MCB 102 BACTERIA

General Characteristics and Structure of Bacteria

1. Prokaryotes

• Prokaryotic cells include two main groups: Eubacteria (BACTERIA) and Archaebacteria (now known as archaea).
• A prokaryotic cell lacks a true nucleus and a membrane surrounding its DNA.
• They lack various internal structures bound with phospholipid membranes.
• They have a simple structure and are relatively small (about 1.0 µm in diameter).

2. Size of Bacteria

• Bacterial cells (singular: bacterium) vary in size, typically ranging from 0.5 µm to 1.0 µm in diameter or width.
• Their small size facilitates fast growth by affecting the rate at which nutrients can enter and are distributed within the cell.

3. Shapes of Bacteria

Bacteria come in many shapes, with the three most common being:

• Coccus (spheres)
• Bacillus (rods)
• Spiral (twisted), including Spirillum, Vibrio, Spirochaete.

Pleiomorphism: Some bacterial shapes vary within a culture, often occurring when nutrients are depleted or waste builds up.

Table: Common Bacterial Shapes and Examples

Name	Shape	Example
Coccus	Spherical	Staphylococcus aureus
Bacillus	Rod-shaped	Escherichia coli
Vibrio	Comma-shaped	Vibrio cholerae
Spirillum	Rigid spiral	Spirillum minus
Spirochaete	Flexible spiral	Treponema pallidum

Other Unusual Shapes

• Star-shaped (e.g., Stella)
• Square (e.g., Haloarcula)

4. Cell Arrangements

Arrangements of Cocci

• Single cells: coccus
• Diplo: pairs of cells (e.g., Diplococcus pneumoniae, now called Streptococcus pneumoniae)
• Strepto: chains (greater than 2 cells/chain) (e.g., Streptococcus pyogenes)
• Tetrad: Division on two planes produces 4 cells in a "square" cluster (e.g., Micrococcus luteus)
• Sarcinae: Division on three planes, 8 cells arranged in a cube (e.g., Sarcina lutea)
• Staphylo: Large groups – irregular, like grape clusters (e.g., Staphylococcus aureus)

Arrangement of Bacilli

Bacilli always divide in just one plane, forming simple patterns of rods joined end to end if they don't separate.

• Single
• Pair: diplobacillus
• Chain: streptobacillus
• Coccobacillus: short rods, sometimes appearing ovoid.

Arrangement of Spirilla

• Spiral bacteria usually remain as single microorganisms; variability comes with having more or fewer corkscrews.
• Includes Vibrio (curved rod), Spirillum (rigid spiral), and Spirochete (flexible spiral).

5. Mode of Nutrition

Bacterial nutritional strategies are broadly categorized as Autotrophic (self-feeders) or Heterotrophic (other-feeders).

1. Autotrophic (Self-feeders)

• Photoautotrophic: Use sunlight energy and fix CO2 to build organic matter (e.g., cyanobacteria, green sulfur, purple sulfur bacteria).
• Chemoautotrophic: Use chemical energy from inorganic compounds and fix CO2 (e.g., nitrifying bacteria).

2. Heterotrophic (Other-feeders)

• Saprophytic: Decompose dead organic matter (e.g., Bacillus subtilis).
• Parasitic: Live on or in a host and may cause disease.
• Symbiotic: Mutually beneficial with a host (e.g., Rhizobium in root nodules, fixing nitrogen for plants while receiving sugars).

6. Reproduction – Binary Fission

• Binary Fission: Asexual reproduction where one cell divides into two identical daughter cells; this is the primary method.
• Horizontal Gene Transfer: Genetic material can be exchanged between cells via:
  • Conjugation: Direct cell contact.
  • Transformation: Uptake of free DNA from the environment.
  • Transduction: Via bacteriophage viruses.

7. Motility

• Flagellar Movement: Many bacteria are motile, propelling themselves through liquid environments using flagella (long, whip-like tails). Most bacilli and spirilla use flagella to swim. Movement by flagella is called swarming when many bacteria move together.
• Gliding/Twitching: Some bacteria slide across surfaces without flagella or twitch using pili (short, hair-like structures) to attach to a solid surface and then retract, pulling the bacteria forward.
• Gliding: Bacteria secrete a slimy substance that helps them glide smoothly over surfaces, similar to twitching but smoother and less jerky.
• Sliding: Caused by the expansion of the bacterial population as cells divide and push against each other.

8. Spore Formation (Endospores)

• Certain Gram-positive genera (notably Bacillus and Clostridium) can form endospores.
• Endospores are metabolically dormant, structurally complex survival structures that resist extreme heat, desiccation, radiation, and most chemical disinfectants.
• They can remain viable for centuries. This is a survival mechanism, not a reproductive process.
• They enable bacteria to "survive" in harsh environmental conditions like dry, elevated temperatures, UV radiation, low/no nutrients, strong acids/bases, and chemical disinfectants.
• Endospores are of great practical importance in food, industrial, and medical microbiology because several species of endospore-forming bacteria are dangerous pathogens.

9. Structure of Bacteria

A. External Structures of Prokaryotic Cells

• Glycocalyx
• Flagella
• Fimbriae and pili

I. Glycocalyx (Capsule or Slime Layer)

• A gelatinous outer layer composed mainly of polysaccharides (sometimes polypeptides) secreted by the bacterium.
• Capsule: Organized, repeating units of organic chemicals firmly attached to the cell surface.
• Slime layer: Loosely attached to the cell surface, used by prokaryotic cells to adhere to external surfaces.

Functions:

• Protection against desiccation and phagocytosis by host immune cells.
• Enhances virulence in pathogenic bacteria (e.g., Streptococcus pneumoniae).
• Aids attachment to surfaces and facilitates biofilm formation.

II. Flagella (Singular: Flagellum)

• Long, slender, thread-like structures that help in locomotion.
• Composed of three main parts:
  • The basal body (a motor embedded in the cell envelope).
  • The hook (a curved joint).
  • The filament (the long whip-like propeller made of the protein flagellin).
• About half of the bacilli and all spiral and curved bacteria are motile by means of flagella.
• Due to their small diameter, flagella cannot be seen in a light microscope unless a special stain is applied.

Arrangements of Bacterial Flagella: The location and number of flagella vary by species and are useful for bacterial identification.

• Monotrichous bacteria: Have one flagellum, often polar (located at an end).
• Amphitrichous bacteria: Have a single flagellum at each pole.
• Lophotrichous bacteria: Have a cluster of flagella at one or both ends.
• Peritrichous bacteria: Have flagella spread evenly over the whole cell surface.

III. Fimbriae

• Short, hair-like protein projections that mediate adhesion to surfaces, host tissues, and other bacteria.
• Much shorter and more numerous than flagella.
• Example: Neisseria gonorrhoeae uses fimbriae to adhere to the body and cluster cells.

IV. Pili (Singular: Pilus)

• Long hollow tubules used to transfer DNA from one cell to another.
• Necessary for bacterial conjugation; they join two bacterial cells and mediate DNA transfer.
• Also known as conjugation pili or sex pili, encoded by plasmids.

B. Bacterial Cell Wall

• A rigid structure that maintains the characteristic shape of each bacterial cell and protects it from the environment.
• It lies just outside the plasma membrane.
• The major component responsible for its rigidity is peptidoglycan (murein).
• Mycoplasma species completely lack a cell wall and rely on sterols in their membrane for stability.

Functions:

• To prevent rupture or osmotic lysis of the cell protoplast.
• Provides structure and shape, and protects the cell from osmotic forces.
• Assists some cells in attaching to other cells or in eluding antimicrobial drugs.

I. Gram-Positive Cell Wall

• Has a relatively thick layer of peptidoglycan.
• Contains unique polyalcohols called teichoic acids.
• Teichoic acids are thought to stabilize the Gram-positive bacterial cell wall.

II. Gram-Negative Cell Walls

• Have only a thin layer of peptidoglycan.
• The bilayer membrane outside the peptidoglycan contains phospholipids, proteins, and lipopolysaccharide (LPS).

III. Gram Staining

• Bacteria are grouped into two categories based on their response to Gram staining, a technique developed in 1884 by Hans Christian Gram.
• The technique involves staining with crystal violet and iodine, then rinsing with alcohol, and finally counter-staining with safranin.
• Based on cell wall structure, bacteria absorb either the purple (crystal violet) or pink (safranin) dye.
• Gram-positive bacteria: Have many layers of peptidoglycan that retain the crystal violet dye, appearing purple under a microscope.
• Gram-negative bacteria: Have an extra layer of lipid (outer membrane) on the outside of the cell wall, which is stripped by alcohol, causing them to lose the crystal violet and appear pink after safranin counter-stain.

C. Cytoplasmic Membrane

• The Cell (plasma) membrane is a phospholipid bilayer lying just inside the cell wall.
• It is selectively permeable and controls the transport of substances in and out of the cell.
• It houses electron transport chain components for cellular respiration and serves as the site of energy (ATP) production, fulfilling roles that mitochondria perform in eukaryotes, because bacteria lack membrane-bound organelles.

Functions:

• Selective Permeability: Determines what enters and exits the cell. Small, non-polar molecules pass easily; larger molecules require transport proteins.
• Protection: Acts as a physical shield, containing cytoplasm, DNA, and organelles, protecting them from the external environment.
• Cell Communication: Contains receptors that detect chemical signals, allowing cells to "talk" and coordinate activities.
• Cell Recognition: Carbohydrate markers on the cell surface allow cells to identify one another, crucial for immune responses and tissue formation.
• Energy Conservation: Site of generation and use of the proton motive force.

D. Cytoplasmic Constituents of Bacterial Cells

• Cytoplasm
• Genetic material: chromosomes and plasmids (DNA)
• Ribosomes
• Inclusions

I. Cytoplasm

• The cytoplasm or protoplasm is the portion of the cell that lies within the cytoplasmic membrane.
• It is gel-like and contains the chromosome, ribosomes, various macromolecules, and small molecules in a water solution.
• Bacterial cytoplasm lacks membrane-bound organelles such as mitochondria, the Golgi apparatus, or the endoplasmic reticulum.

II. The Bacterial Chromosome or Nucleoid

• Typically a single, closed-circle DNA that is concentrated in a nucleoid region.
• It is not membrane-bound as in eukaryotes.
• It can be replicated in a semi-conservative fashion and passed on to progeny cells.

Function:

• Genetic Material Storage: Holds the single, large, circular, double-stranded DNA molecule (typically 1-10 Mb in size).

III. Plasmid

• Small (usually 1-200 kb), double-stranded DNA molecules that can exist independently of the chromosome.
• Both circular and linear plasmids exist, but most known plasmids are circular.
• Plasmids are capable of autonomous replication.
• They are not essential for normal growth but often carry genes conferring survival advantages, such as antibiotic resistance (R-plasmids), toxin production (virulence plasmids), or metabolic capabilities.
• Plasmids are central to the spread of antibiotic resistance in bacterial populations via horizontal gene transfer.
• Episomes: Some plasmids can integrate into the chromosome and are thus replicated with the chromosome.

Classification by function:

• Conjugative plasmid: Mediate conjugation.
• Resistance factor: Carry antibiotic resistance genes.
• Bacteriocin-coding plasmid: Have genes that code for substances (bacteriocins) that kill other bacteria.

IV. Ribosome

• Bacterial ribosomes are 70S particles (comprising a 50S large subunit and 30S small subunit), distributed throughout the cytoplasm.
• They serve as the sites of protein synthesis (translation).
• Ribosomes and their subunits are identified by their sedimentation rate.
• The ribosomes of prokaryotic cells are smaller and lighter than their eukaryotic counterparts.
• Some types of antibiotics exploit this difference by targeting the prokaryotic form and selectively disrupting bacterial protein synthesis (e.g., tetracycline binds to the 30S subunit and prevents tRNA from adding amino acids).

V. Inclusion Bodies

• Granules or vesicles that store reserve materials, allowing bacteria to survive periods of nutrient scarcity.
• Serve as a reserve for lipids, nitrogen, phosphate, starch, and sulfur within the cytoplasm.

Common examples:

• Glycogen Granules: Polymers of glucose used for carbon and energy storage.
• Polyhydroxybutyrate (PHB): Lipid-like carbon and energy storage polymers.
• Volutin (Metachromatic) Granules: Phosphate reserves used in nucleic acid and ATP synthesis.
• Sulfur Granules: Stored by photosynthetic bacteria (e.g., Chromatium) that use hydrogen sulfide instead of water.
• Gas Vesicles: Protein hulls/shells inflated with gases, providing buoyancy (floatation) in aquatic bacteria.
• Parasporal Crystals: Protein structures in endospore-forming bacilli, of unknown function but toxic to certain insects.
• Magnetosomes: Magnetite (iron oxide) particles in aquatic bacteria, used for orienting and migrating along geomagnetic field lines.
• Carboxysomes: Enzymes for autotrophic CO2 fixation.
• Phycobilisomes: Phycobiliproteins in Cyanobacteria, light-harvesting pigments.
• Chlorosomes: Lipid and protein with bacteriochlorophyll in Green bacteria, light-harvesting pigments and antennae.

10. Location of Bacterial Endospore

The size and position of the endospore within the vegetative cell are characteristic of a species and aid in microscopic identification.

• Terminal: Endospores found at the poles of the cell (e.g., Clostridium tetani).
• Subterminal: Endospores positioned between the center and the pole of the cell (e.g., Bacillus subtilis or Clostridium botulinum).
• Central: Endospores located exactly in the middle of the vegetative cell (e.g., Bacillus cereus).

Medically-Important Endospore-Forming Bacteria

• Bacillus anthracis causes anthrax.
• Bacillus cereus causes food poisoning.
• Clostridium tetani causes tetanus.
• Clostridium botulinum causes botulism.
• Clostridium perfringens causes food poisoning and gas gangrene.
• Clostridium difficile causes antibiotic-induced diarrhea and pseudomembranous colitis.