Bacterial Morphology

• Bacteria (bacterium) are prokaryotic unicellular microorganisms.
• They were the first forms of life on Earth, appearing about 4 billion years ago.

Morphology

• Bacteria display a wide diversity of shapes and sizes.
• Bacterial cells are about one-tenth the size of eukaryotic cells, typically 0.5-5.0 micrometers (µm) in length.
• Some species are visible to the naked eye, e.g., Thiomargarita, which is up to half a millimeter long.
• Mycoplasma are among the smallest bacteria, measuring only 0.3 micrometers, as small as the largest viruses.
• Some bacteria may be even smaller, but these ultra microbacteria are not well studied.
• Cocci are spherical bacterial shape.
• Bacilli are rod-shaped bacteria.
• Vibrio bacteria are shaped like slightly curved rods or comma-shaped.
• Spirilla spirial-shaped bacteria
• Spirochaetes are tightly coiled bacteria
• Some species have tetrahedral or cuboidal shapes.
• Some bacteria discovered deep under Earth's crust grow as branching filamentous types.
• The bacterial cell wall determines their shapes.
• This affects bacteria’s ability to acquire nutrients, attach to surfaces, swim through liquids, and escape predators.

Cell Structure

• The protoplast of the bacterial cell is the whole body of living material (protoplasm).
• It is bounded peripherally by the cytoplasmic membrane.

Bacterial Structure

• Bacteria contain a well-developed cell structure, responsible for many of their unique biological functions.
• Bacterial cells contain a cell wall, cell membrane, cytoplasm, ribosomes, plasmids, flagella, and a nucleoid region.
• The Cell Wall is an outer covering that protects the bacterial cell and gives it shape.
• Cytoplasm is a gel-like substance composed mainly of water, enzymes, salts, cell components, and organic molecules.
• The Cell Membrane or Plasma Membrane surrounds the cell's cytoplasm and regulates the flow of substances in and out of the cell.
• The Nucleoid Region is an area of the cytoplasm that contains the single bacterial DNA molecule.
• Inclusions are nonliving components of the cell without metabolic activity or bounding membranes.
• Common inclusions: glycogen, lipid droplets, crystals, and pigments.
• Plasmids are small independent pieces of DNA that encode for advantageous but non-essential traits.
• Plasmids can be easily gained or lost by a bacterium and transferred between bacteria.
• Considered an extra chromosomal DNA in a bacterial cell.
• Flagella are long, whip-like protrusions aiding in cellular locomotion.
• Ribosomes are cell structures responsible for protein production.
• Endospores are bacterial survival structures highly resistant to many chemical and environmental stresses.
• They enable survival in environments that would otherwise be lethal.
• Fimbriae are protein tubes extending from the outer membrane.
• They are generally short and present in high numbers on the bacterial cell surface.
• Fimbriae facilitate attachment of a bacterium to a surface.
• Pili are similar in structure to fimbriae but longer and present in low numbers.
• Pili are involved in bacterial conjugation, known as conjugation pili or sex pili.
• Pili are absent in gram-positive bacteria and present in gram-negative bacteria.

Cell Wall

• The envelope is composed of the plasma membrane and cell wall.
• The bacterial cell wall has peptidoglycan, located immediately outside of the cytoplasmic membrane.
• Peptidoglycan is a polysaccharide backbone of alternating N-Acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) in equal amounts.
• The cell wall provides rigidity, tensile strength, structural support, protection against mechanical stress and infection.
• It aids in diffusion of gases in and out of the cell.
• Gram-positive cell walls are thick, with peptidoglycan/murein forming almost 95% of the cell.
• Gram-positive bacteria have thick cell walls made of many layers of peptidoglycan and teichoic acids.
• Gram-negative cell walls are thin, with a thin peptidoglycan layer (5-10%) adjacent to the cytoplasmic membrane.
• Gram-negative bacteria have thinner cell walls made of few layers of peptidoglycans.
• They are are surrounded by a lipid membrane containing lipopolysacccharides and lipoproteins (outer membrane).

Flagella

• Flagella are extracellular bacterial cell structures responsible for bacterial motility (movement).
• The arrangement of flagella about the bacterial cell is unique to the species.
• Monotrichous: Single flagellum
• Lophotrichous: A tuft of flagella at one pole
• Amphitrichous: Single flagellum at each of two opposite poles
• Peritrichous: Multiple flagella at several locations about the cell

Bacterial Reproduction

• Asexual reproduction via binary fission is common.
• Binary fission results in two genetically identical bacterial cells.
• Budding: external buds from the parent cell.
• Process of Binary Fission in Prokaryotes
  • The bacterium is in a state where the DNA is tightly coiled.
  • The DNA uncoils and replicates.
  • As the bacterium grows, the DNA is pulled to opposite poles.
  • A new cell wall starts to grow, separating the bacterium.
  • The new cell wall fully develops, splitting the bacterium completely.

Sexual Reproduction

• There is no meiosis (formation of gametes and zygote).
• It involves the transfer of genetic material (DNA) from a donor cell to a recipient cell.
• This process is called genetic recombination and can occur in three ways:
  • Transformation: uptake of liberated DNA from a destroyed cell
  • Transduction: transfer of DNA fragments via a bacteriophage
  • Conjugation: transfer of plasmid DNA through sexual contact between two bacterial cells.
    • The donor cell transfers plasmid DNA (small circular DNA) to the recipient cell through a conjugation tube.
    • The recipient cell replicates the transferred DNA, cells separate, and the plasmid forms a circle.

Bacterial Growth

• Bacterial growth follows four phases in a high-nutrient environment.
  • Lag phase:
    • When cells adapt to the new environment,
    • Showing slow growth while preparing for rapid growth.
    • Characterized by high biosynthesis rates.
  • Log phase (exponential phase):
    • Marked by rapid exponential growth.
    • The rate at which cells grow is known as the growth rate (k).
    • The time it takes the cells to double is the generation time.
  • Stationary phase:
    • Nutrients are depleted, limiting growth.
    • Cells reduce metabolic activity and consume non-essential cellular proteins.
    • Transition from rapid growth to stress response.
  • Declining/death phase:
    • Bacteria run out of nutrients and die.

Metabolism

• Bacteria feed themselves in a variety of ways.
  • Heterotrophic bacteria cannot fix carbon and use organic carbon for growth.
    • Photoheterotrophs: use light for energy.
    • Chemoheterotrophs: use chemical energy.
  • Autotrophic bacteria make their own food from simple substances in their surroundings.
    • Chemoautotrophs use chemical energy to convert carbon dioxide and water into carbohydrates and sugars.
    • Photoautotrophs use sunlight to turn light energy into chemical energy, containing cyanobacteria to convert carbon dioxide and sunlight into oxygen and sugar

Bacterial Environments

• Aerobes (aerobic bacteria) grow only in the presence of oxygen.
• Anaerobes (anaerobic bacteria) grow only without oxygen.
• Facultative anaerobes grow with or without oxygen, preferring oxygen for respiration.

Bacteria and Disease

• Most bacteria play a positive role in recycling essential substances.
• Some bacteria are pathogenic, causing disease in plants and animals.
• In humans, infections occur through contaminated food/water, inhalation, or sexually.
• Bacteria vary in:
  • Pathogenicity: ability to cause disease
  • Virulence: degree of pathogenicity
  • Invasiveness: ability to invade host tissues or fluids
  • Toxicity: ability to create a clinical response due to exotoxin or endotoxin