Bacterial Structures Overview

Bacterial Structures

Capsules are a discrete layer of polysaccharides outside the cell wall.
Glycocalyx, a type of capsule, is a thin layer of tangled polysaccharide fibers on the cell surface.
Capsules are generally composed of polysaccharides, containing amino sugars or peptides.
Capsules mediate the adherence of cells to surfaces.
Capsules protect bacterial cells from engulfment by predatory protozoa or white blood cells (phagocytes) or antimicrobial agents.
Capsules protect cells from drying or desiccation in certain soil bacteria.
Cell walls provide strength and rigidity to the cell and are permeable to solutes.
Bacterial murein is a type of peptidoglycan, a polymer of sugars cross-linked by short chains of amino acids.
All bacterial peptidoglycans contain N-acetylmuramic acid.
Gram-positive bacteria retain the crystal violet dye in the Gram stain procedure; their cell walls are thick (15-80 nanometers) and consist of several layers of peptidoglycan.
Teichoic acids are molecules in the Gram-positive cell wall that are perpendicular to peptidoglycan sheets.
Gram-negative bacteria do not retain the crystal violet dye; their cell walls are thin (10 nanometers) and consist of a single layer of peptidoglycan surrounded by a membrane.
The outer membrane of Gram-negative bacteria invariably contains lipopolysaccharide (LPS or endotoxin), which is toxic to animals.
In E. coli and other Gram-negative bacteria, peptidoglycan's backbone is made of alternating molecules of N-acetylglucosamine (G) and N-acetylmuramic acid (M).
The glycan backbone of peptidoglycan can be broken by the enzyme lysozyme that is found in animal tissue and secretions and in phagocyte granules.
The exact site of lysozyme cleavage is the beta 1,4 bond between N-acetylmuramic acid (M) and N-acetylglucosamine (G).

Cytoplasmic Membrane

The cytoplasmic membrane of bacterial cells is a delicate and plastic structure completely enclosing the cell cytoplasm.
The bacterial membrane is composed of 40% phospholipids and 60% proteins.
Phospholipids are amphoteric molecules, meaning they have a water-soluble hydrophilic region (glycerol head) attached to two insoluble hydrophilic fatty acid "tails."
Cytoplasm helps in cellular growth, metabolism, and replication.
Cytoplasm is the storehouse for all types of chemicals and components needed to sustain bacterial life.
DNA is not enclosed inside a membrane-bound nucleus but instead resides inside the bacterial cytoplasm.
Bacterial DNA is not packaged using histones to form chromatin, as in eukaryotes.

Plasmids

Plasmids are often stranded and circular.
Plasmids can contain varying lengths of genetic information.
Different types of plasmids perform various functions, including virulence, environmental adaptability, persistence, metabolic functions, and nutrient utilization.
Plasmids allow for the biodegradation of a variety of toxic substances like toluene and other organic hydrocarbons, herbicides, and pesticides.

Ribosomes

Ribosomes are the site of protein synthesis in all living organisms.
Prokaryotes have 70S ribosomes, while eukaryotes have larger 80S ribosomes in their cytosol.
Prokaryotic ribosomes consist of 30S and 50S subunits.

RNA and Ribosomal Proteins

The 30S subunit contains 16S rRNA and about 21 proteins.
The 50S subunit contains 5S and 23 rRNA and about 34 proteins.

Intracellular Membranes

Mesosomes are folds present inside the plasma membrane.
Mesosomes play a vital role in cellular respiration, DNA replication, cell division, and separation of chromosomes.
Mesosomes also perform the role of Golgi bodies and mitochondria.
Chromatophores are intracellular membranes found in phototrophic bacteria, primarily for photosynthesis. They contain bacteriochlorophyll pigments and carotenoids.

Pili and Fimbriae

Pili (sometimes called attachment pili) are protein tubes extending from the outer membrane, found in many bacterial species.
Pili are generally short and numerous, facilitating bacterial attachment to surfaces.
Pili are hair-like structures composed of single-aggregating proteins (pilin).
Pili are shorter, thinner, and straighter than flagella.
Several pili are similar in structure to fimbriae but are much longer and present in fewer numbers on the bacterial cell.
Pili are involved in bacterial conjugation (sex pili).

Flagella

Flagella are filamentous protein structures that provide swimming movement for motile prokaryotes.
Flagella are attached to the cell surface.
Prokaryotic flagella are much thinner than eukaryotic flagella.
The diameter of a prokaryotic flagellum is about 7-15 µm and 20 nm in diameter.

Bacterial Cell Arrangement

The arrangement of flagella varies; typical arrangements include monotrichous, lophotrichous, amphitrichous, and peritrichous.

Endospores

Certain genera of Gram-positive bacteria form highly resistant dormant structures called endospores.
Endospores don't undergo reproduction; they are formed only in some cases.
They can withstand extreme conditions like UV light, gamma radiation, detergents, heat, freezing, pressure, and desiccation.
The spore has a cortex layer beneath the spore coat composed of peptidoglycan.
Variations in endospore morphology include terminal, lateral, and central endospores.

Bacterial Reproduction

Bacteria reproduce asexually via binary fission.
Cell elongates and DNA is replicated.
Cell wall and plasma membrane begin to divide.
A cross-wall forms around the divided DNA.
Cells separate.

Bacterial Staining

Basic stains are used to examine bacteria.
Basic stains bind electrostatically to negatively charged molecules like polysaccharides, proteins, and nucleic acids.
Commonly used basic stains include crystal violet, safranin, and methylene blue.