MICRO-EXAM 3

Alexander Flemming

1928- made penicillin

Antimicrobials

1. antibioticls/antibacterial-bacteria
2. antivirals- viruses
3. antiparasitic agents- parasites
4. antifungals-fungi

natual antibiotics

produced by living organisms

synthetic antibiotics

created in a lab

semi-synthetic antibiotics

natural antibiotics modified by biochemist

ideal chemotherapeutic agent

1\.Does not induce drug allergy in the host/patient

–10% of the population allergic to penicillin

2\.Does not induce drug resistance in the target organism/pathogen

–Drug resistance is a major problem in the control of disease with chemotherapeutic agents today.

3\.Exhibits a high degree of __**selective toxicity**__;

selective toxicity

Exploitation of differences in cell morphology between target and host organisms

Ex. b-lactam antibiotics target cell wall (peptidoglycan synthesis) of a bacterial cell

chemotherapeutic index

 __**selective toxicity**__;                                   

   possesses a high therapeutic ratio as                         measured by

broad spectrum

less discrimination and targets a wide range of organisms

narrow spectrum

only targets a select group while leaving others unharmed

roles of cell wall synthesis inhibitors

•Major means of bacterial control

•Highly favorable chemotherapeutic indices

•But not without problems

•Hypersensitivity (allergies; CDC figures around 10%)

•Increasing number of resistance mechanisms are constantly evolving in bacteria

CW synthesis inhibitors

1\.b-lactams

  1a. Penicillins

  1b. Cephalosporins

  1c. Monobactams

  1d. Carbapenems

2\.Glycopeptides

3\.Polypeptides

4\.Acid-fast Drugs

peptidoglycan synthesis

•Complex, multi-step process

•Involves a large number of enzymes, structural membrane proteins, transporters, etc.

•Intracellular and extracellular phases

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Transpeptidase (TP)-penicillin binding protein synthesis of cross link

b-lactam antibiotics

mech: competitively bind to transpeptidase and prevent peptidoglycan crosslinking

B-lactams: penicillin

Examples: Penicillin G, Penicillin V

Spectrum: primarily G+ organisms

Additional Information: susceptible to b-lactamase  

B-lactamase

which are enzymes that bind to β-lactam antibiotics and destroy them; this and the limited spectrum of penicillin have led to the creation of many semi-synthetic penicillins

B-lactams: Semi-synthetic penicillin

“-cillin”

Methicillin, Oxacillin, Cloxacillin, Dicloxacillin, Nafcillin

Spectrum: primarily G+

Additional Information: increase resistance to b-lactamases

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Ampicillin, Amoxicillin, Piperacillin

Spectrum: Broad (G+/G-)

Additional Information: often combined with β-lactamase inhibitor

response to B-lactamase

•Inhibitor binds to β-lactamase permanently, enhancing activity and effectiveness of penicillin

–Augmentin/Clavulin is a combination of amoxacillin (broad spectrum, semi-synthetic penicillin) and clavulanate (β-lactamase inhibitor)

B-lactams: Cephalosporin

Examples start with “Cef-”

or “Ceph-”

Cefotaxime   Cephalexin

Ceftriaxone   Cephalothin

Cefuroxime   Cephamycin

Spectrum of B-lactams: Cephalosporin

Spectrum

1st generation: Primarily gram-positive coverage, some gram negative

2nd generation: Improved gram-negative coverage

3rd and 4th generation: Extended to most gram-negative

5th generation: Treatment for MRSA

Additional Information: more resistant to b-lactamases, less likely to produce allergy than penicillin

B-lactams monobactam

Example: Aztreonma

Spectrum: primarily aerobic G-

Additional Information: resistant to β-lactamase, can be nebulized to treat lung infections

B-lactams carbapenem

Examples: Imipenem, Meropenem

Spectrum: Broad (G+ and G-)

Additional Information: tend to be less susceptible to bacterial resistance mechanisms but can be toxic to host, typically reserved for those with drug-resistant pathogens

glycopeptides

Examples: Vancomycin, Dalbavancin, Oritavancin

Mechanism: binds to the D-Ala-D-Ala moiety of side chain, prevents transpeptidases (TP aka PBP) from creating crosslink and transglycosylases (TG) from creating NAM-NAG polymer synthesis

Spectrum: G+

Additional Information: useful against MRSA, can be more toxic than other cell wall inhibitors

polypeptides: bacitracin

Mechanism: interferes with bactoprenol, which transports peptidoglycan precursors across membrane for external synthesis

Spectrum: G+

Additional Information: common in topical antibiotic creams

acid fast cell wall synthesis inhibitors - ioniazid

interferes with synthesis of mycolic acid

acid fast cell wall synthesis inhibitors- ethambutol

interferes with synthesis of arabinogalactan

acid fast cell wall synthesis inhibitors- Cycloserine

Mechanism: interferes with formation of peptidoglycan sidechain formation

Additional Information: can be useful against G+

cell membrane inhibitors

1\.Polypeptides

2\.Ionophores

3\.Bacteriocins

4\.Acid-fast Drugs

Polypeptides: Polymyxins

Examples: Polymyxin B, Polymyxin E (Colistin)

Mechanism: disrupt the membrane and increase permeability, causing cell lysis

Spectrum: G-

Additional Information: Polymyxin B often found in topical antibiotic creams, Polymyxin E is considered a “last-resort” antibiotic for multidrug resistant gram negative bacteria, has increased toxicity to host

pyrazinamide

Mechanism: diffuse into cells and pyrazinamidase converts to pyrazinoic acid, causing destructive physiological changes

Spectrum: Acid Fast

protein synthesis inhibitors

1\.Oxazolidinones

2\.Aminoglycosides

3\.Tetracyclines

4\.Macrolides

5\.Lincosamides

6\.No specific class

  Chloramphenicol

  Mupirocin

  Streptogramin

Ribosome function

polypeptide synthesis

oxazolidinones

Examples: Linezolid, Tedizolid

Mechanism: synthetic peptide that binds to 23S rRNA of 50S subunit and prevents initiation of ribosomal assembly

Spectrum: G+

Additional Information: substitute treatment for drug-resistant *S. aureus*

aminoglycosides

Examples: Amikacin, Capreomycin, Gentamicin, Kanamycin, Neomycin, Paromomycin, Spiramycin, Streptomycin, Tobramycin

Mechanism: irreversible binding to 30S subunit, blocks translation and causes incorporation of incorrect amino acids into proteins

Spectrum: G-

Additional Information: do contain some renal and CNS side effects, Neosporin typically is Neomycin, Polymyxin B, and Bacitracin

tetracyclines

Examples end in “-cycline: Doxycycline, Minocycline, Tetracycline, Tigecycline

Mechanism: binds to 30S subunit and inhibit tRNA from entering A-site

Spectrum: Broad (G+ and G-) as well as cell wall-less

Additional Information: reduced usage due to wide-spread resistance and liver/renal toxicity among other side effects, though still effective against atypical infections and used to treat severe acne, not recommended for pregnant/nursing or children

macrolides

Examples: Azithromycin, Clarithromycin, Erythromycin, Natamycin, Telithromycin

Mechanism: bind reversibly to 23S rRNA in P-site of the 50S subunit and inhibit both peptidyl transferase and translocation

Spectrum: Broad (G+ and some G-)

Additional Information: topical forms exist, have anti-inflammatory properties and can be used to treat diseases like COPD, some of the most prescribed antibiotics in USA

Lincosamides

Examples: Clindamycin

Mechanism: bind to 23S rRNA of 50S subunit and prevent translocation

Spectrum: G+ and anaerobic G-

Additional Information: increases risk of opportunistic *Clostridioides difficile* colitis (aka *Clostridium difficile* or *C. diff.)*

Chloramphenical

Mechanism: bind to 23S rRNA in 50S subunit and prevents peptidyl transferase

Spectrum: Broad (G+ and G-)

Additional Information: severe side-effects (aplastic anemia, bone marrow suppression, neurological damage, kidney, liver, and GI issues), many drug interactions, used in eye drops but rarely preferred IV/orally over safer drugs

mupirocin

Mechanism: bind to tRNAIle and prevents isoleucine incorporation into polypeptide chain

Spectrum: G+

Additional Information: topical antibiotic, used to treat MRSA

streptogramin

Combination drug (Dalfopristin1 and Quinupristin2)

Mechanism: bind to the 50S subunit and prevent elongation (peptidyl transferase1 and translocation2)

Spectrum: Broad (G+ and G-)

Additional Information: often effective at treating vancomycin resistant organisms, bacteriostatic individually but bactericidal when administered together

nucleic acid synthesis inhibitors

1\.Quinolones

2\.Nitroimidazoles

3\.Acid-fast Drugs

quinolones

Examples: Ciprofloxacin, Levofloxacin, Moxifloxacin, Ofloxacin, Nalidixic Acid

Mechanism: target DNA Gyrase and Topoisomerase IV, prevent DNA replication

Spectrum: Broad (G+ and G-)

Additional Information: many side effects, rarely given to children/elderly/pregnant/nursing

nitroimidazoles

Examples end in “-nidazole”: Benznidazole, Metronidazole, Tinidazole

Mechanism: produce ROS that damage DNA, depletes thiol found in many enzymes/cofactors

Spectrum: Anaerobic bacteria as well as some parasitic infections

acid fast nucleic acid synthesis inhibitors -clofazimine

Mechanism: bind to guanine, prevents replication and transcription

acid fast nucleic acid synthesis inhibitors- rifamycin

Examples: Rifampin, Rifaximin

Mechanism: bind to RNA Polymerase, prevents transcription

Additional Information: can be used to treat other select G+ and G- infections

metabolite inhibitors

1\.Antifolates          

  1a. Sulfonamides

  1b. Trimethoprim

Acid-fast Drugs

Antifolates: Sulfonamides

Examples begin with “Sulfa-”: Sulfadiazine, Sulfadoxine, Sulfamethoxazole, Sulfanilamide

Mechanism: competitively inhibit Dihydropteroate Synthetase

Spectrum: Broad (G+ and G-)

Additional Information: Second generation enhance effectiveness of the inhibition and improve pharmacokinetics

antifolates: trimethoprim

Mechanism: competitively inhibits Dihydrofolate Reductase

Spectrum: Broad (G+ and G-)

Additional Information: Sulfonamides and Trimethoprim are often given together as      Co-Trimoxazole to prevent resistance via mutation

dapsone

Mechanism: competitively inhibits Dihydropteroate Synthetase

Spectrum: Acid Fast

Additional Information: has anti-inflammatory properties

bedaquiline

Mechanism: blocks ATP Synthase

Spectrum: Acid Fast

Additional Information: used to treat multidrug-resistant tuberculosis (MDR-TB)

MRSA

Methicillin Resistant *Staphylococcus aureus*

Staphylococcus aureus

•35-40% of the population are carriers for *S. aureus*

–2-3% are MRSA carriers in general population

–5-6% of hospitalized patients have \[or get\] MRSA

–

•\~1.2 million “Staph” infections in the U.S. annually (\~10% become invasive)

–12 million “soft skin/tissue infections”, often undiagnosed

daptomycin (cubicin)

•targets gram-positive cell membrane approved in 2003

–Effective against MRSA

–Potential severe side-effects

–Daptomycin resistant *S. aureus* was first observed in 2005

•Mutations within the cell membrane render the drug ineffective

ceftaroline (teflaro)

•is a beta-lactam that can inhibit PBP2A found in MRSA, approved in 2010

–Risk for hypersensitivity as well as *C. difficile* infection

–Resistance detected in 2011 due to mutations in the PBP2A protein

Why does this resistance evolve bacteria?

1. mutations
2. DNA transfer and genetic recombination


1. transformation
2. transduction
3. conjugation

plasmid promiscuity

bacteria like to combine beneficial genes from multiple plasmids together

conjugation between bacteria in the same organism

given exposure to antibiotics, gut bacteria can become resistant and pass resistance to infectious bacteria

last resort treatment

polymyxins (colistin)

cause of antibiotic resistance

1. over-prescribing of antibiotics
2. patients not finishing their treatment
3. over-use of antibiotics in livestock and fish farming
4. poor infection control in hospitals and clinics
5. lack of hygiene and poor sanitation
6. lack of new antibiotics being developed

prevent drug resistance

1. public awareness
2. sanitation and hygiene
3. antibiotics in agriculture and the environment
4. vaccines and alternatives
5. rapid diagnostic
6. human capitol