General Bacteriology Overview

General Bacteriology

• Microbiology is the study of small living organisms like bacteria, fungi, and viruses.
• Organisms were classified into Protista (bacteria, fungi, protozoa, and algae) before the electron microscope.
• After the electron microscope, Prokaryotes (Bacteria, Rickettsia, Chlamydia) and Eukaryotes (Fungi, protozoa, plant and animal cells), and Viruses are included.

Bacterial Structure

• Bacteria have a capsule, cell wall, plasma membrane, nucleoid, cytoplasm, and ribosomes.

Classification of Bacteria

• Morphology: Bacteria can be spherical (cocci), rod-shaped (bacilli), or spiral (spirochetes). Specific arrangements exist (diplococci, streptococci, staphylococci).
• Stained according to gram staining: Gram-positive and gram-negative.
• Metabolism and nutrition
• Orders, families, genera, species, and groups

Structure of the Bacterial Cell

• Essential structures: cell wall, cytoplasmic membrane, and protoplasm (cytoplasm and the nuclear material).
• Accessory structures: capsule and slime layer, fimbriae (pili), flagella, spores, and plasmids.

1- Cell Wall

• Rigid structure that maintains bacterial shape.
• Prevents osmotic rupture or lysis.
• Plays a role in cell division.
• Virulence factor: LPS (endotoxin) is present in Gram-negative bacteria; Teichoic acids are in Gram-positive bacteria, enabling adherence to host cells. Protein A (Staph) and Protein M (Strept) are anti-phagocytic.
• Medial importance: Major antigen, important for classification according to staining, and a good target for antibiotics (penicillin and cephalosporins).

Gram-positive vs. Gram-negative

• Gram-positive: Thick peptidoglycan layer (50%), teichoic acid, absent outer membrane.
• Gram-negative: Thin peptidoglycan layer (5-10%), outer membrane with lipopolysaccharide (LPS), periplasmic space.

2- Cytoplasmic Membrane

• Semi-permeable, delicate membrane.
• Maintains constant environment.
• Chemical composition: phospholipid bilayer with proteins.
• Function: -Selective permeability to molecules. -Excretion of hydrolytic enzymes and toxins -Site of respiration, energy production, and electron transport. -Plays a role in DNA replication and cell wall biosynthesis.

Mesosomes

• Invaginations of the cell membrane.
• Play a role in respiration and cell division.

3- Bacterial Cytoplasm

• Viscous, watery, soft gel-like substance.
• Contains cytoplasmic inclusion granules (e.g., meta-polyphosphate).
• Contains ribosomes (70S, 30S and 50S subunits) responsible for protein synthesis.
• Contains other types of RNA (t-RNA, m-RNA).

4- Bacterial Nucleus

• Single, circular, double-stranded DNA.
• Supercoiled to fit within the cell
• Binary fission method of multiplication; chromosome attaches to the mesosome.
• Forms two identical copies of the DNA.

5- Plasmids

• Extra-chromosomal, circular, double-stranded DNA molecules.
• Replicate independently of the host chromosome.
• Multiple copies are present in the cell.
• Do not encode essential bacterial functions.
• Can be classified as conjugative or non-conjugative.

B-Accessory Structures

• Capsule: Firmly attached layer of polysaccharides or polypeptide (mucoid colonies).
• Slime Layer (Glycocalyx): Loosely attached.
• Capsule and slime function as a virulence factor, aiding in attachment and preventing phagocytosis.
• Function:
  • Adherence to host surfaces
  • Protection against immune system
  • Resistance to antibiotics.

Flagella

• Long, thin filaments composed of flagellin protein.
• Enable motility in the bacteria.
• Present in different distributions.
• Different types of flagella distributions exist (monotrichous, amphitrichous, lophotrichous, peritrichous, and atrichous).

Pili (Fimbriae)

• Short, hair-like protein filaments.
• Enable adhesion to host cells, increasing pathogenicity.
• Two types of Pili found; ordinary pili and sex pili.
• Ordinary pili used for colonization.
• Sex pili used for bacterial conjugation.

Spores

• Highly resistant resting stage.
• Formed under unfavorable conditions (e.g., nutrient depletion, heat, dryness).
• Resistant to heat, chemicals, and desiccation.
• Marked by structural components like calcium dipicolinate and impermeable layer.
• Medial Importance -Extraordinary resistance to heat & chemical methods of sterilization and boiling. -Can generate into vegetative forms, causing disease. -Use as indicators of sterilization and as a biological weapon.

Demonstrations

• Various staining techniques for visualizing bacterial structures.
• Capsules can be visualized using special staining techniques.