Bacteriology Flashcards
Prokaryotic bacterial cell
- Bacteria are unicellular
- Lacks membrane-bound nucleus
- No membrane-bound organelles
- Multiply by binary fission
Size, shape, arrangement of bacterial cells
- Most bacteria are 0.2 to 2 um in diameter, 2 to 8 um in length
- Morphology: cell shape
- Major cell morphologies:
- Coccus: spherical or ovoid(strepto/staphylo)
- Rod: cylindrical shape(bacillus)
- Spirilla: sprial shape
- Many variations on basic morphological types; pleomorphic bacteria can assume several shapes
Cytoplasmic Membrane in bacteria
- Cytoplasmic membrane: thin structure that surround the cell
- 6-8 nm thick
- vital barrier that separates cytoplasm from environment
- highly selective permeable barrier; enables concentration of specific metabolites and excretion of waste products
Composition of membranes
- General structure is phospholipid bilayer
- Contain both hydrophobic and hydrophilic components
- Can exist in many different chemical forms as result of variation in groups attaced to glycerol backbone
- Fatty acids point inward to form hydrophobic environment; hydrophilic portions remain exposed to external environment or cytoplasm
Major functions of cytoplasmic membrane
- Prevents leakage and function as gateway for transport in and out of cell
- Site of many proteins involved in transport, bioenergetics, and chemotaxis
- Site of generation and use of proton motive force
Peptidoglycan
- Rigid layer that provides strength to cell wal
- Polysaccharide composed of N-acetylglucosamine and N-acetylmuramic acid
- Amino acids
- Lysine or diaminopimelic acid(DAP)
Bacterial cell wall
- Semi-rigid structure gives shape
- Protects cell from rupture die to osmotic stress
- Composed of peptidoglycan: repeating units of NAG and NAM linked by peptide bridges
- Peptide composition and thickness vary among species
Peptidoglycan
Gram negative: composed of glycan backbone, L-ala, D-glu, DAP, - D-ala
Gram positive:
composed of glycan backbone, L-ala, D-glu-NH2, L-lys, D-ala, links next chain with Gly inter bridge
2 Types of Bacterial Cell Wall
- Gram-positive
- Gram-negative
Cell Walls of Gram-Positive Bacteria
- Peptidoglycan
- Cytoplasm
- Membrane
Cell Walls of Gram-Negative Bacteria
- Peptidoglycan
- Cytoplasm
- Cytoplasmic membrae
- Periplasm
- Outer membrane(lipopolysaccharide and protein)
Gram-positive cell walls
- Contain up to 90% peptidoglycan
- very thin(<15nm) periplasmic space
- Common to have teichoic acids( - charged attracts Na, Ca, and Mg) embedded in cell wall
- Lipoteichoic acids: teichoic acids covalently bound to membrane lipids
- Provide rigidity to cell wall by attracting cations and role in biofilm formation
Outer membrane of gram-negative bacteria
- Total cell wall contain 10% peptidoglycan
- Most of cell wall composed of outer membrane(lipopolysaccharide layer)
- Consist of lipid A, core polysaccharide, and O-polysaccharide
- LPS replaces most of phospholipids in outer half of outer membrane
- Endotoxin: toxic component of LPS
- Porins: channels for movement of hydrophilic low molecular-weight substances
- Periplasm: space located between cytoplasmic and outer membranes
- 15 nm wide
- Contents have gel-like consistency
- Houses many proteins
Gram-positive/negative cell walls
Gram positive: Thick PG(10-100nm), Teichoic acid linked to PG, Single membrane(only cytoplasmic), Lipoteichoic acid anchored to membrane, Very thin
- Gram-negative: Thin PG, No TA or LTA, inner membrane and outer membrane, periplasmic space, lipopolysaccharide
Structures external to cell wall
- Capsule
- Flagella
- Fimbriae
Capsules and slime layers
- Polysaccharide layers
- Assist in attachment to surfaces
- Aid in evasion of immune system
- Resist desiccation
Fimbriae
- Filamentous protein structures
- Enable organisms to stick to surfaces or form pellicles
Pili
- Filamentous protein structures
- Typically longer than fimbriae
- Assist in surface attachment
- Facilitate genetic exchange between cells
- Type IV pili involved in twitching motility
Gram Stain
- Bacterial suspension dried on glass slide
- Crystal violet for 1 minute, wash off
- Iodine 1 minute, wash off
- 95% alcohol for 10 seconds. wash off(alcohol removes lipids in outer membrane in gram positive)
- Safranin for 30 seconds(stains cell red in gram negative)
- Gram positive = blue/violet; gram-negative = red
Gram stain
- Crystal violet + iodine = I stabilizes crystal violet
- In G-, de-colorizer dissolves lipid layer, diffuses through cell wall and decolorizes crystal violet. 2nd stain taken up
- In G+, wall is too thick to allow uptake of de-colorizing agent and remains violet
Gram positive and gram negative bacteria
- G+: mostly cocci(staphylococci, streptococci, pneumococci, enterococci), some bacilli(bacillus, clostridia, corynebacteria)
- G-: Mostly bacilli(E. coli, H. pylori, Salmonella, V. cholerae). Some cocci(neisseria, moraxella)
