1. Bacteria:

Bacteria are unicellular organisms that are found in a wide variety of shapes, sizes, and structures. They are some of the most abundant organisms on Earth and can live in almost every type of environment. The general morphology of bacteria can be divided into several characteristics, which are:

Shape (Morphology):

1. Coccus:
   • Shape: Spherical or round-shaped.
   • Arrangement: Can be seen individually (e.g., Staphylococcus), in pairs (diplococci, e.g., Streptococcus pneumoniae), chains (streptococci), or clusters (staphylococci).
   • Examples: Staphylococcus aureus, Streptococcus pyogenes.
2. Bacillus:
   • Shape: Rod-shaped, cylindrical.
   • Arrangement: Can be single, in pairs (diplobacilli), or in chains (streptobacilli).
   • Examples: Escherichia coli, Bacillus subtilis.
3. Spirillum:
   • Shape: Spiral or helical (twisted like a corkscrew).
   • Movement: Can move by rotating like a corkscrew, which helps in motility.
   • Examples: Spirillum volutans, Treponema pallidum (causes syphilis, which is more tightly coiled).
4. Vibrio:
   • Shape: Comma-shaped or curved rods.
   • Examples: Vibrio cholerae, Vibrio parahaemolyticus.
5. Filamentous:
   • Shape: Long, thread-like structures (often branched).
   • Examples: Streptomyces (a genus of Actinobacteria that produces antibiotics).
6. Pleomorphic:
   • Shape: These bacteria do not have a fixed shape and can vary in size and form.
   • Examples: Mycoplasma (which lacks a cell wall).

Size:

• Typical Range: Bacteria generally range in size from 0.5 to 5 micrometers in length. For example:
  • Escherichia coli has a size of around 1-2 micrometers.
  • Staphylococcus aureus is typically around 0.5-1 micrometer in diameter.
  • Some bacteria, such as Epulopiscium species, can be as large as 500 micrometers in length (found in fish intestines).
• Factors Affecting Size:
  • The size of bacteria can be influenced by environmental factors like nutrient availability, habitat, and growth conditions.

Structure:

1. Cell Wall:
   • The bacterial cell wall provides structural support and shape to the cell. It is composed primarily of peptidoglycan, which is a polymer of sugars and amino acids.
   • Gram-Positive Bacteria: Have a thick peptidoglycan layer which retains the crystal violet stain during Gram staining, making them appear purple.
     • Examples: Staphylococcus aureus, Bacillus anthracis.
   • Gram-Negative Bacteria: Have a thin peptidoglycan layer surrounded by an outer membrane, which does not retain the crystal violet stain during Gram staining, making them appear pink.
     • Examples: Escherichia coli, Salmonella enterica.
2. Cytoplasmic Membrane (Plasma Membrane):
   • A phospholipid bilayer with embedded proteins. It acts as a barrier controlling the entry and exit of substances.
   • Contains enzymes involved in cellular respiration and metabolism in many bacteria.
3. Cytoplasm:
   • The cytoplasm is a gel-like substance containing enzymes, proteins, ribosomes, and nutrients essential for metabolic processes.
   • It contains the nucleoid, which holds the chromosomal DNA.
4. Chromosomal DNA:
   • Bacteria usually possess a single, circular chromosome made of DNA, which is not enclosed in a membrane-bound nucleus. The nucleoid region in the cytoplasm holds the bacterial DNA.
5. Plasmids (Optional):
   • Plasmids are small, circular pieces of extrachromosomal DNA that are independent of the chromosomal DNA. They often carry genes for antibiotic resistance or virulence factors.
   • Plasmids can be transferred between bacteria via conjugation, which is significant in the spread of antibiotic resistance.
6. Ribosomes:
   • Bacterial ribosomes (70S) are smaller than those in eukaryotes (80S), but they play a vital role in protein synthesis.
7. Capsule (Optional):
   • A capsule is an additional layer outside the cell wall made of polysaccharides or proteins. It protects bacteria from phagocytosis and helps in attachment to surfaces.
   • Examples: Streptococcus pneumoniae, Klebsiella pneumoniae.
8. Endospores (Optional):
   • Some bacteria, such as Bacillus and Clostridium, can form endospores when environmental conditions are harsh. These spores are highly resistant to heat, desiccation, and chemicals.
   • Examples: Bacillus anthracis, Clostridium botulinum.

Flagella and Pili (Appendages for Motility and Attachment):

1. Flagella:
   • Flagella are long, thin structures that provide motility to the bacterial cell. They rotate to propel the bacterium forward, allowing it to move toward favorable conditions (chemotaxis).
   • Parts:
     • Filament: Composed of the protein flagellin.
     • Hook: Connects the filament to the basal body.
     • Basal Body: Embedded in the membrane and functions like a motor to rotate the flagellum.
   • Types of Flagella:
     • Monotrichous: Single flagellum at one end.
     • Lophotrichous: Tuft of flagella at one or both ends.
     • Amphitrichous: Flagella at both ends.
     • Peritrichous: Flagella distributed over the surface.
2. Pili (Fimbriae):
   • Pili are shorter than flagella and involved in attachment to surfaces and in conjugation (the transfer of genetic material between bacteria).
   • Conjugation Pili: These pili are specialized for the transfer of genetic material.
   • Made of protein subunits called pilin.

2. Mollicutes (Mycoplasmas):

Mollicutes represent a unique group of bacteria that lack a cell wall, a characteristic that sets them apart from most other bacteria.

Shape (Morphology):

• Pleomorphic: Mollicutes are highly variable in shape. They can adopt a variety of forms due to the absence of a rigid cell wall.
• Common Forms: Coccoid (round), bacillary (rod-shaped), or filamentous (thread-like).
• Example: Mycoplasma pneumoniae can be spherical or filamentous.

Size:

• Mollicutes are among the smallest bacteria, with some species, such as Mycoplasma genitalium, measuring 0.2 micrometers in diameter.
• Due to their small size, they are among the few organisms that can pass through filters that normally retain other bacteria.

Lack of Cell Wall:

• Mollicutes do not possess a peptidoglycan cell wall, which means they do not respond to Gram staining (they are neither Gram-positive nor Gram-negative).
• This feature makes them resistant to penicillin and other antibiotics that target cell wall synthesis.

Plasma Membrane:

• Mollicutes possess a sterol-rich plasma membrane, which helps maintain cell shape and integrity in the absence of a cell wall.
• The plasma membrane is crucial for maintaining the cell’s flexibility and stability.

Motility and Adhesion:

• Some mollicutes exhibit gliding motility, which allows them to move on surfaces, although they lack flagella.
• Pili are involved in attachment to host cells or surfaces. They use surface proteins to adhere to epithelial cells, which is crucial for parasitism and infection.

Reproduction:

• Mollicutes reproduce through binary fission like other bacteria. However, their division can be irregular due to their pleomorphic nature.

Nutritional Requirements:

• Mollicutes are often nutritionally fastidious, meaning they require rich media for growth.
• Some species, like Mycoplasma pneumoniae, are parasitic and rely on the host for nutrients such as amino acids, lipids, and nucleotides.

Key Differences Between Bacteria and Mollicutes: