3 - Antibiotics 1

Question 1

A burn patient in the ICU i found to be suffering from a serious bloodstream infection caused by a gram-negative aeorbe identified as pseudomonas aeruginosa. Susceptibility tests on the isolate reveal resistance to amoxicillin. The therapy that would like likely be effective in treating that infection is…?

Answer 1

Amoxicillin and Clavulanic acid

It’s a gram negative bacteria so it has a B-lactam ring. Clavulanic acid is a B lactamase inhibitor so administering it with amoxicillin allows it to be effective again.

Question 2

What are antibiotics? What is their clinical use?

Answer 2

Molecules that kill bacteria or inhibit their growth. -Can be of biological origin or of (semi) synthetic origin. Clinical use: inhibit specific cellular processes in bacterial cells. Exhibit toxic effects on bacteria by not on humans (selective toxicity).

Question 3

What is selective toxicity?

Answer 3

Antibiotics exert their activity by inhibiting gene products found in bacterial cells, but not in humans. Ideally such targets are completely absent from humans such as enzymes that make PG or if present in human cells, possess unique properties in bacteria that can be exploited to confer specificity (ribosomes)

Question 4

What are disinfectants toxic to? What are they used for?

Answer 4

Toxic to humans and bacteria (ie bleach) Nonspecific effects Used to eliminate organisms on inanimate objects and surfaces.

Question 5

What are antiseptics toxic to? What are they used for?

Answer 5

Generally toxic for bacteria. Non-specific effects (protein denaturation). Too toxic for systemic effects in humans (peroxides, alcohols) but OK for topical use.

Question 6

Antibiotics can be classified as _______ or ________. Define each term.

Answer 6

Bacteriostatic: inhibit growth of bacteria but do not kill them - rely on immune system to eradicate Bactericidal: kill bacteria directly - important for immunocompromised pts.

Question 7

What is the pharmacology/bioavailability of antibiotics? What is spectrum activity?

Answer 7

Not all antibiotics penetrate all tissues equally. To be effective, an antibiotic need to get to the site of infection at therapeutic levels. Spectrum activity: collection of bacterial species that is susceptible to a given antibiotic.

Question 8

Describe the different spectrums of activity of antibiotics?

Answer 8

Narrow spectrum: effective against a relatively small group of bacteria (ie gram + bacteria) Broad spectrum: effective against a wide range of bacteria (ie gram + and gram -)

Question 9

What is advantage and disadvantage to using broad spectrum antibiotics?

Answer 9

Advantage: can be used when infectious agent is unknown or in an emergency Disadvantage: affects many members of natural microbiota, leading to undesirable secondary effects such as diarrhea and antibiotic resistance.

Question 10

What does susceptibility mean? How do we determine susceptibility?

Answer 10

Bacteria are susceptible if their growth can be inhibited by concentrations of the antibiotic that can be readily achieved at the site of infection. Determinations of susceptibility made from pure cultures from patients.

Question 11

Why do we need to measure susceptibility?

Answer 11

Infectious bacteria must be susceptible for an antibiotic to be clinically effective. Not all isolates of a given bacteria are susceptible to an antibiotic so you need to identify the antibiotic susceptibility profile for that specific isolate to determine treatment.

Question 12

How do we measure susceptibility of bacteria?

Answer 12

We grow the bacteria in a liquid medium to determine: minimum inhibitory concentration (MIC): the lowest concentration of antibiotic that inhibits growth minimum bactericidal concentration (MBC): lowest concentration of antibiotic that kills a defined proportion of bacterial population after specified time. (to be effective an antibiotic must achieve or exceed these at the site of infection in the host).

Question 13

What are toxic side effects associated with: -Tetracycline -Streptomycin -Chloramphenicol -Penecillin

Answer 13

Tetracyclin: discoloration of teeth Streptomycin: auditory damage Chloramphenicol: anemia Penecillin: anaphylactic shock

Question 14

What are two adverse effects that have the potential to occur with all antibiotics?

Answer 14

Alteration of normal flora causing antibiotic associated diarrhea/enterocolitis. (C diff a common cause of this). Selection for antibiotic resistance from heritable changes in bacterial genotype that confers enhanced growth in the presence of an antibiotic.

Question 15

Why is C. difficile an antibiotic associated disease?

Answer 15

Some people may have C diff but at only a small levels, so when they are on antibiotics that kill their normal gut flora, the C diff has a chance to grow and produce toxins A and B that can cause diarrhea.

Question 16

What are the two ways by which antibiotics usually become resistant?

Answer 16

1. Horizontal gene transfer: acquisition of foreign DNA encoding resistance genes; can enable rapid emergence of multi-drug resistant strains 2. Spontaneous mutations: strong selection for growth superimposed on large populations of bacteria can lead to emergence of rare resistant mutations.

Question 17

What are the three basic mechanisms by which bacteria overcome inhibition by antibiotics?

Answer 17

1. Modification (inactivation) of antibiotic molecule itself: cleavage of B-lactams by B-lactamase.
2. Modification (reprogramming) of antibiotic target: point mutations in ribosomes or PBPs.
3. Reduction of antibiotic concentration/prevent access to target: efflux pumps eject abx frmo cell and can have broad specificity.

Question 18

What are the main cellular processes that are targeted by antibiotics?

Answer 18

Peptidoglycan Synthesis RNAP and RNA synthesis Key metabolic reactions Ribosomes and protein synthesis DNA replication RNA replication and repair

Question 19

What is the structure of PG? How does the structure of the cell wall of gram - bacteria impact the effectiveness of antibiotics?

Answer 19

Polymer of alternating sugars (GlcNAc-MurNAc) crosslinked via peptide side chains.

In gram negative bacteria antibiotics need to get through the porins in the outer membrane to get to the PG and have it’s effects. OM is a permeability barrier that restricts the access of antimicrobials into the cell.

Question 20

Why is PG synthesis a good target for antibiotics?

Answer 20

It must be synthesized during cell growth, and it provides critical structural integrity to bacterial cells that protects them for lysis–therefore growing cells will lyse if PG synthesis is inhibited. PG and the enzymes that synthesize it are unique to bacteria. High specificity and low toxicity for host.

Question 21

What are the three stages of PG biosynthesis? What is the structure of peptide side chains in PG?

Answer 21

1. Synthesis of MurNAc-pentapeptide precursors in the cytoplasm.
2. Lipid linkage and transport of disaccharide precursors across the membrane
3. Polymerization and cross-linking of precursors into PG occurs extracellularly.

Peptide side chains end in D-ala D-ala

Question 22

What are the key enzymes in peptidoglycan synthesis called?

Answer 22

Penicillin binding proteins (PBPs): catalyze the polymerization and cross linking of peptidoglycan

Question 23

How does the B-lactam family of antibiotics work?

Answer 23

B-lactams bind penicillin binding proteins (PBPs) and inactivate them to prevent peptidoglycan crosslinking of the peptide side chains (bactericidal). This inhibits bacterial growth. (all four types of B-lactams do this)

Question 24

What are the four basic types of B-lactams?

Answer 24

Penicillin

Cephalosporin

Carbapenem

Monobactam

Question 25

What does the structure of B-lactams mimic?

Answer 25

The natural substrate of penicillin binding protiens (PBPs)

Question 26

How do the four basic types of B-lactams differ? What effect does this have?

Answer 26

By modification at the R groups, this alters properties of the antibiotic and alters properties like spectrum of activity or pharmacology, but NOT the mechanism of action.

Question 27

B-lactams can cause ______ in some people, possibly severe.

Answer 27

Hypersensitivity (allergic reactions).

Question 28

What drugs are in the penecillin family of B lactams?

Answer 28

End in “cillin”

Penicillin

Ampicillin

Amoxicillin

Methicillin

Oxacillin

Ticarcillin

Piperacilin

Question 29

What drugs are in the Cephalosporin family of B-lactams?

Answer 29

Start with “cef” or “cepha”

Cefazolin

Cephalexin

Cefuroxime

Cefoxitin

Ceftriaxone

Ceftazimide

Cefepime

Question 30

What drugs are in the Carbapenem family of B-lactams?

Answer 30

End in “enem”

Imipenem

Meropenem

Ertapenem

Doripenem

Question 31

What drug is in the Monobactam family of B-lactams?

Answer 31

Aztreonam

Question 32

What is the primary mechanism of resistance to B-lactam antibiotics? What bacteria has this encoded in their genome? In what type of bacteria are these normally found?

Answer 32

Production of an enzyme caleld a B-lactamase that catalyzes the enzymatic inactiation of B-lactam antibiotics that cleave the B-lactam ring of the antibiotic and make it ineffective.

Psuedomonas aeruginosa has this in their chromosome and it can be induced by present of an antitbiotic.

Normally found in gram - bacteria: (ESBL’s) extended spectrum B-lactamases with broad specificities for B-lactams have been found.

Question 33

Besides B-lactamase, what is a second mechanism of resistance to B-lactam antibiotics?

Answer 33

Reduced permeability that prevents B-lactam antibiotics from accessing PBPs (penecillin binding proteins)

This is intrinsic resistance found in some gram -

Mutations in porins can reduce access to periplasmic space.

Question 34

Other than B-lactamases and reduced permeability, what is a third mechanism of resistance to B-lactam antibiotics? Where is this commonly found?

Answer 34

Altered PBPs (penicillin binding proteins) that prevent binding of B-lactam antibiotics.

Commonly found in methicillin resistant staph aureas (MRSA) which is the aquisition of low-affinity PBP2a, encoded by mecA on the mec cassette. This variant is not inactivated by B lactam antibiotics.

Question 35

What can be done to treat bacteria that make B-lactamase? What are names of these drugs?

Answer 35

B-lactamase inhibitors can be given with B-lactam antibiotics to make them more effective: clavulanic acid, sulbactam, and tazobactam.

These bind to B-lactamases and are released very slowly (forms inactive complex), which inhibits B-lactamase activity.

Question 36

B-lactam antibiotics work by _____? What type of bacteria are they affective against?

Answer 36

Inactivating PBPs and are affective against gram + and gram - bacteria.

Question 37

What is the difference between how gram + and gram - develop resistance?

Answer 37

Gram +: predom through altered PBPs (nothing done to drug, just prevent drug from binding to targets.

Gram -: we see B lactamases that destroys the B lactam drugs and lets normal PBPs do their job to make PG.

Question 38

What is the difference between how we overcome resistance of gram + and gram - bacteria?

Answer 38

Gram - overcome: use drug combo that includes B lactam drug with B lactamase inhibitor

Gram +: we are hosed. All you can do is try a different drug.