Bacterial Structure

Question 1

prokaryotes lack _____ and divide via _____

Answer 1

lack nuclear membrane and organelles

divide via transverse fission (2 identical daughters)

Question 2

shapes:
bacilli =
cocci =
spirochetes =

Answer 2

shapes:
bacilli = rods
cocci = spheres (like berries)
spirochetes = spirals

Question 3

what are mreB and parM required for in bacteria?

Answer 3

actin homologues required for chromosome/plasmid segregation

Question 4

the 3 layers of the bacterial envelope:

Answer 4

1. plasma membrane (lipid bilayer + proteins)
2. cell wall (peptidoglycan)
3. structures outside wall

protects, provides virulence, target of antibodies

Question 5

where does respiratory metabolism occur in bacteria?

Answer 5

plasma membrane of bacteria contains respiratory chain equivalent to mitochondrial inner membrane

Question 6

how are peptidoglycans assembled in bacteria?

Answer 6

1. NAM (N-acetylmuramic acid) and NAG (N-acetylglucosamine) disaccharide assembled with peptide in cytosol on lipid carrier
2. exported across plasma membrane, assembled into long chains
3. chains cross-linked via peptide bonds between side chains —> essential for strength and osmotic protection

Question 7

the amino acids in peptidoglycan are D or L amino acids?

Answer 7

D amino acids are found in prokaryotes

[L amino acids are found in eukaryotes]

Question 8

how does penicillin work?

Answer 8

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

Question 9

with Gram staining, which bacteria stain purple, which pink? explain why

Answer 9

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)

Question 10

what is the general procedure of gram staining?

Answer 10

1. heat fix bacteria (polysaccharides become sticky)
2. stain with Gentian or Crystal violet
3. add iodine, which complexes with dye (Gram+ purple are fixed - thick peptidoglycan wall)
4. decolorize to get dye out of Gram- (thin peptidoglycan wall)
5. counterstain with safranin (pink dye, fixes Gram-)

Question 11

what are 4 components you will find in gram+ bacteria envelope?

Answer 11

1. thick peptidoglycan
2. fimbriae/pili: adhere to mammalian cells and ETC matrix (also in Gram-)
3. teichoic acid: polymers of sugar alcohols linked by phosphate groups
4. group carbohydrates (Lancefield carbohydrates): linked to peptidoglycan on some Gram+ bacteria

Question 12

what are the 3 layers of Gram- bacteria envelope? what do they contain?

Answer 12

1. outer plasma membrane with LPS attached (highly impermeable)
2. periplasm: space between outer/inner plasma membrane, contains thin peptidoglycan
3. inner plasma membrane containing respiratory chain complexes

Question 13

what are the 3 components of lipopolysaccharide (LPS), found attached to the outer plasma membrane of gram- bacteria?

Answer 13

1. O antigen (sugar repeats, long length keeps MAC of complement far away from membrane)
2. core (oligosaccharide)
3. lipid A (endotoxin, disaccharide + fatty acids)

Question 14

what is the structure of porin proteins, which allow solutes to traverse the outer plasma membrane of gram- bacteria?

Answer 14

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

Question 15

which part of LPS prevents destruction from MAC complex of complement, and how does it do so? (on Gram- bacteria outer plasma membrane)

Answer 15

O antigen: polymer of 3-5 sugars

long length keeps MAC far away from membrane (can’t reach)

Question 16

endotoxin vs exotoxin vs enterotoxin

Answer 16

endotoxin: polysaccharide, part of cell wall

exotoxin: secreted protein

enterotoxin: exotoxin in the GI

Question 17

how does Lipid A endotoxin of LPS in Gram- bacteria cause toxicity?

Answer 17

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)

Question 18

what is a Jarish-Herxheimer reaction?

Answer 18

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)

Question 19

what is the function of flagella vs pili vs conjugation pili in bacteria?

Answer 19

flagella (H antigen) = locomotion
pili (fimbrae) = adhesion
conjugation pili = DNA transfer

Question 20

how does flagella of bacteria provide locomotion?

Answer 20

basal body rotates in envelope via H+ influx

rotating flagellum propels cell, and reversing direction changes direction

(counter-clockwise = smooth, clockwise = tumbling)

Question 21

why are bacterial fimbriae/pili so important for virulence?

Answer 21

protein fimbriae/pili bind membrane proteins of mammalian cells and ETC matrix

covalently linked to peptidoglycan, appear as fuzz in EM

Question 22

capsules protect bacteria from phagocytes - what are they made of?

Answer 22

K antigen, mostly high molecular weight polysaccharides

often many antigenic types per species

not visible in Gram stain, requires EM

Question 23

bacterial spores are formed in response to ____ and are made only by some ____ bacteria

Answer 23

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

Question 24

selective vs differential media for growing bacteria?

Answer 24

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)

Question 25

alpha vs beta vs gamma hemolysis on blood agar plates

also, what kind of plates are blood agar plates?

Answer 25

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

Question 26

how is MacConkey agar both selective and differential?

Answer 26

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

Question 27

describe how mannitol salt agar is both selective and differential?

Answer 27

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