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Microbial Structure: Cell Envelope Part 2 (Function of Cell Wall (provides…
Microbial Structure: Cell Envelope Part 2
Function of Cell Wall
provides structure and stability for the cell
protection from osmotic pressure change
partially responsible for morphology and arrangement of bacterial cells
more rigid than membrane
thick cell wall=coccus; thin cell wall=bacillus
provides points for attachment (colonization) and receptors for viruses (bacteriophage)
target of antibiotics/ chemotherapy agents
reduce toxicity because human cells don't have cell walls; wont attach human cells
responsible for some disease/symptoms; may contain toxic molecules
important for classification based on antigens on cell wall and gram pos. vs gram neg.
Composition of Peptidoglycan
glycan backbone; NAG and NAM attach through dehydration synthesis; connect through B->1,4 glycosidic linkage
side chain: not every NAM has one
Gram pos: L-alanine, D-glutamic acid, L-lysine. D-alanine (80% w/ side chain)
Gram neg: L-alanine, D-glutamic acid, DAP, D-alanine (35-40% with side chain)
interbridge (crosslink): pentaglycine (5 glycines) connect D-alanine to DAP/L-lysine
direct peptide linkages in gram neg; amino acid spacers in gram pos
Cell Wall: Layers of Peptidoglycan
teichoic acids: embedded in peptidoglycan; makes cell wall more stable; holds cell together when it grows (requires cell wall to break); important for cell adhesion
lipotechoic acid: span entire peptidoglycan and anchor it down into the cell membrane
Gram Staining: purple dye adheres to peptidoglycan (stays in gram positive and washes out of gram negative); red dye stains gram negative
Gram Positive: purple, thick peptidoglycan; Gram Negative: pink, thin peptidoglycan, no techoic acid, inner and outer membrane, LPS
LPS: sugar layer attached to outer membrane of gram negative bacteria
core polysaccharide: consistent in all
O-specific polysaccharide: repeated region specific to species
LOS: no repeating O-antigen (Neisseria spp, Haemophilus spp.)
lipid A: phospholipid embedded in lipid bilayer
Endotoxins: part of outer cell wall
liberated when cell dies and cell wall breaks apart
enter blood stream and cause septic shock; cuts off blood supply to organs
Bacterial Porins: proteins that form a hole in the outer cell membran; not selective; allows things to reach inner membran (gram neg.)
Acid fast: membrane, thin peptidoglycan, thick lipid layer
Mycobacterium and Nocardia
Cell Wall-less
Mycoplasma and Ureaplasma
L forms of bacteria: start with a cell wall and lose it due to environment (antibiotics)
protoplasts: start as gram pos.
spheroplasts: start as gram neg.
no set shape; can't protect themselves; hard to grow in lab
Glycocalyx: material surrounding cell; thick sticky sugar coat
slime layer: temporary, less consistent, can be on some parts of cell and not on others
capsule: organized, permanent, uniform around entire cell, doesn't break off as often; made of polysaccharides and proteins
Glycocalyx functions
protection against dehydration, toxins, bacteriophage infection; prevents white blood cells from grabbing them bc glycocalyx falls off= nothing to hold on to
adherence: cell-cell recognition and interaction depends on attachment; attach to develop disease; form biofilm (increase resistance)
streptococcus mutans: biofilm causes dental plaque
classification from antigens and carbohydrate reserves