Bacterial Structure Flashcards
prokaryotes lack _____ and divide via _____
lack nuclear membrane and organelles
divide via transverse fission (2 identical daughters)
shapes:
bacilli =
cocci =
spirochetes =
shapes:
bacilli = rods
cocci = spheres (like berries)
spirochetes = spirals
what are mreB and parM required for in bacteria?
actin homologues required for chromosome/plasmid segregation
the 3 layers of the bacterial envelope:
- plasma membrane (lipid bilayer + proteins)
- cell wall (peptidoglycan)
- structures outside wall
protects, provides virulence, target of antibodies
where does respiratory metabolism occur in bacteria?
plasma membrane of bacteria contains respiratory chain equivalent to mitochondrial inner membrane
how are peptidoglycans assembled in bacteria?
- NAM (N-acetylmuramic acid) and NAG (N-acetylglucosamine) disaccharide assembled with peptide in cytosol on lipid carrier
- exported across plasma membrane, assembled into long chains
- chains cross-linked via peptide bonds between side chains —> essential for strength and osmotic protection
the amino acids in peptidoglycan are D or L amino acids?
D amino acids are found in prokaryotes
[L amino acids are found in eukaryotes]
how does penicillin work?
penicillin is analogue of D-ala-D-ala of peptidoglycan side chain —> binds active site of cross-linking enzyme and forms irreversible covalent bond
new peptidoglycan is formed without cross-links —> osmotic lysis
*note that penicillin works on growing cells only, which are creating new peptidoglycan formation
with Gram staining, which bacteria stain purple, which pink? explain why
Gram+ = purple, due to thick peptidoglycan layer which holds in dye (Crystal violet or Gentian)
Gram- = pink, due to thin peptidoglycan layer which cannot hold dye (pink safranin dye is added later so you can see bacteria at all)
what is the general procedure of gram staining?
- heat fix bacteria (polysaccharides become sticky)
- stain with Gentian or Crystal violet
- add iodine, which complexes with dye (Gram+ purple are fixed - thick peptidoglycan wall)
- decolorize to get dye out of Gram- (thin peptidoglycan wall)
- counterstain with safranin (pink dye, fixes Gram-)
what are 4 components you will find in gram+ bacteria envelope?
- thick peptidoglycan
- fimbriae/pili: adhere to mammalian cells and ETC matrix (also in Gram-)
- teichoic acid: polymers of sugar alcohols linked by phosphate groups
- group carbohydrates (Lancefield carbohydrates): linked to peptidoglycan on some Gram+ bacteria
what are the 3 layers of Gram- bacteria envelope? what do they contain?
- outer plasma membrane with LPS attached (highly impermeable)
- periplasm: space between outer/inner plasma membrane, contains thin peptidoglycan
- inner plasma membrane containing respiratory chain complexes
what are the 3 components of lipopolysaccharide (LPS), found attached to the outer plasma membrane of gram- bacteria?
- O antigen (sugar repeats, long length keeps MAC of complement far away from membrane)
- core (oligosaccharide)
- lipid A (endotoxin, disaccharide + fatty acids)
what is the structure of porin proteins, which allow solutes to traverse the outer plasma membrane of gram- bacteria?
monomer is a cylinder, walls of beta sheets, that is made into trimmer
hydrophobic exterior, hydrophilic interior
pore diameter determines ability of antibiotics to enter cell
which part of LPS prevents destruction from MAC complex of complement, and how does it do so? (on Gram- bacteria outer plasma membrane)
O antigen: polymer of 3-5 sugars
long length keeps MAC far away from membrane (can’t reach)
endotoxin vs exotoxin vs enterotoxin
endotoxin: polysaccharide, part of cell wall
exotoxin: secreted protein
enterotoxin: exotoxin in the GI
how does Lipid A endotoxin of LPS in Gram- bacteria cause toxicity?
TLR receptors on monocytes bind LPS —> inflammatory cytokines
in septicemia (bacteria in bloodstream) - systemic reaction —> septic shock, organ dysfunction, death
*note that Lipid A endotoxin is released only when cell lyses and present in dead or alive bacteria (not removed via sterilization)
what is a Jarish-Herxheimer reaction?
reaction (fever, hypotension) to Lipid A endotoxin of Gram- bacteria (part of LPS in outer membrane) following antibiotic treatment (causes endotoxin release)
remember that Lipid A is only released when cell lyses and is present in both living and dead bacteria (not removed via sterilization)
what is the function of flagella vs pili vs conjugation pili in bacteria?
flagella (H antigen) = locomotion
pili (fimbrae) = adhesion
conjugation pili = DNA transfer
how does flagella of bacteria provide locomotion?
basal body rotates in envelope via H+ influx
rotating flagellum propels cell, and reversing direction changes direction
(counter-clockwise = smooth, clockwise = tumbling)
why are bacterial fimbriae/pili so important for virulence?
protein fimbriae/pili bind membrane proteins of mammalian cells and ETC matrix
covalently linked to peptidoglycan, appear as fuzz in EM
capsules protect bacteria from phagocytes - what are they made of?
K antigen, mostly high molecular weight polysaccharides
often many antigenic types per species
not visible in Gram stain, requires EM
bacterial spores are formed in response to ____ and are made only by some ____ bacteria
formed in response to starvation, only by some Gram+ rods
specialized cells for dispersal, metabolically inert, omnipresent in soil/plants, very resistant (requires autoclave to destroy)
break open in rich media and cell continues to divide
selective vs differential media for growing bacteria?
selective: selects for or against certain types (contain antibiotics or missing key nutrients, etc)
differential: distinguishes between types of bacteria (pH indicators for formation, hemolysis on blood plates, etc)
alpha vs beta vs gamma hemolysis on blood agar plates
also, what kind of plates are blood agar plates?
blood agar: 5% sheep blood, differential media (looks for capability to lyse RBC)
alpha hemolytic: partial hemolysis, green
beta hemolytic: complete hemolytic, yellow
gamma hemolytic: no hemolysis, no color change
how is MacConkey agar both selective and differential?
MacConkey agar: contains bile salts, crystal violet, pH indicator
selective: grows only Gram- bacteria (bile salts and crystal violet)
differential: pH indicator differentiates lactose fermenting vs non-lactose fermenting bacteria
describe how mannitol salt agar is both selective and differential?
mannitol salt agar: contains mannitol, high NaCl concentration, pH indicator dyes
selective: many bacteria cannot survive high salt environment
differential: only certain bacteria can metabolize mannitol
