Bacterial Structures and Classification

Bacterial Structures/Components

• Bacteria come in various shapes and arrangements, useful for identification
• Bacillus (bacilli, pl.) are rod shaped
• Coccus (cocci, pl.) are spherical
• Curved or spiral shapes include vibrio (curved), spirilla (rigid helix), spirochete (flexible helix)
• Different arrangements exist, such as diplo-, strepto-, staphylo-, and tetrads
• Morphology refers to the shapes and arrangements of bacteria

Bacterial Classification

• Classification organizes living things into groups based on similarities and differences. This is important for identifying known and unknown organisms, and understanding their evolutionary relationships.
• Taxonomy is the science of classification
• Phylogenetics tracks evolutionary descent using genetic material
• Carl Linnaeus developed a hierarchical classification system that grouped organisms based on physical characteristics
• Binomial nomenclature uses two names to identify organisms: Genus species (e.g., Escherichia coli).

Bacterial Structures/Components: Learning Objectives

• Describe the structure and function of bacterial cell components
• Differentiate between different bacterial cell shapes and arrangements
• Apply binomial nomenclature to name bacterial species
• Compare and contrast Gram-positive and Gram-negative bacteria

Bacterial Cell Anatomy

• A bacterial cell has an envelope surrounding the cytoplasm, which contains the cell's components
• The plasma membrane controls the passage of substances into and out of the cell.
• The cell wall is composed of peptidoglycan and prevents the cell from bursting.
• The outer membrane (present only in Gram-negative bacteria) is an additional layer outside the cell wall
• A capsule or slime layer may surround the cell wall.
• Cytoplasmic membrane, wall, and outer-membrane together, constitute cell-envelope
• Ribosomes are responsible for protein synthesis
• Flagella are for motility. Different arrangements exist (e.g., monotrichous, amphitrichous, lophotrichous, peritrichous)
• Pili (singular is pilus) are shorter, thinner filamentous structures for attachment, and conjugation
• Plasmids are small, circular dsDNA fragments that can replicate independently and carry additional genes.
• Nucleoid is the region of the cell containing the circular chromosome.
• Endospores are tough, resistant structures that help bacteria survive harsh conditions.
• Bacteria reproduce through the process of binary fission

Gram-Positive vs. Gram-Negative Bacteria

• Gram-positive bacteria have a thick peptidoglycan layer, stain purple with Gram stain, and lack an outer membrane. They are susceptible to penicillin
• Gram-negative bacteria have a thin peptidoglycan layer and stain pink/red with Gram stain. They have an outer membrane and are not susceptible to penicillin.

Gram Staining

• Gram staining is a process to distinguish between Gram-positive and Gram-negative bacteria.
• Critical in choosing antibiotics (narrow or broad spectrum)

Additional Information

• Microbes are primarily classified by morphological characteristics, differential staining, biochemical tests, and DNA-based methods.
• Human classification is a hierarchical system