Action of penicillins

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Action of penicillins

• 1. mode of action
• 2. family of ß lactams
• 3. route of traverse across cell envelope

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1. Antibacterial agents

• Cell wall
• Cell membrane
• Intracellular metabolism
• Nucleic acid biosynthesis
• Protein synthesis
• New targets genomics

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Selective toxicity

• Ability of a biological agent to inhibit or
  arrest a pathogen without injury to the host
• unique targets in procaryotes
• Bactericidal
• kills
• Bacteriostatic
• inhibitory host defences

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Antibiotic

• Chemical substance produced by one spp
• Inhibits in small amounts growth of another
  micro-organism
• Discovery of penicillin
• A. Fleming
• benzylpenicillin
• Penicillium notatum

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2. ß lactams

• Constrained 4-membered structure
• ß lactam ring
• Thiazolidine ring
• sulphur-containing
• 5- or 6-membered ring
• Substututions
• free amino group ß lactam ring
• free carboxyl group thiazolidine ring

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ß lactams

• Penicillins
• ampicillin, amoxicillin, methicillin
• Cephalosporins
• 1 cephalothin, cephaloridine, cephalexin
• 2 cephamandole, cefuroxime, cefaclor
• 3 cefotaxime, ceftriaxone, ceftazidine

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ß lactams

• Different properties to inhibit different m-o
• Diffuse across OM / porins
• Cleavage of ß lactam ring
• ß lactamase ? inactivated antibiotic
• Targets
• enzymes of peptidoglycan biosynthesis
• PBPs

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PBPs

• Single target ? multiple targets
• Expt 1
• Growing E. coli 14C-penicillin
• Pen binds to target(s)
• Collect cells lysate identify labelled material
• SDS-PAGE / autoradiography
• Multiple proteins 14C-penicillin
• All 14C-proteins are in CM

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PBPs

• Isolation affinity chromatography
• Inert matrix penicillin
• Expt 2
• Lysate of E. coli applied to matrixpenicillin
• Some proteins bind to penicillin
• Elute, assay in vitro activities
• PBPs penicillin-sensitive enzymes

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Penicillin-sensitive enzymes
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3. Route across cell envelope

• OM porins
• Periplasm degrading enzymes
• CM enzymes of p/g assembly PBPs

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Desirable properties

• Penetrates OM via porins
• Free diffusion
• Hydrophilic compounds
• Resist ß lactamase
• Bind with high affinity to targets PBPs

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Infectious antibiotic resistance

• ß lactamase
• Plasmid-encoded
• Promiscuous
• inter species transfer intra species transfer
• Proliferation of resistance elements
• Selective advantage

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keywords

• Micro-organisms
• Streptomyces jumojeniensis, actinomycete
• bacteria Proteus vulgaris, Escherichia coli,
  Klebsiella pneumoniae, Serratia marcescens,
  Bacteriodes fragilis

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keywords

• Anti-microbials
• Penicillins ampicillin
• Cephalosporins cephamycin C, cefmetazole,
  cefalothin, cefazolin
• Nitrocefin
• Aminoglycosides

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keywords

• Terms
• Soil samples, ß lactamase, antibiotic activity,
  OM, cell wall, peptidoglycan, Gram-negative and
  Gram-positive, barrier layer, periplasm, MICs
  (minimum inhibitory concentrations 3-13 and 100
  µg/ml, vacuoles, blebs, spheroplasts,
  bacteriolysis, ghosts, snake-forms,
  indole-positive, opportunistic pathogen,
  septicemia, 7 PBPs in CM, PBP2- spheres, PBP3-
  filaments, t.s. phenotype, affinity for PBPs,
  autoradiography