Bacterial Structure and Classification Flashcards
Classification of bacteria is largely determined by what?
structure
pathogenic properties of bacteria can be dependent on specific structure such as ___
pili and capsules
the structures unique to bacteria are also the bases and targets of ____
antimicrobial therapy
what are the 2 major shapes of bacteria?
rods (bacilli) and spheres (cocci)
chains of cocci =
streptococci
cocci in pairs =
diplococci
cocci in grape-like clusters =
staphylococci
which type of cocci is most infectious?
staphylococci
comma-shaped bacteria =
vibrio
corkscrew shape bacteria =
spirilli and spirochetes
the bacterial genome usually comprises what?
single, circular DNA chromosome that lacks introns and is not bound by histone proteins
do bacteria have cell walls?
yes
what are bacterial cell walls composed of?
peptidoglycan
the cell wall provides ___ for the bacteria and also serves as ___ for antimicrobial therapy
provides rigidity; major target
how do the cell walls of gram positive and gram negative bacteria differ?
gram positive: very thick, heavily cross-linked, lies external to cytoplasmic membrane, contains teichoic and lipoteichoic acid (up to 50%)
gram negative: thin, lightly cross-linked, lies bw inner cytoplasmic and outer membrane in periplasmic space
the bacterial lipid membrane lacks what?
sterols
what is special about the membrane of gram negative bacteria?
they have 2 lipid membranes
what is the role of pili (fimbrae)?
1) attach bacteria to host cell surfaces (major)
2) ‘sex pilus’ used to transmit genetic material from one bacterium to another during conjugation
what is the role of flagella?
allow motile bacteria to swim
flagella bear what antigen which is used in serotyping?
H
do all bacteria have pili? flagella?
no; no
what are capsules?
external structures composed of either polysaccharide or polypeptides
what is the role of capsules?
protect encapsulated bacteria from phagocytosis
not all bacteria are encapsulated, but nearly all bacterium causing ___ contains a capsule
meningitis
how can capsules be visualized?
by their ability to exclude dyes from immediate proximity of bacterium
what are spores?
dehydrated, dormant forms of bacteria that allow them to survive during harsh conditions
can both gram negative and gram positive bacteria form spores?
only gram positive rods
the cytoplasmic membrane of gram pos/neg bacteria contains many ___ necessary to bring in nutrients.
transport proteins
the outer membrane of gram negative bacteria serves as an additional ___
permeability barrier
is the outer membrane of gram negative bacteria charged? why or why not?
yes, negatively charged which helps bacteria evade phagocytosis, hinder antibiotic uptake, and avoid action of complement
the outer membrane of gram negative bacteria contains ___ to allow import of nutrients
porins
the periplasm is home to ___ some of which destroy antibiotics
degradative enzymes
unique to the outer membrane of gram neg. bacteria is ___, found in the outer leaflet of the membrane.
lipopolysaccharide (LPS)
what does LPS contain?
lipid A (toxic phospholipid, aka endotoxin) core polysaccharide O antigen polysaccharide (major surface antigen of gram neg. bacteria)
lipid A is recognized by the ___ which elicits what response?
innate immune system –> cytokine storm leading to septic shock
the successful pathogenesis by some bacteria depends on their ability to ___
secrete proteins or other substances from the cell
describe the Type I protein secretion system
- members of ABC transporter family
- encoded by all gram neg bacteria
- proteins are directly secreted into EC environment from cytoplasm
what is the significance of the type I secretion system with regards to bacterial pathogenesis?
antimicrobial drugs can be expelled from the cell using these systems, thus promoting resistance to the antibiotic
describe the Type II protein secretion system
- found in all gram negative bacteria
- serve as the general secretory pathway, used to deliver proteins to periplasm and EC spaces
- proteins first secreted to the periplasm before being secreted across outer membrane
describe the Type III protein secretion system
- found only in pathogens
- hydrophobic segments of the Type III
system span cytoplasmic and outer
membranes of gram negative cell as well as
plasma membrane of animal cell
why are type III secretion systems referred to as molecular syringes?
bc their structure allows direct injection of toxins and other virulence factors into cytosol of a targeted animal cell
what are the bases for gram staining?
differing peptidoglycan depths and extent of cross-linking
in gram staining, the extensive cell wall of Gram ___ bacteria retains the crystal violet while the thin layer of peptidoglycan found in Gram ___ bacteria does not.
positive; negative
peptidoglycan is a major target for ___
antimicrobial therapy
Peptidoglycan is composed of a backbone of repeating disaccharides comprising ___
N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG)
what enzyme links the disaccharides in peptidoglycan to form long chains?
transglycosylase
what is attached to the NAM residue of
each disaccharide?
pentapeptide composed of L- and D-isomers of amino acids;
what enzymes are involved in cross-linking?
transpeptidases (penicillin-binding proteins) and carboxypeptidases
how does the cross-linking differ bw gram
positive and gram negative bacteria?
simple bond in Gram negative bacteria, but in Gram positive bacteria, a pentaglycine interpeptide links the polypeptides
NAM-pentapeptides contain a terminal D-alanine pair which is critical for what?
crosslinking process making them a target for antimicrobials
cell wall assembly begins where?
cytoplasm
what are the steps of cell wall assembly that occur in the cytoplasm?
individual NAM-NAG disaccharides linked to a pentapeptide side chain are synthesized and attached via a diphosphate linkage to a lipid carrier
how do the cytoplasmic cell wall components make it out of the cytoplasm?
the lipid carrier is localized in the cytoplasmic membrane and is used to transport individual disaccharide pentapeptide subunits across the membrane to the external face of the cytoplasmic membrane
once external to the cytoplasmic membrane, what happens to the cell wall components?
subunit is incorporated into the growing
cell wall by the action of glycosidases and
transpeptidases
once external to the cytoplasmic membrane, what happens to the lipid carrier? why is this significant?
lipid carrier recycles to
the interior leaflet of the cytoplasmic membrane to recharge with new cargo, but only after dephosphorylation –> dephos. is antimicrobial target
what is lysozyme?
a glycosidase that hydrolyzes the bond bw NAM and NAG, which disrupts the sugar backbone (instead of the pentapeptide cross-links)
where can lysozyme be found?
tears, saliva, in lysosomes of phagocytic cells
what was the first beta-lactam that was discovered?
penicillin
what is a beta-lactam? why is it important?
drugs containing a beta-lactam ring, which structurally resembles the D-alanine D-alanine terminal pair of the pentapeptide side chains; bc of this resemblance, the beta-lactams are bound by the transpeptidases (i.e., penicillin-binding proteins) of the bacteria and prevent cross-linking from occurring
how does the mechanism of action of the glycopeptide vancomycin differ from beta-lactams?
directly binds the D-alanine D-alanine pair while the cell wall subunit is still attached to its lipid carrier, blocking its availability for subsequent incorporation into the growing cell wall.
what is problem with beta-lactams and glycopeptides?
resistance
how do bacteria resist beta-lactams?
the beta-lactam ring is cleaved by beta-lactamases produced in resistant
bacteria; other resistant bacteria produce
mutated transpeptidases that no longer bind beta-lactams
how do bacteria resist glycopeptides like vancomycin?
D-alanine D-lactone is synthesized by bacteria instead of D-alanine D-alanine. The new pair is still recognized by transpeptidases, but not vancomycin
beta-lactams require what for activity?
cell growth
when beta-lactams inhibit the cross-linking of new subunits, cells ___ due to a loss of structural
integrity
lyse
what is the growth property of bacteria most associated with classification?
response to oxygen
aerobic bacteria =
require oxygen and use respiration for growth
anaerobic bacteria =
inhibited or killed in the presence of oxygen and use fermentation exclusively for metabolism
facultative bacteria =
represent the majority of pathogens and use respiration in the presence of oxygen and use fermentation in its absence
microaerophilic baceria =
grow in the absence of oxygen, but grow optimally at low oxygen concentrations (5-10%)
the basis of classification of bacteria includes:
Morphology Arrangement Staining properties Growth properties Fermentation properties Other enzymatic and virulence properties Antigenicity Genotype
