Microbiology Notes: Revenge of Microbes (Chapter 5)

Microbes – Ch. 5

*Most bacteria are covered by a rigid cell wall
-Defines shape
-Keeps structural integrity
-Necessary because most bacteria live in liquids
~If H2O flowed into cell – cause it to burst and die
*Antibiotic that impairs strength of cell wall will kill bacterium by causing it to burst
*Main component of cell wall = Complex mesh-like polymer → Peptidoglycan
-Gives cell wall it’s strength
-Hans Christian Gram: Inventor of staining procedure
-Chain link fence
~Glycan strands are cross-linked with chains of amino acids-peptides
-Assembled on surface of cell (travel to outside from cytoplasm)
*Antibiotics penicillin and vancomycin undermine stability of strong meshwork structure by preventing cross-links from forming
-Prevent last step in Peptidoglycan synthesis (cross-linking on outside)
-Need not cross the cytoplasmic membrane and enter the cytoplasm to work; they need only reach
membrane surface

*Gram Positive and Gram Negative
-Blue/Purple stain = Positive
-Pink/Lt. Red stain = Negative
*Gram positive has Peptidoglycan thick cell wall (greater than 30 layers)
*Gram negative has same cell wall (5 or less layers)
-Strengthened by second (outer) membrane
-Not consisted of two layers of phospholipids (phospholipid and lipopolysaccride)
-Open channels in outer membrane (pores) to cytoplasmic membrane = Periplasmic Space
-Openings can prevent larger antibiotics from entering; making gram-negative bacteria more resistant to
certain antibiotics than gram-positive
-Pores can become smaller from porin proteins

*Penicillins, cephalosporins, carbapenems, and monobactams – Mainstays in antibiotic therapy
-Beta-lactam ring: 3 carbon atoms and 1 nitrogen atom = ACTIVE portion of these antibiotics to kill bacteria
-Called → Beta-lactam Antibiotics
*Beta-lactam Antibiotics:
-Kill bacteria because they undermine the structure of Peptidoglycan sufficiently to cause the bacteria to
explode due to internal pressures no longer countered by the Peptidoglycan “girdle”
-Weakened cell wall that lacks stabilizing cross-links cannot contain internal pressure of bacteria, and burst
-Beta-lactam binds to cross-linking enzymes and inactivates them
-In trying to split the Beta-lactam antibiotic, the enzyme gets stuck and then the antibiotic gets stuck to the
enzyme → prevents enzyme from further cross-linking the Peptidoglycan

*Bacteria resistant to Beta-lactam Antibiotics
-Bacteria developed enzyme called Beta-lactamase: splits Beta-lactam ring of penicillin, rending antibiotic inactive
-Beta-lactamase evolved from proteins that catalyze cross-linking of Peptidoglycan (proteins target of penicillin)
*Beta-lactamase: Protein that has evolved in its structure to the point where it is able to complete the reaction, releasing a penicillin molecule with a broken Beta-lactam ring (dispose of penicillin before inactivating cross-linking of pep. Wall)
-Gram-positive = surface of the cytoplasmic membrane on outside, so Beta-lactamase can diffuse away from cell
-Gram-negative = Beta-lactamase trapped btw. Cytoplasmic membrane and outer membrane → “Guard dog”
~Mutate: outer membrane pores so opening is narrower
~Restricts ability of penicillin and similar molecules to diffuse

*Fighting against Beta-lactamases
-Develop forms of penicillin that are no longer attacked effectively by B-lacta.
-Hard for Beta-lactamase to bind to new antibiotics and deactivate them

-Beta-lactamases mutate to degrade new antibiotics (evolve-Extended-spectrum Beta-lactamases)
*Use combinations of antibiotics and inhibitor of Beta-lactamase
-Amoxicillin and inhibitor = clavulanic acid (binding ability)
*Bacteria – no longer used Beta-lactamase to fight antibiotics
-Bacteria with mutant penicillin-binding proteins (PBP)
-Now seen in both gram-positive and gram-negative bacteria

*Vancomycin: Antibiotic inhibited bacterial cell wall synthesis (penicillin)
-Drawbacks –
1. Initial preparations = brown color; number of contaminants
2. Only effective against gram-positive bacteria
~Big, rangy molecule that cannot diffuse through outer membrane porins of gram-negative bacteria
-Initially not used, because wanted a ‘catch-all’ (single antibiotic for gram-positive and gram-negative)
~1960s/1970s – gram-positive bacteria decreased in # of cases

*1980s – Gram-positive bacteria came back (staphylococci and streptococci)
-Explanation = Use of antibiotics that were most effective against gram-negative bacteria allowed gram-positive bacteria to reclaim role in diseases
-Purer preparation (front-line antibiotic)

IMPLICATION: Spectrum of disease-causing bacteria can alter
~Gram-positive to gram-negative (vice-versa)

*How Vancomycin works
-Inhibits cross-linking reaction that finishes construction of the Peptidoglycan cell wall
~Binds to peptides to become part of the cross-linked Peptidoglycan structure
-Interferes with interaction → Stops enzymes from working
~Physically prevents enzymes from carrying out cross-linking
*Resistance of Vancomycin
-Involves several genes encoding several proteins that comprise a pathway for changing the Peptidoglycan cross-linking peptides into a form that no longer binds with Vancomycin but will still be cross-linked by bacterial enzymes
~Gene encodes enzyme that degrades original terminal D-alanine-D-alanine part of cross-linking peptide
-VRSA: Vancomycin-resistant Staphylococcus aureus
-Probiotics: preparations of presumably beneficial bacteria that are consumed daily in powder or unpasteurized yogurt
~Do not harm
~Most resistant to Vancomycin (trait cannot be transmitted)
*Protecting Vancomycin from overuse
-Prevent overuse so lifetime of these now-vital antibiotics can be prolonged until a suitable substitute is found
-Importance of constant vigilance to protect front-line antibiotics from abuse in settings other than hospitals

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