Bacterial Structure Flashcards
Bacterial Shape
Bacteria are classified by their shape into three fundamental groups which are?
:
Cocci: These bacteria are round.
Bacilli: These bacteria are rod-shaped.
Spirochetes: These bacteria are spiral-shaped.
- Additionally, some bacteria do not have a fixed shape and are referred to as pleomorphic (many-shaped). The shape of a bacterium is determined by its rigid cell wall, which provides structural integrity and defines its form. The microscopic appearance of a bacterium, influenced by its shape, is crucial for its identification.
Arrangement of Bacteria
Apart from their shape, bacteria also exhibit characteristic arrangements that aid in their identification. Which are?
Diplococci: Certain cocci occur in pairs.
Streptococci: Some cocci form chains.
Staphylococci: Other cocci are found in grape-like clusters.
These arrangements result from the orientation and degree of attachment of the bacteria during cell division. The patterns formed can be an important diagnostic tool.
Arrangement of Bacteria
Apart from their shape, bacteria also exhibit characteristic arrangements that aid in their identification. Which are?
Diplococci: Certain cocci occur in pairs.
Streptococci: Some cocci form chains.
Staphylococci: Other cocci are found in grape-like clusters.
These arrangements result from the orientation and degree of attachment of the bacteria during cell division. The patterns formed can be an important diagnostic tool.
Bacterial Size
Bacteria vary significantly in size, ranging from about 0.2 to 5 micrometers (μm):
Classify them based on size
Smallest Bacteria: Mycoplasma species are among the smallest bacteria, about the same size as the largest viruses (poxviruses). These are the smallest organisms capable of existing outside a host.
Largest Bacteria Rods: For example, Bacillus anthracis can be as long as some yeasts and human red blood cells, which are approximately 7 micrometers in size.
Bacterial Shapes and Identification
Bacterial classification based on shape helps microbiologists identify and study different bacteria. The primary shapes are?
The primary shapes are cocci, bacilli, and spirochetes:
Cocci (round) can be further identified by their arrangement into pairs, chains, or clusters. For example, Streptococcus (chains) can cause strep throat, while Staphylococcus (clusters) includes species that can cause skin infections.
Bacilli (rod-shaped) include species like Escherichia coli, which is commonly found in the intestines of humans and animals.
Spirochetes (spiral-shaped) include Treponema pallidum, the causative agent of syphilis.
Pleomorphic Bacteria
Pleomorphic bacteria can change shape and are not restricted to a single form. This flexibility can sometimes complicate identification and treatment because these bacteria can adapt to various environments.
The arrangement of bacteria (diplococci, streptococci, staphylococci) provides additional clues to their identification
List and example of each of the above
Diplococci: Neisseria gonorrhoeae (causes gonorrhea) is an example of diplococci.
Streptococci: Streptococcus pyogenes (causes strep throat) is an example of streptococci.
Staphylococci: Staphylococcus aureus (causes various infections, including MRSA) is an example of staphylococci.
Size Variation in Bacteria
Understanding the size variation among bacteria is crucial for microbiologists, especially when using microscopes and other diagnostic tools:
List Examples of size variations
Mycoplasma species, being among the smallest, lack a cell wall, which contributes to their small size and unique characteristics.
Bacillus anthracis, known for its size, is also notable for its role in anthrax, a serious infectious disease.
The structure of bacterial cells, encompassing their shape, arrangement, and size, plays a vital role in the identification and understanding of these microorganisms. The rigid cell wall determines the shape, while arrangements result from bacterial division processes. The size of bacteria varies widely, with some being as small as large viruses, and others as large as yeast cells or red blood cells. These characteristics are essential for microbiologists in diagnosing and studying bacterial infections and behaviors.
Bacteria are unicellular prokaryotic organisms, meaning they lack a defined nucleus and membrane-bound organelles. The structure of bacteria is composed of three primary parts: ____, _____&_____
the outer layer (cell envelope), the cell interior, and additional structures.
What can you tell me about Outer Layer (Cell Envelope)
Outer Layer (Cell Envelope)
The cell envelope includes two main components:
Cell Wall: The cell wall provides structural support and maintains the shape of the bacterium. It acts as a barrier against external physical and chemical factors.
Plasma Membrane: Situated beneath the cell wall, the plasma membrane controls the movement of substances into and out of the cell, maintaining the internal environment.
The cell envelope serves as both a structural and physiological barrier, protecting the bacterial cell from osmotic lysis (bursting due to pressure) and environmental threats, while also giving the bacterium its characteristic shape.
Cell Interior
The internal structure of a bacterial cell, known as the _____, and it includes:
protoplasm
Cytoplasm: The gel-like substance within the cell where metabolic reactions occur.
Cytoplasmic Inclusions:
Mesosome: Involved in cell division and DNA replication.
Ribosomes: Sites of protein synthesis.
Inclusion Granules: Storage sites for nutrients and other substances.
Bacteria also possess a single circular DNA molecule that contains all the genetic information necessary for the cell’s functions and replication.
Additional Structures
Bacteria may also have several additional structures that aid in various functions:
- Capsule: A protective outer layer that enhances the bacterium’s ability to cause disease (pathogenicity) by preventing phagocytosis.
- Flagella: Long, whip-like appendages that enable bacterial motility.
- Fimbriae: Short, hair-like structures that allow bacteria to adhere to surfaces and other cells.
- Spores: Highly resistant structures formed under adverse conditions, allowing bacteria to survive extreme environments.
Cell Wall: Composed mainly of _____, it protects against mechanical stress and osmotic pressure.
In Gram-positive bacteria, the cell wall is ____, whereas in Gram-negative bacteria, it is ____ but surrounded by an additional outer membrane.
peptidoglycan
thick, whereas in Gram-negative bacteria, it is thinner but surrounded by an additional outer membrane.
Plasma Membrane: Consists of a phospholipid bilayer with embedded proteins that regulate nutrient uptake, waste elimination, and signal transduction
The cell envelope serves as both a structural and physiological barrier, protecting the bacterial cell from osmotic lysis (bursting due to pressure) and environmental threats, while also giving the bacterium its characteristic shape.
Internal Components
The internal components are essential for metabolic activities and genetic functions:
Cytoplasm: Contains enzymes and nutrients necessary for biochemical reactions.
Mesosome: An invagination of the plasma membrane, involved in processes like cell wall formation during cell division.
Ribosomes: Translate mRNA into proteins, crucial for the cell’s structural and functional needs.
Inclusion Granules: Store carbon, phosphate, and other nutrients that can be used when resources are scarce.
Additional Structural Features
These structures provide specialized functions that enhance bacterial adaptability and pathogenicity:
Capsule: Composed of polysaccharides, it provides an extra layer of protection and can help bacteria evade the host’s immune system.
Flagella: Enable bacteria to move toward favorable environments (chemotaxis) and away from hostile conditions.
Fimbriae: Facilitate attachment to host tissues, which is crucial for colonization and infection.
Spores: Allow bacteria to withstand extreme conditions such as heat, desiccation, and chemical exposure. When favorable conditions return, spores can germinate and return to their vegetative state.
Cell Wall (Peptidoglycan)
The bacterial cell wall is a rigid and chemically complex structure that lies between the cell membrane and the capsule or slime layer. It plays a critical role in maintaining the shape of the bacterial cell and protecting it from changes in osmotic pressure.
The cell wall accounts for about __to__% of the cell’s dry weight.
20-30%
What can you tell me about the Structure and Composition of bacteria
The major component of the bacterial cell wall is the peptidoglycan layer. Peptidoglycan is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane. The two primary sugar derivatives in peptidoglycan are:
- N-acetyl glucosamine (NAG)
- N-acetyl muramic acid (NAM)
These sugars are linked together in long chains, and the chains are cross-linked by short peptides to provide additional strength and rigidity.
Classify bacterias based on Cell Wall Structure and note their composition
Bacteria are classified into Gram-positive and Gram-negative based on the structure of their cell walls, which can be determined using the Gram staining method.
Gram-Positive Bacteria:
- Cell Wall Structure: They have a thick peptidoglycan layer, which retains the crystal violet stain used in Gram staining, making them appear purple under a microscope.
- Components: Besides peptidoglycan, the cell wall contains teichoic acids and lipoteichoic acids, which contribute to the wall’s rigidity and regulate the movement of cations.
Gram-Negative Bacteria:
- Cell Wall Structure: They have a thin peptidoglycan layer located between the inner cytoplasmic membrane and an outer membrane. During Gram staining, the crystal violet stain is washed out, and these bacteria take up the counterstain (safranin), appearing pink.
- Components: The outer membrane contains lipopolysaccharides (LPS), which contribute to the structural integrity and act as a barrier to certain antibiotics. The space between the inner and outer membranes is called the periplasmic space, where various enzymes and proteins are located.
Peptidoglycan Layer:
NAG and NAM: The sugars NAG and NAM alternate in chains, linked by β-1,4-glycosidic bonds.
Cross-Linking Peptides: Short peptide chains attached to the NAM residues form cross-links between glycan chains, providing mechanical strength.
What can you tell me about the structure of gram +ve & -ve structure
Gram-Positive Cell Wall:
Thick Peptidoglycan Layer: Comprising multiple layers of peptidoglycan (up to 90% of the cell wall).
Teichoic and Lipoteichoic Acids: These polymers of glycerol or ribitol phosphate are covalently linked to peptidoglycan and extend through and beyond the cell wall, providing rigidity and involved in ion regulation.
Gram-Negative Cell Wall:
Thin Peptidoglycan Layer: Only a few layers thick (about 10% of the cell wall).
Outer Membrane: Composed of a lipid bilayer containing lipopolysaccharides (LPS), phospholipids, and proteins such as porins (which allow the passage of small molecules).
Periplasmic Space: Contains the thin peptidoglycan layer and a variety of enzymes involved in nutrient acquisition, electron transport, and peptidoglycan synthesis.
What are the Functions of the Cell Wall
Function of the Cell Wall
Shape Maintenance: The cell wall determines the characteristic shape of the bacterium, whether it is spherical (cocci), rod-shaped (bacilli), or spiral (spirochetes).
Protection Against Osmotic Pressure: By providing structural integrity, the cell wall prevents the cell from lysing (bursting) in hypotonic environments.
Barrier to Environmental Stress: The cell wall acts as a protective barrier against mechanical damage and harmful chemicals.
Conclusion
The bacterial cell wall is a vital structure that ensures the integrity, shape, and survival of bacteria in various environments. Its complex composition, primarily based on peptidoglycan, provides mechanical strength and protection against osmotic lysis. The distinction between Gram-positive and Gram-negative bacteria, based on cell wall structure, is crucial for understanding bacterial classification, physiology, and the mechanisms of action of different antibiotics.
Differentiate Between Gram-Positive and Gram-Negative Cell Walls
Gram-Positive Cell Wall
Thickness: The cell wall is thick, measuring approximately 15-80 nm.
Peptidoglycan Layer: Present in abundance, forming a thick, multilayered structure.
Lipid Content: Contains 2-5% lipids.
Teichoic Acid: Present, contributing to the wall’s rigidity and involved in ion regulation.
Effect of Lysozyme Treatment: Leads to the formation of a protoplast, a cell devoid of its wall but surrounded by the plasma membrane.
Gram-Negative Cell Wall
Thickness: The cell wall is thin, measuring approximately 2 nm.
Peptidoglycan Layer: Significantly less, forming a thin, single-layered structure located in the periplasmic space.
Lipid Content: Contains 15-20% lipids, primarily in the outer membrane.
Teichoic Acid: Absent.
Effect of Lysozyme Treatment: Leads to the formation of a spheroplast, a cell with a partially removed cell wall but retaining the outer membrane.
Gram-Positive Cell Wall
Structure: Composed of multiple layers of peptidoglycan which make up to 90% of the cell wall.
Components: Includes teichoic acids and lipoteichoic acids which are covalently bound to the peptidoglycan and extend through and beyond the cell wall.
Lysozyme Effect: Lysozyme cleaves the β-1,4-glycosidic bonds between NAG and NAM in peptidoglycan, leading to the formation of protoplasts.
Gram-Negative Cell Wall
Structure: Composed of a thin layer of peptidoglycan, located in the periplasmic space between the inner cytoplasmic membrane and the outer membrane.
Components: The outer membrane contains lipopolysaccharides (LPS), phospholipids, and proteins such as porins. The periplasmic space houses various enzymes and proteins.
Lysozyme Effect: Lysozyme partially degrades the peptidoglycan layer, leading to the formation of spheroplasts, which retain the outer membrane.
