Antimicrobial AgentsIdeal Antimicrobial AgentsAntibiotics:Cell Wall Active Agents:Protein synthesis inhibitorsDNA Synthesis and MaintenanceAntibiotic ClassificationUnique microbial targetsCell WallsGram Positive OrganismsGram Negative OrganismsPenicillin “…chance favors only the prepared mind” PasteurPenicillinsSlide 14Transpeptidase ReactionSlide 16Cell Wall Synthesis and PenicillinStereo ChemistrySo Why Doesn’t Penicillin Kill All Bacteria?Beta-lactamasesHumans Fight Back!More Selective Pressure…Bacteria RespondCell Wall Synthesis and MRSAHumans Respond Again!Vancomycin MechanismVancomycin ResistanceSlide 28Protein Synthesis InhibitorsProtein Synthesis InhibitionResistance to protein synthesis inhibitorsSlide 32Quinolone ResistanceDNA SynthesisSulfonamidesAntimicrobialsAntimicrobial AgentsMicro 301Alan J. Lesse, M.D.Ideal Antimicrobial Agents•Only destroys the offending microbe•Targets microbial processes that are not present in the host•Have limited or no toxicity to the host•Long half-life of the drug•Both oral and intravenous formulation•Have little effect on normal host flora•Not be susceptible to microbial resistance mechanismsAntibiotics:•Only effective against bacteria,•Many common classes of antibiotics based on their mechanism of action.Cell Wall Active Agents:•Penicillins•Cephalosporins•Monobactams•Carbepenems•GlycopeptidesProtein synthesis inhibitors•50 s ribosome–Erythromycin–Clindamycin–Chloramphenicol–Streptogramins–Oxazolidinones•30 s ribosomes–Aminoglycosides–Tetracyclines•DNA-dependent RNA-polymerase inhibition–RifampinDNA Synthesis and Maintenance•Sulfonamides and trimethoprim•QuinolonesAntibiotic Classification•Cell Wall Active Agents–Penicillins–Cephalosporins–Monobactams–Carbepenems–Glycopeptides–Lipopetides–Lipoglycopeptides•Protein Synthesis Inhibitors–50 s Ribosome•Macrolides•Ketolides•Lincomycins•Chloramphenicol•Streptogramins•Oxazolidinones•Protein Synthesis Inhibitors –30 s Ribosome•Aminoglycosides•Tetracyclines•Glycylcycline–DNA-dependent RNA-polymerase•Rifampin•DNA Synthesis and Maintenance–Sulfonamides and trimethoprim–QuinolonesUnique microbial targets•Cell Wall–Bacteria make cell wall with peptidoglycan, repeating structure of N-acetylmuramic acid and N-acetylglucosamineCell WallsGram Positive OrganismsGram Negative OrganismsPenicillin “…chance favors only the prepared mind” Pasteur•Discover by Alexander Fleming–Was working on isolating lysozyme from Staphylococcus•It is said he did not believe in chemical disinfection–One floor below, another investigator was working on Penicillium–Plated his cultures but did not place in incubator–Went on vacation–On return, he noticed that there were no bacteria growing near the moldPenicillins•Fleming notices that bacteria are inhibited from growing in the region around a contaminating mold on a bacterial culture plate.http://www.nobel.se/medicine/educational/penicillin/readmore.htmlPenicillins•Fleming notices that bacteria are inhibited from growing in the region around a contaminating mold on a bacterial culture plate.http://www.nobel.se/medicine/educational/penicillin/readmore.htmlTranspeptidase ReactionPenicillins•Binding of the penicillin to the transpeptidase results in acylation of the transpeptidase, causing irreversible inhibition of the transpeptidase. The transpeptidase and other enzymes are members of the protein family of penicillin-binding proteinsCell Wall Synthesis and PenicillinStereo Chemistry•The three-dimensional structure of penicillin looks like d-alanine-d-alanineSo Why Doesn’t Penicillin Kill All Bacteria?•Penicillin doesn’t bind to all penicillin binding proteins•Bacteria have ways of destroying penicillinBeta-lactamases•Beta-lactamases hydrolyze the beta-lactam ring and render the penicillin ineffective. These can be coded on extra-chromosomal elements called plasmids, or on the chromosome.•These beta-lactamases can change the specificity for the antibiotic with one nucleotide change!!! Makes change easy.Humans Fight Back!•Chemists alter the structure of the penicillin and these structures resist destruction via beta-lactamases•Closely related chemical compounds (cephalosporins, monobactams, carbapenems) are isolated or synthesized that resist degradation by beta-lactamasesMore Selective Pressure…•Point 1- you can never sterilize the host.•Point 2- the bugs are always smarter than we are.Bacteria RespondCell Wall Synthesis and MRSAHumans Respond Again!Vancomycin MechanismVancomycin hydrogen bonds with the D-alanine-D-alanine structure and covers the binding site to the PBP2 inhibiting transglycolationhttp://www.omedon.co.uk/vrsa/pg/Vancomycin Resistance•Beta-lactamases do not degrade vancomycin•Not all bacteria use D-alanine-D-alanine on their cell wall precursors•If bacteria exchange genes to synthesize a different type of cell wall precursor (no D-ala-D-ala) then they would be vancomycin resistantVancomycin ResistanceClinical Infectious Diseases 2006; 42:S25–34Protein Synthesis Inhibitors•Target bacterial ribosomes, not human ribosomes:Protein Synthesis Inhibition•30 s ribosomes–Aminoglycosides and tetracyclines•Problems with toxicity•50s ribosomes–Erythromycins, clindamycin, oxazolidinonesResistance to protein synthesis inhibitors•Alter ribosomal binding site•Pump antibiotics out of cellDNA Synthesis and Maintenance•Quinolones alter DNA gyrase-responsible for folding and unfolding the DNA•Quinolones target 2 enzymes–DNA gyrase (gyrA and gyrB)–Topoisomerase IV (parC and parE)–parC mutants may be sensitive to drugs like levofloxacin but provide a background for the 2nd hit and resistance.Quinolone Resistance00.511.522.533.540.5 1 2Percentage of Strains with parC Mutations at Different Levo MIC's (All S)1992-1996(214)1999-2000(528)0150000300000450000Annual Scripts forLevofloxacin199819992001DNA SynthesisSulfonamides•Sulfonamides are structural analogs of PABA that competitively inhibit dihydropteroate synthase. •Organisms sensitive to the sulfonamides require extracellular PABA in order to form dihydrofolic acid which is essential in the production of nucleic acids. •Mammalian cells (and some bacteria) lack the enzymes required for folate synthesis and depend upon exogenous sources of folate; therefore, they are not susceptible to sulfonamides.Antimicrobials•Just a few