Bacterial culture and growth

• Bacteria - prokaryotic cells that lack nucleus and membrane bound organelles
• Cell envelope - several layers surrounding the bacterial cell; consists of:

  1. Capsule
  2. Cell wall
  3. Plasma membrane
• Capsule - acts as shield against phagocytosis and helps in adhesion to other cells; it is an important virulent factor
• Lack a capsule = less virulent
• Cell wall

  • on the outher of cell membrane
  • for protection
  • consists of peptidoglycan
• Gram-positive bacteria - has thick peptidoglycan
• Gram-negative bacteria - has thin peptidoglycan
• Plasma membrane - most important layer; encloses the cytoplasm
• Cytoplasm - gel-like susbtance made up of water, cell components, enxymes, and organic molecules
• Plasma membrane is selectively permeable
• Ribosomes - site of protein synthesis
• Ribosomes in cytoplasm - produces proteins destined to remain inside the cell
• Ribosomes attached to plasma membrane - make proteins for cell envelope and get transported outside the cell
• Nucleoid - irregularly shaped region that contains the genetic material or chromosomes
• Chromosome - mostly single circle of double stranded DNA; contains genes needed for protein synthesis
• Plasmids - small double stranded DNA independent of the chromosome; relatively few genes and genetic information is non-essential; confer a selective advantage to the bacteria
• External Structures

  • Pili - hair-like structures that allow bacteria to attach to other cells
  • Fimbriae - smaller/shorter pili
  • Flagella - long, whip-like portrusions that allow movement of bacteria
• Nucleolus - where ribosomal RNA is being produced
• S-layer - variably demonstrated protein layer; gives protection and immunologic function; rigid coat
• Capsule - sticky outermost layer made of polysaccharides
• Peptidoglycan layer - alternating amino sugars cross linked by transpeptidase; thick with teichoic acid
• Gram-negative bacteria has cell wall made up of lipopolysaccharides
• Gram-stain - dye is non-covalently bound to negative charge molecules in the cells and forms a macromolecular complex with teichoic acid
• Gram positive: plasma membrane -> peptidoglycan layer -> capsule
• Gram negative: plasma membrane -> thin peptidoglycan layer -> outer membrane -> capsule
• Cytoplasmic membrane - selectively permeable bilayer made of proteins and lipids
• Pili/Fimbriae - composed of glycoproteins for adhesion to cell surface
• Sex pili - for attachment during conjugation
• Somatic pili - for attachment; Sex pili - for DNA exchange
• Flagella - long, rope-like appendages made up of flaggelin; anchored to basal bodies in the plasma membrane
• Binary fission - the process of bacterial reproduction
• Lag phase

  • 1-3 days
  • No increase in no. of cells
• Log (exponential) phase:

  • No. of cells increase exponentially
  • Generation time doubles
  • Most metabollically active
  • Cell count is proportional to colony count
• Stationary phase:

  • Growth slows down
  • Total no. of cells reaches a maximum
  • Cell count is not equal to colony count
  • Rate of cell division = rate of cell death
• Death phase

  • Rate of cell division < rate of cell death
  • Total depletion of growth requirements
  • Spore formation due to death of cells
• Five main requirements to sustain life:

  1. Nutrients
  2. pH
  3. Oxygen & Carbon Dioxide
  4. Temperature
  5. Salt
• Undemanding eaters - can be cultured on simple media
• Demanding eaters - require more complex media that contain growth factors
• Obligate aerobes - require oxygen
• An example of obligate aerobes is Pseudomonas spp.