Bacterial Cell Structure and Morphology

Bacterial Morphology

• Cell morphology refers to the shape and arrangement of a cell's parts
• Bacteria can have various shapes, including:
  • Coccus (spherical)
  • Bacillus (rod-like)
  • Pleomorphic (many shaped)
  • Spirillum (spiral)
  • Filamentous (branched)
• Most human pathogen bacteria are between 0.2-5 μm in size
• Cell morphology does not predict physiology, ecology, or phylogeny of a prokaryotic cell
• Cell morphology can vary depending on growth conditions, such as optimization for nutrient uptake or swimming motility

Cell Morphology vs Colony Morphology

• Cell morphology refers to the shape of individual cells when examined by light microscopy
• Colony morphology refers to the shape and characteristics of colonies when grown on petri plates
• Examples of colony morphology include:
  • Staphylococcus aureus: large, opaque, round, and creamy white to yellowish colonies
  • Proteus sp.: swarming behavior on blood agar
  • Bacillus cereus: "ground-glass" colonies displaying beta-hemolysis on blood agar

Cell Wall

• The cell envelope consists of the cytoplasmic membrane and the cell wall
• The cell wall provides structural integrity to the cell and protects it from internal turgor pressure
• The cell wall is composed of peptidoglycan (also known as murein), a rigid layer that provides strength and shape
• Peptidoglycan is a polysaccharide composed of poly-N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM)
• Peptidoglycan is exclusive to bacterial cells and is located outside the cytoplasmic membrane

Cell Wall and Antibiotics

• Several antibiotics block cell wall synthesis, which is required for bacterial survival and absent in eukaryotes
• This selective toxicity allows antibiotics to target bacterial cells without harming human cells

Types of Cell Wall

• There are two main types of cell walls:
  • Gram-negative: two layers, including lipopolysaccharide (LPS) and peptidoglycan
  • Gram-positive: one layer, consisting of peptidoglycan

Gram-Positive Cell Wall

• The gram-positive cell wall is composed of a thick, multilayer of peptidoglycan (~95%)
• Teichoic acids, which are polymers of modified ribose or glycerol connected by phosphates, are embedded in the peptidoglycan layer
• Lipoteichoic acids, which are teichoic acids covalently bound to membrane lipids, are also present
• Teichoic acids have antigenic properties and can determine the serotype of bacteria

Cell Division

• Cell division involves the growth and extension of cell wall components, followed by the production of a septum to divide the daughter cells
• The septum consists of two membranes separated by two layers of peptidoglycan
• Septum formation is initiated at midcell, at a site defined by protein complexes affixed to a protein filament ring that lines the inside of the cytoplasmic membrane

Rate of Growth

• Bacterial populations undergo exponential (logarithmic) growth
• The generation time is the time required for microbial cells to double in number

Growth Cycle or Phases of Growth

• Bacteria in a broth culture can be modeled with 4 different phases:
  • Lag phase: interval between inoculation and beginning of growth
  • Log phase (exponential): period characterized by doubling in cell number
  • Stationary phase: growth rate of population is zero, and growth rate and death are equal
  • Death phase: bacteria run out of nutrients and die

Bacterial Physiology

• Physiological characteristics of bacteria are encoded in their genetic material
• Physiological properties are affected by environmental factors, such as:
  • Atmosphere: oxygen requirements, including aerobic, anaerobic, facultative, and aerotolerant anaerobic bacteria
  • Temperature: psychrophile, mesophile, and hyperthermophile bacteria
  • pH: neutrophiles, acidophiles, and alkalophiles
  • Nutrition: bacteria require a source of energy and raw materials to build proteins, structures, and membranes

Bacterial Metabolism

• Catabolism: energy-releasing metabolic reactions that break down molecules into smaller units
• Anabolism: set of metabolic pathways that construct molecules from smaller units, requiring energy and often powered by the hydrolysis of ATP
• Bacterial respiration can be aerobic or anaerobic, using oxygen or alternative terminal electron acceptors
• Aerobic respiration uses the tricarboxylic acid (TCA) cycle and produces more ATP than anaerobic respiration
• Anaerobic respiration uses alternative terminal electron acceptors and produces less ATP
• Fermentation is a type of anaerobic respiration that uses substrate-level phosphorylation to directly synthesize ATP from an energy-rich intermediate