Chapter 9: Antimicrobial Chemotherapy

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Microbiology Exam 2 Review

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Paul Enrich

worked with dyes that specifically bind to microbial cells, reasoned that dyes could selectively destroy pathogens but not harm human cells, developed treatment for syphilis 

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The first antibiotic was ___ and it was rediscovered by ____

penicillin, Alexander Fleming

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Selective toxicity

ability of a chemotherapeutic agent to kill or inhibit a microbial pathogen while providing little to no damage to the host

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Antibiotics are produced by ____

fungi/bacteria

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How is degree of selective toxicity expressed?

  1. Therapeutic dose

  2. Toxic dose

  3. Therapeutic index

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Therapeutic dose

drug level required for treatment of a particular infection

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Toxic dose

drug level at which agent is too toxic for the host

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therapeutic index

ratio of the toxic dose to the therapeutic dose

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The (smaller or larger) the therapeutic index, the better the chemotherapeutic agent?

the larger the index, the better the agent

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narrow spectrum drugs

drugs only effective against a limited variety of pathogens

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broad spectrum drugs

target many different kinds of bacteria

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True or False: Although a cidal agent kills the target pathogen, it may be static at low levels

True: remember the concentration of the microbial agent effects the affectiveness

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minimal inhibitory concentration (MIC)

lowest concentration of a drug that prevent the growth of a particular pathogen

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minimal lethal concentration (MLC)

lowest drug concentration that kills the pathogen

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<p>dilution susceptibility tests</p>

dilution susceptibility tests

MIC and MLC values can be determined be serially diluting the drug and adding consistent number of bacteria

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<p>Disc diffusion assay </p>

Disc diffusion assay

Able to monitor for resistance or susceptibility to multiple antibiotics, includes zone of inhibition

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<p>E-Test</p>

E-Test

Determine MIC on one test, strip has increasing concentrations of antibiotics

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A challenge to develop new antibiotics is finding bacterial _____ or _____ not already targeted

structures or processes

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The most selective antibiotics are those that interfere with _______ synthesis

bacterial cell wall

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The most crucial feature of the penicillin molecule is _____ ?

B-lactam ring (beta lactam ring)

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Transpeptidase

enzyme that forms peptidoglycan cross-links to add new units to a growing bacterial cell wall

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What part of penicillin’s structure resembles the amino acids at the end of the peptidoglycan units side chain, D-alanyl-D-alanine? Hint: it is the terminal end of the penicillin molecule.

B-lactam ring

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How does the B-lactam ring work to block bacterial cell wall formation ?

It’s structural similarity to D-alanyl-D-alanine, the terminal structure of peptidoglycan peptide chains, causes it to bind to transpeptidase blocking cell wall formation

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What does PBP stand for?

Pencillin-binding proteins

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What occurs after penicillin blocks cell wall formation?

Osmotic lysis.

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True or False: Penicillins act only on growing bacteria

True: penicillin act on peptidoglycan synthesis, and therefore are only affective on bacteria actively synthesizing new peptidoglycan

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<p>What does this diagram show?</p>

What does this diagram show?

Transpeptidase (aka PBP) forming peptide bonds/cross-links between parallal strands of peptidoglycan subunits

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<p>What is the function of the light green molecule that is engulfing the blue enzyme? Hint: the blue enzyme is a PBP</p>

What is the function of the light green molecule that is engulfing the blue enzyme? Hint: the blue enzyme is a PBP

The light green represents penicillin’s B-lactam ring preventing transpeptidation and cell wall synthesis

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<p>What does the dark blue A represent?</p>

What does the dark blue A represent?

The A is bacterial autolysins (B-lactam rings increase the activity of bacterial autolysins which contribute to peptidoglycan degredation)

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Penicillinase

enzymes produced by penicillin resistant bacteria

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How do penicillinase work to inactivate penicillin?

They hydrolyze a bond in the B-lactam ring

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What three consequences to bacterial cells are associated with Penicillin?

  1. Inhibition of cell wall production

  2. Disruption of transpeptidase

  3. Swelling and eventually lysis of cell

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What kind of inhibition is demonstrated with penicillin’s B-lactam ring and transpeptidase (PBP) D-alanyl-D-alanine end?

Competitive inhibition

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What is B-lactam on peptidase similar in structure to? Hint: what does it competitively inhibit?

D-alanyl-D-alanine, terminal structure on peptide chains

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vancomycin

glycopeptide antibiotic produced by bacterium streptomyces orientalis

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How is vancomycin blocking peptidoglycan synthesis different than penicllin?

Instead of binding to PBPs like B-lactam antibiotics, vancomycin binds to the enzyme’s substrate itself

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True or False: vancomycin is bacteriacidal only for Gram-negative bacteria

False: Vancomycin is only effective against Gram-positive bacteria because it cannot penetrate the Gram-negative outer membrane

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what does vancomycin bind to?

D-alanyl-d-alanine terminal sequence

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protein synthesis inhibitors

inhibition of protein synthesis by binding bacterial ribosomal proteins or rRNA

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what are two examples of cell wall synthesis inhibitors

Penicillin and vancomycin

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aminoglycosides

bind to 30s ribosomal subunit and stops peptide bond formation

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tetracyclines

four-ring structure with a variety of side chains attached

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Tetracyclines and Aminglycosides target the ___ subunit of the ribosome

30 S

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True or False: Tetracyclines are broad spectrum and bacteriostatic

True: tetracyclines are effective against most bacteria, but do not kill them (only inhibit growth)

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True or False: Aminoglycosides, like Tetracyclines, are bacteriostatic antibiotics

False: Aminoglycosides are bacteriocidal, which means that bacterial cells are killed not just inhibited

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Aminoglycosides treat Gram _______ bacterial cells

Negative

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Metabolic antagonists

drugs act as antimetabolites, as they block the functioning of metabolic pathways as they are structural analogs of the substrates for enzymes in the pathways (competitive inhibition)

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Metabolic antagonists are (broad or narrow) spectrum?

They are broad spectrum and can prevent metabolic activity across various bacteria

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Sulfa drugs (sulfonamides)

used for bacteria that make their own folic acid, these antibiotics prevents the synthesis of purines in bacteria that use PABA to make folic acid (competitive inhibition, drug is structural analog to PABA)

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what enzymes does nucleic acid synthesis inhibition target?

Inhibition of DNA polymerase, topoisomerases, RNA polymerases

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What enzyme(s) specifically do Fluoroquinolones inhibit?

Dna polymerase and topoisomerase

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What enzyme(s) specifically do Rifamycin target?

RNA polymerase

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Why are drugs that work to inhibit nucleic acid synthesis not as selectively toxic as other antibiotics?

Bacteria and eukaryotes do not differ greatly with respect to nucleic acid synthesis

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How do most antiviral drugs work?

inhibiting virus-specific enzymes and virus replication cycle processes

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What are the two types of drug resistance methods?

Intrinsic and Acquired

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Intrinsic resistance

resistance due to a property bacteria already possesses, example: resistanve to B-lactam antibiotics due to lacking a cell wall

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Acquired resistance

Change in the genome of a bacterium that converts it to one that is resistant to an antibiotic, example” acquiring mutations or genetic material

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drug-tolerant bacteria

lack mechanisms for anti-biotic resistance and therefore “ignore” the presence of antibiotics

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Acquired drug resistance mechanisms (5)

  1. destroying the antibiotic

  2. adding modifying groups to the antibiotic

  3. modifying target so it no longer bind antibiotic

  4. pumps antibiotic out of the cell

  5. using alternative biochemical reactions

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How does drug resistance develop?

due to flexibility of genomes, mutations can occur and resistance genes can transfer 

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what is the function of effux pumps?

pumping the drug out of the cell after it has entered, usually drug/proton antiporters

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What are we doing to contribute to antibiotic resistance? (3)

  1. Overprescribing antibiotics

  2. Not taking the correct dosage of antibiotics

  3. Overusing antibiotics in agriculture

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How can we fight against antibiotic resistance?

  1. Co-administering drugs

  2. Synergy between antibiotics of different classes

  3. Developing faster and better diagnostics

  4. New strategies to fight/prevent infections

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What is the difference between an E-test and the Kirby-Bauer assay? (i-clicker question)

An E-test can determine the concentration of antibiotic needed to inhibit growth