Antimicrobial Therapy

1
Antimicrobial Therapy

• Chapter 10

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History of Antimicrobials

• 1600s
• Quinine for malaria
• Emetine for amebiasis (Entamoeba histolytica)
• 1900-1910
• Arsphenamines for syphilis
• 1935
• Sulfonamides - broadly active
• 1940
• Penicillin - substantially more active than sulfa
  drugs
• Originally discovered in 1929 by Alexander
  Fleming (Scottish)
• Nobel Prize, 1945
• Knighted, 1944
• Produced by fungus Penicillium chrysogenum

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Mechanisms of Action of Anitmicrobial Drugs

• Selective toxicity
• Antimicrobials must be toxic to the microbe, but
  not to the host
• Unfortunately, no such antibiotic exists
• Mechanisms of action
• Cell wall synthesis inhibitors
• Cell membrane inhibitors
• Protein synthesis inhibitors
• Nucleic acid synthesis inhibitors
• Metabolic Pathways

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Cell Wall Inhibitors

• Cell wall
• Outer layer of bacterial cell
• Barrier to outside
• Maintains osmotic pressure
• Peptidoglycan (polymer)
• Polysaccharide and cross-linked peptides
  (transpeptidation)
• N-acetylglucosamine (NAG)
• N-acetylmuramic acid (NAM)
• Only found in bacteria
• Synthesis of peptidoglycan layer is performed by
  several enzymes
• Gram have substantially thicker peptidoglycan
  layer

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Cell Wall Inhibitors

• Penicillin and Cephalosporin
• Highly insoluble in natural form
• Usually converted to a salt to increase
  solubility
• Contains a ß-lactam ring that interferes with
  cell wall synthesis
• Penicillin is first bound by cellular penicillin
  binding receptors (PBP)
• This binding interferes with transpeptidation
  reaction
• This prevents peptidoglycan synthesis

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Cell Wall Inhibitors
Semisynthetic penicillins
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Cell Membrane Function Inhibitors

• The cell membrane is a biochemically-rich
  compartment
• Polymyxins
• Contain detergent-like (amphipathic) cyclic
  peptides
• These damage membranes containing
  phosphatidylethanolamine
• Novobiocin - inhibits teichoic acid synthesis
• Ionophores - disrupt ion transport
• Discharge membrane potential
• Disrupts oxidative phosphorylation

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Protein Synthesis Inhibitors
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Protein Synthesis Inhibitors

• Most interfere with ribosomes
• By preventing ribosome function, polypeptide
  synthesis is inhibited
• Compounds
• Aminoglycosides (e.g., streptomycin)
• Bind to 30S subunit
• Interferes with initiation complex
• mRNA localization to P site
• fMet tRNA
• Incorrect amino acid is incorporated into
  polypeptide
• Tetracyclines
• Bind to 30S subunit
• Prevents IF3 binding
• No tRNA binding

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Protein Synthesis Inhibitors

• Others
• Macrolides - initiation complex, translocation
• Azalides - initiation complex, translocation
• Ketolides - initiation complex, translocation
• Lincomycins - initiation complex, translocation
• Glycylcyclines - Tet analogs bind with higher
  affinity
• Chloramphenicol - Inhibits peptidyl transferase
• Streptogramins - Irreversible binding to 50S
  subunit unknown mechanism
• Oxazolidinones - Inhibit fMet tRNA binding to P
  site

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Nucleic Acid Synthesis Inhibitors

• Types
• DNA/RNA polymerase inhibitors
• Base analogs
• Rifampin
• Binds with high affinity to ß subunit of
  DNA-dependent RNA polymerase
• Prevents RNA synthesis
• Quinolones - inhibit bacterial DNA gyrase
• Sulfonamides
• Structural homologs of p-aminobenzoic acid (PABA)
• PABA is required for folic acid synthesis by
  dihydropteroate synthetase (DHPS)
• Folic acid is a nucleotide precursor
• Sulfa compounds compete with PABA for the active
  site of DHPS

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Nucleic Acid Synthesis Inhibitors
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Resistance to Antimicrobial Drugs

• Mechanisms of resistance
• Enzymes that cleave or otherwise inactivate
  antibiotics
• ß-lactamases
• Changes in bacterial permeabilities
• Prevents entry of antibiotic into cell
• Mutation in target molecule
• Alter binding characteristics of the antibiotics
• Alteration of metabolic pathways
• Some resistant bacteria can acquire PABA from the
  environment
• Molecular pumps (efflux systems)
• Secretion systems that export antibiotics faster
  than the rate of import

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Nongenetic Origins of Drug Resistance

• Low replication rates
• Antibiotic is metabolized or neutralized before
  it act
• Mycobacteria spp.
• Alteration of cellular physiology
• Bacterial L forms are cell wall-free
• Streptococcus spp., Treponema spp., Bacillius
  spp., others
• Colonization of sites where antibiotics cannot
  reach
• Gentamicin cannot enter cells
• Salmonella are thus resistant to gentamicin

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Genetic Origins of Drug Resistance

• Chromosomal Resistance
• Genes that regulate susceptibility
• Often found in enzymes, rRNA and secretion system
  genes
• Mutations in RNApol render it resistant to the
  effects of rifampin
• Efflux pumps with specificity for antibiotics
• Found in all bacteria
• All possess large hydrophobic cavity for binding
  antibiotics

Five efflux pumps (antiporters) that regulate
antibiotic resistance (Paulsen, 2003)
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Genetic Origins of Drug Resistance

• Extrachromosomal Resistance
• Often account for interspecies acquisition of
  resistance
• Contribute to multi-drug resistance (MDR)
• Genetic elements are
• Plasmids
• Transposons
• Conjugation
• Transduction
• Transformation

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Drug Resistance
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Antimicrobial Activity In Vivo

• Drug-Pathogen Relationships
• Environment
• State of metabolic activity slow-growing or
  dormant bacteria less susceptible
• Distribution of drug CNS is often exclusionary
• Location of organisms Some drugs do not enter
  host cells
• Interfering substances pH, damaged tissues, etc.
• Concentration
• Absorption some cannot be taken orally
• Distribution some accumulate in certain tissues
• Variability of concentration peaks and troughs
• Postantibiotic effect delayed regrowth of
  bacteria

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Antimicrobial Activity In Vivo

• Host-Pathogen Relationships
• Alteration of tissue response
• Suppression of microbe can reduce inflammatory
  responses
• Alteration of immune response
• Prevention of autoimmune antibodies (e.g.,
  rheumatic fever)
• Alteration of microbial flora
• Expansion of harmful flora (e.g., C. difficile)

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Clinical Use of Antibiotics

• Selection of appropriate antibiotic
• Accurate diagnosis is critical
• Susceptibility testing should be performed if
• Isolated microbe is often antibiotic resistant
• Infection would likely be fatal if incorrect drug
  is selected
• Need rapidly bactericidal activity (e.g.,
  endocarditis)
• Susceptibility testing is often performed with
  antibiotic discs
• A large zone of clearance suggest sensitivity

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Minimal Inhibitory Concentration

• The MIC determines the dose of antibiotic
  necessary to kill or retard bacteria
• It is usually done as a tube test (i.e., liquid
  phase)
• Serial dilutions of an antibiotic is made, then a
  defined number of bacteria are added to the tubes
• Tubes are read the following day (or days) for
  the endpoint

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Minimal Inhibitory Concentration
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Dangers of Indiscriminate Use

• In some countries antibiotics are available OTC
• This has led to the emergence of antibiotic
  resistance
• Often the wrong antibiotic is used
• The full regimen is not completed
• Hypersensitivities (e.g., penicillin anaphylaxis)
• Hepatotoxicity
• Changes in normal flora

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Antimicrobial Chemoprophylaxis

• Exposure to specific pathogens (e.g., N.
  meningitidis)
• Health-related susceptibilities
• Heart disease/valve replacement
• Respiratory disease (e.g., influenza, measles)
• Recurrent urinary tract infections
• Opportunistic infections
• Post surgery
• Disinfectants
• Medical devices (e.g., catheters)

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Antifungal Drugs
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Antiprotozoal and Antihelminth Drugs
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Antiprotozoal and Antihelminth Drugs
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Toxic Side Effects

• Penicillins Hypersensitivity
• Cephalosporins Hypersensitivity, nephritis,
  hemolytic anemia
• Tetracyclines Discoloring of teeth
• Chloramphenicol Disruption of RBC production
• Erythromycins Hepatitis
• Vancomycin Deafness, leukopenia, renal damage
• Sulfonamides Hemolytic anemia, bone marrow
  depression