Antibiotics: Protein Synthesis, Nucleic Acid Synthesis and Metabolism

Antibiotics: Protein Synthesis, Nucleic Acid Synthesis and Metabolism
Principles and Definitions
Selectivity
Selectivty8 toxicity9
Therapeutic index
Toxic dose/ Effective dose
Categories of antibiotics
Bacteriostatic
Duration of treatment sufficient for host defenses
Bactericidal
Usually antibiotic of choice
Principles and Definitions
Selectivity
Therapeutic index
Categories of antibiotics
Use of bacteriostatic vs bactericidal antibiotic
Therapeutic index better for bacteriostatic antibiotic
Resistance to bactericidal antibiotic
Protein toxin mediates disease – use bacteriostatic protein synthesis inhibitor
Principles and Definitions
Antibiotic susceptibility testing (in vitro)
Minimum inhibitory concentration (MIC)
Lowest concentration that results in inhibition of visible growth
Minimum bactericidal concentration (MBC)
Lowest concentration that kills 99.9% of the original inoculum
Antibiotic Susceptibility Testing
Zone Diameter Standards for Disk Diffusion Tests
Principles and Definitions
Combination therapy
Prevent emergence of resistant strains
Temporary treatment until diagnosis is made
Antibiotic synergism
Penicillins and aminoglycosides
CAUTION: Antibiotic antagonism
Penicillins and bacteriostatic antibiotics
Antibiotics vs chemotherapeutic agents vs antimicrobials
Antibiotics that Inhibit Protein Synthesis
Review of Initiation of Protein Synthesis
Review of Elongation of Protein Synthesis
Protein Synthesis

Microbe Library -American Society for Microbiology
www.microbelibrary.org
Survey of Antibiotics
Protein Synthesis Inhibitors
Mostly bacteriostatic
Selectivity due to differences in prokaryotic and eukaryotic ribosomes
Some toxicity - eukaryotic 70S ribosomes
Antimicrobials that Bind to the 30S Ribosomal Subunit
Aminoglycosides (bactericidal)
streptomycin, kanamycin, gentamicin, tobramycin, amikacin, netilmicin, neomycin (topical)
Mode of action - The aminoglycosides irreversibly bind to the 16S ribosomal RNA and freeze the 30S initiation complex (30S-mRNA-tRNA) so that no further initiation can occur. They also slow down protein synthesis that has already initiated and induce misreading of the mRNA. By binding to the 16 S r-RNA the aminoglycosides increase the affinity of the A site for t-RNA regardless of the anticodon specificity. May also destabilize bacterial membranes.
Aminoglycosides (bactericidal)
streptomycin, kanamycin, gentamicin, tobramycin, amikacin, netilmicin, neomycin (topical)
Spectrum of Activity -Many gram-negative and some gram-positive bacteria; Not useful for anaerobic (oxygen required for uptake of antibiotic) or intracellular bacteria.
Resistance - Common
Synergy - The aminoglycosides synergize with $-lactam antibiotics. The $-lactams inhibit cell wall synthesis and thereby increase the permeability of the aminoglycosides.
Tetracyclines (bacteriostatic)
tetracycline, minocycline and doxycycline
Mode of action - The tetracyclines reversibly bind to the 30S ribosome and inhibit binding of aminoacyl-t-RNA to the acceptor site on the 70S ribosome.

Spectrum of activity - Broad spectrum; Useful against intracellular bacteria

Resistance - Common

Adverse effects - Destruction of normal intestinal flora resulting in increased secondary infections; staining and impairment of the structure of bone and teeth.
Spectinomycin (bacteriostatic)
Mode of action - Spectinomycin reversibly interferes with m-RNA interaction with the 30S ribosome. It is structurally similar to the aminoglycosides but does not cause misreading of mRNA.

Spectrum of activity - Used in the treatment of penicillin-resistant Neisseria gonorrhoeae

Resistance - Rare in Neisseria gonorrhoeae
Antimicrobials that Bind to the 50S Ribosomal Subunit
Chloramphenicol, Lincomycin, Clindamycin (bacteriostatic)
Mode of action - These antimicrobials bind to the 50S ribosome and inhibit peptidyl transferase activity.

Spectrum of activity - Chloramphenicol - Broad range; Lincomycin and clindamycin - Restricted range

Resistance - Common

Adverse effects - Chloramphenicol is toxic (bone marrow suppression) but is used in the treatment of bacterial meningitis.
Macrolides (bacteriostatic)
erythromycin, clarithromycin, azithromycin, spiramycin
Mode of action - The macrolides inhibit translocation.

Spectrum of activity - Gram-positive bacteria, Mycoplasma, Legionella

Resistance - Common
Antimicrobials that Interfere with Elongation Factors
Selectivity due to differences in prokaryotic and eukaryotic
elongation factors
Fusidic acid (bacteriostatic)
Mode of action - Fusidic acid binds to elongation factor G (EF-G) and inhibits release of EF-G from the EF-G/GDP complex.

Spectrum of activity - Gram-positive cocci
Inhibitors of Nucleic Acid Synthesis
Inhibitors of RNA Synthesis
Selectivity due to differences between prokaryotic and eukaryotic
RNA polymerase
Rifampin, Rifamycin, Rifampicin, Rifabutin (bactericidal)
Mode of action - These antimicrobials bind to DNA-dependent RNA polymerase and inhibit initiation of mRNA synthesis.

Spectrum of activity - Broad spectrum but is used most commonly in the treatment of tuberculosis

Resistance - Common

Combination therapy - Since resistance is common, rifampin is usually used in combination therapy.
Inhibitors of DNA Synthesis
Selectivity due to differences between prokaryotic and eukaryotic enzymes
Quinolones (bactericidal)
nalidixic acid, ciprofloxacin, ofloxacin, norfloxacin, levofloxacin, lomefloxacin, sparfloxacin
Mode of action - These antimicrobials bind to the A subunit of DNA gyrase (topoisomerase) and prevent supercoiling of DNA, thereby inhibiting DNA synthesis.

Spectrum of activity - Gram-positive cocci and urinary tract infections

Resistance - Common for nalidixic acid; developing for ciprofloxacin
Antimetabolite Antimicrobials
Inhibitors of Folic Acid Synthesis
Basis of Selectivity
Review of Folic Acid Metabolism
Sulfonamides, Sulfones (bacteriostatic)
Mode of action - These antimicrobials are analogues of para-aminobenzoic acid and competitively inhibit formation of dihydropteroic acid.

Spectrum of activity - Broad range activity against gram-positive and gram-negative bacteria; used primarily in urinary tract and Nocardia infections.

Resistance - Common

Combination therapy - The sulfonamides are used in combination with trimethoprim; this combination blocks two distinct steps in folic acid metabolism and prevents the emergence of resistant strains.
Trimethoprim, Methotrexate, Pyrimethamine (bacteriostatic)

Mode of action - These antimicrobials binds to dihydrofolate reductase and inhibit formation of tetrahydrofolic acid.

Spectrum of activity - Broad range activity against gram-positive and gram-negative bacteria; used primarily in urinary tract and Nocardia infections.

Resistance - Common

Combination therapy - These antimicrobials are used in combination with the sulfonamides; this combination blocks two distinct steps in folic acid metabolism and prevents the emergence of resistant strains.
Anti-Mycobacterial Antibiotics
Para-aminosalicylic acid (PSA) (bacteriostatic)

Mode of action - Similar to sulfonamides

Spectrum of activity - Specific for Mycobacterium tuberculosis
Dapsone (bacteriostatic)

Mode of action - Similar to sulfonamides

Spectrum of activity - Used in treatment of leprosy (Mycobacterium leprae)
Isoniazid (INH) (bacteriostatic )
Mode of action - Isoniazid inhibits synthesis of mycolic acids.

Spectrum of activity - Used in treatment of tuberculosis

Resistance - Has developed
Antimicrobial Drug Resistance
Principles and Definitions
Clinical resistance vs actual resistance
Resistance can arise by mutation or by gene transfer (e.g. acquisition of a plasmid)
Resistance provides a selective advantage
Resistance can result from single or multiple steps
Cross resistance vs multiple resistance
Cross resistance -- Single mechanism-- closely related antibiotics
Multiple resistance -- Multiple mechanisms -- unrelated antibiotics
Antimicrobial Drug Resistance
Mechanisms
Altered permeability
Altered influx
Gram negative bacteria
Antimicrobial Drug Resistance
Mechanisms
Altered permeability
Altered efflux
tetracycline
Antimicrobial Drug Resistance
Mechanisms
Inactivation
-lactamase
Chloramphenicol acetyl transferase
Antimicrobial Drug Resistance
Mechanisms
Altered target site
Penicillin binding proteins (penicillins)
RNA polymerase (rifampin)
30S ribosome (streptomycin)
Antimicrobial Drug Resistance
Mechanisms
Replacement of a sensitive pathway
Acquisition of a resistant enzyme (sulfonamides, trimethoprim)