
Since bacteria have no mitochondria, phosphorylation (the addition of a phosphate (PO4) group to an organic molecule) occurs in the cell membrane. Outside the cell membrane is a rigid cell wall which keeps the cell intact.
In the cell walls of Gram positive bacteria, there is a thick peptidoglycan layer (which is much thicker than the cell wall peptidoglycan layer in Gram negative bacteria.)
Gram negative bacteria possess an extra outer membrane which forms a major permeability barrier.
The space between the inner and outer membranes is known as the periplasmic space. Gram positive bacteria do not have a periplasmic space.
Spheroplasts and protoplasts
Bacteria that do not have cell walls can be made in the laboratory with enzymes that dissolve the cell wall or interfere with the production of peptidoglycan. Such bacteria are called spheroplasts and protoplasts.
Bacteria structure and morphology
Bacteria are simple compared to eukaryotes. Nevertheless, they do possess distinct structural units. (Not all of the items listed below will be present in all bacteria.)
Flagella
Locomotory organelles that are embedded in the cell membrane.
Flagella move a bacterium by rotating, much like a propeller.
Flagella consist of various proteins, such as flagellin.
Pili or fimbriae
Hair-like projections that are involved in sexual conjugation and adhesion to host cells that will be infected.
Plasmids
Extra-chromosomal DNA that is present in multiple copies.
Involved in disease/host infection and resistance to antibiotics
Sometimes involved in bacterial replication